Energy consumption analysis of sheep milk cooling systems

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UNIVERSITÀ DEGLI STUDI DI SASSARI

SCUOLA DI DOTTORATO DI RICERCA

Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle Produzioni Alimentari

Indirizzo in Scienze e Tecnologie Zootecniche

Ciclo XXV

ENERGY CONSUMPTION ANALYSIS OF SHEEP MILK

COOLING SYSTEMS

Direttore della Scuola

Prof.ssa Alba Pusino

Referente di Indirizzo

Prof. Nicolò P.P. Macciotta

Docente Guida

Prof. Antonio Pazzona

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ABSTRACT

In this study the energy consumption of milk cooling systems for sheep milk was quantified, depending on number of milkings and performance class. The aim was to produce updated data about the incidence of energy consumption for milk cooling, useful for energy auditing research in livestock. The cost in electricity bills and the weight on the current price of sheep milk were calculated. The experimental work was carried out on 22 milk cooling systems in Sardinia, equipped with open-type tank and direct expansion system. A performance test was performed to determine the cooling time, monitoring the milk cooling energy consumption simultaneously. Factors affecting energy consumption were identified, such as the number of milkings, power/volume ratio and performance class. The average energy consumption was 1.795 kWh/100 l for two milkings and 2.427 kWh/100 l for four milkings tanks. The energy consumption for storage of cooled milk was estimated averagely 0.120 kWh/100 l. Malfunctioning systems in the sample consumed averagely 26% more than those with regular cooling time. The electricity cost for cooling accounts for 0.63% on the current price of sheep milk. The study highlights the need for regular maintenance in old tanks and a modernisation of milk cooling systems in sheep breeding farms in Sardinia, influenced by the introduction of a milk quality payment scheme, taking into account the importance of a correct and fast cooling process.

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RIASSUNTO

In questo studio è stato quantificato il consumo di energia elettrica degli impianti per la refrigerazione del latte ovino, in funzione del numero di mungiture e della classe di prestazione degli impianti. L’obiettivo è stato quello di produrre dati aggiornati circa l’incidenza del consumo elettrico per la refrigerazione del latte, nell’ambito delle ricerche di energy auditing nel settore zootecnico. Si è inoltre appurato il relativo costo in bolletta elettrica ed il peso percentuale sul prezzo attuale del latte ovino, nell’ottica di un suo prossimo regime di pagamento a qualità. Il lavoro sperimentale è stato eseguito su 22 impianti in Sardegna, dotati di serbatoio di tipo aperto ad espansione diretta. Su di essi sono stati effettuati controlli prestazionali tesi ad appurare il tempo di refrigerazione, ed un monitoraggio dei consumi elettrici, tramite strumentazione specificamente assemblata. Sono stati individuati i fattori che incidono sui consumi elettrici, come il numero di mungiture dell’impianto, il rapporto potenza/volume e la classe di prestazione. I consumi elettrici medi dei tank in classe II, maggiormente diffusi sul territorio, si sono attestati su 1,795 kW/100 l per gli impianti a due mungiture e di 2,427 kWh/100 l per quelli a quattro mungiture. I consumi per la conservazione del latte refrigerato alla temperatura di 4-5°C sono stati stimati mediamente in 0,120 kWh/100 l. Sono stati inoltre messi in evidenza i consumi degli impianti malfunzionanti presenti nel campione, maggiori mediamente del 26% rispetto a quelli con tempi di refrigerazione regolari. Il costo dell’energia elettrica per la refrigerazione incide per lo 0,63% sul prezzo attuale del latte ovino. Lo studio mette in evidenza la necessità di una manutenzione regolare in impianti con età elevata e un rinnovamento impiantistico nelle aziende zootecniche ovine della Sardegna, condizionato dall’introduzione di un regime di pagamento basato sulla qualità del latte, che tenga in attenta considerazione l’importanza di un corretto e rapido processo di refrigerazione.

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I. INTRODUCTION

In the modern agriculture, like in other economic sectors, energy consumption and its supply sources are gaining a strong importance. The technological progress allowed replacing a large amount of labor force with an engine force that led to an increasing use of different energy sources. Even dairy farming is walking through a process of intense productive a technological renovation. The productivity of the farms shows that there has been an increasing concentration of farms in large enterprises. The availability of facilities and equipment increased, with a reduction of the workforce and its gradual replacement by systems with a steadily higher energy demand. Consequently, even if the energy cost still represents a high portion of total production costs, the amount of energy consumed will assume a stronger role, together with the gradual and increasing technology transfer in livestock production sector.

The need for saving or self-produce energy is due to several European regulations that have interpreted the so-called "20-20-20" of the Kyoto Protocol, whose measures were revised and strengthened in the recent United Nations Conference Rio +20. The aim is to reduce the carbon dioxide (CO2) emissions causing global warming, rationalising energy consumption and increasing power production ratio from renewable energy sources by 20% within 2012, compared to emissions in 1990. Furthermore the Kyoto Protocol introduced three mechanisms regulating CO2 emissions, through the establishment of the so called “emission credits”:

Clean Development Mechanism (CDM) allows industrialized countries to implement projects developing environmental benefits in terms of reducing greenhouse gas emissions and economic and social development, generating emission credits (CERs) for the countries that promote interventions;

Joint Implementation (JI) allows industrialized countries to implement projects to reduce greenhouse gas emissions in another country and use credits, together with the host country;

Emissions Trading (ET) allows the exchange of emission credits between industrialized countries, a country that has achieved a reduction of their greenhouse gas emissions than its target can thus yield (using ET) such “credits"

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to a country that, on the contrary, has not been able to meet its commitments to reduce greenhouse gas emissions (United Nations, 1998).

Europe was the continent that developed the world’s first Emission Trading System (ETS) in order to achieve the targets of the Kyoto Protocol. In fact the European Union (EU) was aware of the negative effects of economic and demographic growth to environmental resources, causing damage to the biosphere. Furthermore an internal and external policy was strategic to secure energy sources and increase energy efficiency, through development of alternative supplies and renewable energy systems, since about 50% of EU energy requirements currently derive from imported products (EU Parliamentary Meeting, 2006). The EU interpreted the Kyoto Protocol and met its needs through the Green Paper in 2006.

The Green Paper is considered the most important regulation in developing EU energy policy. The aims of the document are to contrast the climate change by promoting renewable energy sources with low CO2emissions; diffusion of technologies for energy efficiency; coordinating and securing the EU supply sources by diversifying the energy mix. The paper states that the economic sustainability of energy produced inside the EU borders could be achieved only with an open energy market, based on the competition between energy supply companies. In fact the main objective of the EU internal energy market is to promote the competitiveness of EU industry, which requires a stable and predictable regulatory framework in the long-term period (Commission of the European Communities, 2006).

The EU also supports a common European CO2 tax and a modernised common agricultural policy that gradually transfers public subsidies to biofuels and biomass energy production, or to technologies regarding the Carbon Capture and Sequestration (CCS). However the main EU economies are still characterised by a high degree of energy import: United Kingdom imports only 10% of the energy consumed, but Spain and Italy import respectively 80 and 85%. Due to differences in the size of economies in the EU, substantial variation exists in total energy consumption: Germany, the largest energy consumer, uses 13 times more energy than Hungary, the smallest consumer. Germany is also the largest energy importer in absolute terms, though its degree of

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1. ENERGY DEMAND IN THE MODERN DAIRY FARM

The estimation of the energy needs for the national livestock sector is quite complex. The Italian production system is heterogeneous in terms of structure and basic farming. Moreover, energy demanding technologies applied to the production process, change basically depending on the structural characteristics and the productivity of the livestock. These characteristics, which are the most important variables explaining the different energy needs of dairy farms, are only partially known (Rossi et al., 2009). The multifunctionality of the agricultural sector is characterized by a systematic approach available to contributions from different disciplines and alternative technologies, which are concurrent with each other though. The production of renewable energy developed a growing interest in the agricultural sector, especially the biomass cultivation and the installation of photovoltaic systems, which has become a priority for many farming activities. The model of distributed generation, the energy efficiency of the process during crop production, the modernisation of facilities and the establishment of a functional framework to promote technological innovation and environmental sustainability, are targets which received a steady growing interest within the agricultural sector. The development of energy efficiency regulations in the agricultural field requires the availability of new skills, such as plant biology experts, to optimize the production process for energy saving and environmental monitoring of the agro-food chain (ENEA, 2010). The interventions regarding this sector in order to improve energy efficiency estimate a potential energy saving of 1.4 Mtoe in 2016 and 5.5 Mtoe within 2020. In fact the agricultural sector is suitable for application of technologies to produce energy in Italy, as it accounts for only 3% on the national demand for primary energy, with 3.3 Mtoe of primary energy consumed (ENEA, 2011) and 2% for domestic energy consumption (Terna, 2011). Agriculture requires low power installations and is characterized by a high availability of space for placing energy production systems. Therefore it is possible to predict a future scenario in which agriculture, with the diffused generation, becomes largely energy self-sufficient. The benefits will be both environmental and economic.

Energy consumption for the agricultural sector in Sardinia in 2010 amounted to 197.5 GWh, equal to 11.7% of national energy demand for agriculture (Table 1). The regions

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where energy consumption is concentrated are Lombardy and Emilia Romagna, where they remained basically stable over the last two years, and currently account for 1.8% on the total national energy consumption.

Table 1. National and regional energy consumption for the economic-productive sectors in 2009 and 2010. Data in GWh (Terna, 2011).

Agriculture Industry Service Domestic TOTAL

Regions

2009 2010 2009 2010 2009 2010 2009 2010 2009 2010 Piedmont 309.3 309.0 12,451.1 13,153.4 6,806.2 6,901.0 4,993.7 5,070.2 24,560.3 25,433.6

Valle d'Aosta 3.5 4.4 354.7 429.5 301.8 334.9 162.0 184.0 822.0 952.8

Lombardy 849.4 840.5 31,437.8 34,279.2 18,461.4 18,717.1 11,800.4 12,044.8 62,549.1 65,881.7 Trentino Alto Adige 241.1 232.0 2,333.0 2,489.9 2,452.7 2,594.6 1,228.4 1,261.3 6,255.2 6,577.8 Veneto 617.4 618.8 14,971.0 15,447.4 7,949.2 8,059.1 5,558.7 5,621.9 29,096.3 29,747.2 Friuli Venezia Giulia 125.4 123.3 5,143.2 5,841.9 2,339.8 2,329.3 1,395.9 1,426.1 9,004.2 9,720.5 Liguria 34.2 34.0 1,535.9 1,634.2 2,963.2 2,953.4 1,907.1 1,930.4 6,440.4 6,552.1 Emilia Romagna 933.0 924.5 11,400.5 12,163.6 8,476.1 8,939.1 5,275.5 5,283.7 26,085.2 27,310.9 Northern Italy 3,113.4 3,086.5 79,627.2 85,439.0 49,750.3 50,828.8 32,321.7 32,822.3 164,812.6 172,176.6 Tuscany 283.6 287.1 8,661.3 8,955.1 6,579.5 6,619.1 4,369.5 4,402.0 19,893.9 20,263.2 Umbria 101.3 104.0 2,994.4 3,178.8 1,291.7 1,311.8 977.6 980.4 5,364.9 5,575.0 Marche 127.1 124.8 3,273.4 3,231.7 2,367.8 2,387.5 1,643.0 1,643.7 7,411.4 7,387.6 Lazio 330.8 328.2 4,737.8 4,829.7 10,930.9 10,983.7 7,118.6 7,112.3 23,118.1 23,253.8 Central Italy 842.8 844.0 19,667.0 20,195.3 21,169.8 21,302.1 14,108.7 14,138.3 55,788.3 56,479.7 Abruzzo 82.5 83.6 2,953.1 2,988.4 1,946.4 1,949.6 1,269.7 1,323.2 6,251.7 6,344.7 Molise 29.6 30.8 723.4 698.6 380.1 379.9 300.4 302.5 1,433.5 1,411.7 Campania 267.7 271.3 4,830.9 5,001.7 6,210.4 6,289.7 5,829.0 5,891.3 17,138.0 17,454.0 Puglia 514.8 510.8 7,192.5 8,230.6 4,459.6 4,515.5 4,260.6 4,265.3 16,427.5 17,522.2 Basilicata 67.0 63.1 1,491.7 1,499.9 616.5 598.3 522.6 525.4 2,697.9 2,686.6 Calabria 122.2 117.9 956.0 959.6 2,324.6 2,327.3 2,147.5 2,143.5 5,550.3 5,548.3 Sicily 406.7 404.9 6,724.9 7,157.5 5,564.9 5,676.2 5,874.9 5,848.2 18,571.4 19,086.9 Sardinia 203.0 197.5 6,339.3 6,268.7 2,412.3 2,417.1 2,289.2 2,290.5 11,243.9 11,173.8 Southern Italy 1,693.7 1,679.8 31,211.8 32,805.0 23,914.8 24,153.6 22,494.0 22,589.8 79,314.2 81,228.3 ITALY 5,649.9 5,610.3 130,505.9 138,439.3 94,834.9 96,284.5 68,924.4 69,550.5 299,915.2 309,884.5

Dairy sheep farming is still one the leading sectors in the Sardinian economy. In 2011 Sardinia held 3,008,467 sheep, 52% of national ewes population (increased by 7% compared to 2000) and 251,375 cattle. Sardinia is also the Italian region with the highest number of goats, with 237,320 animals surveyed in 2011, with an increase by

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Table 2. Milk delivered to milk processing facilities (q). Data per Italian region (ISTAT 2010). Regions Cow milk Sheep milk Goat milk Buffalo milk TOTAL

Piedmont 8,283,282 17,275 23,977 7,978 8,332,512 Valle d'Aosta 324,411 - 3,687 - 328,098 Lombardy 40,985,825 2,910 42,295 20,423 41,051,453 Liguria 233,446 - 1,185 - 234,631 Trentino-Alto Adige 5,145,290 - 9,700 - 5,154,990 Bolzano 3,648,418 - 4,889 - 3,653,307 Trento 1,496,872 - 4,811 - 1,501,683 Veneto 10,038,990 2,772 20,589 5,850 10,068,201 Friuli-Venezia Giulia 1,905,028 - 91 10,572 1,915,691 Emilia-Romagna 21,752,820 12,046 - - 21,764,866 Tuscany 727,186 687,862 1,151 2,772 1,418,971 Umbria 631,729 29,613 114 - 661,456 Marche 564,737 34,112 - 2,620 601,469 Lazio 4,521,244 416,504 16,097 266,950 5,220,795 Abruzzo 318,593 34,278 348 - 353,219 Molise 707,421 - - 333 707,754 Campania 2,416,010 19,826 554 1,420,178 3,856,568 Puglia 2,369,246 36,336 8,239 10,217 2,424,038 Basilicata 241,510 1,258 3,333 765 246,866 Calabria 601,107 23,886 250 46 625,289 Sicily 1,580,213 171,195 3,686 25,651 1,780,745 Sardinia 2,383,726 2,832,349 114,052 220 5,330,347 ITALY 105,731,814 4,322,222 249,348 1,774,575 112,077,95

Nowadays the economic budget of dairy farms is burdened by high power and heat demand caused, in most cases, by the excessive size of plants and machineries. Considering the current global economic situation, where the cost of prime materials and production factors are not followed by an adequate increase of the agricultural product price, the success of agro-livestock depends mainly on the ability of the farmer to reduce production costs including energy, through the introduction of new technologies and good farming practices, management systems and machineries (Rossi et al., 2009). Rationalizing energy consumption through conservation and self-energy production allows both to reduce or eliminate farming costs that reduce profit margins, and to diversify income for the farmer, improving the market competitiveness of the company.

In order to identify strategies for higher energy efficiency, it is possible to carry out energy auditing studies, which can systematically evaluate the efficiency of the organization of the power management system, monitoring both heat and electrical energy consumption of all users. The energy auditing study identifies the critical points

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to reduce energy consumption and make the company more energy efficient. Through these studies, possible sources of leakage or waste can be found, in order to provide decision support for optimising consumption, considering also the national incentive mechanisms for the implementation of technologies to self-produce and save energy. The energy consumption data allows the estimation of the annual energy requirement of the company, providing suggestions on the size of energy production systems, such as photovoltaic, solar thermal, wind or biogas. These technologies should be chosen depending on the size of the company, facilities and infrastructures available, final destination of the generated energy, environmental issues etc.. Installations for renewable energy production are widely promoted by the Italian national government. After planning the energy production plant, a technical-economic analysis of the investment can be performed in order to quantify the convenience with the main economic indicators (Net present value, internal rate of return, payback time etc.).

2. ENERGY DEMAND FOR MILK COOLING

Many studies regarding energy auditing in the farms can be found in literature, especially concerning cattle farms. Some studies are focused on energy consumption and energy efficiency of the breeding system, both in terms of direct energy (fuels, lubricants, electricity, gas etc.) and indirect energy consumption, that is the energy required to provide the production factors used in the farm (Marijke et al., 2006). Other papers consider only the direct consumption of electricity and thermal energy for the operations of buildings of the farm, identifying the users with the greatest impact on energy consumption. In a dairy farm the largest impact seems to be the milk cooling (43% of total consumption of energy in the milking parlor), the heat for the washing water (27%) and the vacuum pump (15%), while other utilities seem to be secondary (Institut de l'Elevage, 2009). On the other hand if the milking operation takes place via robotic systems, the most demanding electrical loads are the vacuum pump of the milking plant, because of the higher number of operating hours per day, followed by the

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the same trend there are studies establishing that the consumption of electricity due to the milk tank, water heating and vacuum pump, is equal to 36-55% of the electricity used in the whole company (Peebles et al., 1994). Therefore milk cooling is one of the main energy users in a cattle farm. The electrical energy consumption for this operation can be estimated 10-18% of the whole energy consumed; the amount ranges from 1.76 to 2.42 kWh/100 kg of milk (Peebles et al., 1994). In other studies the cooling of cow milk on annual basis is the most energy-consuming operation, with 96.7 kWh/cow, corresponding to 1.1 kWh/100 l milk refrigerated at 4°C, equal to approximately 21-24% of the whole electricity consumption of the farm, depending on consistency of livestock (Murgia et al., 2008).

In order to estimate the energy demand for milk cooling, evaluations using objective parameters should be performed, such as energy efficiency indicators, which allow identifying critical operations, suggesting the adoption of energy saving measures. These energy efficiency indicators can be expressed in term of "Energy Cost" (EP, Energy Price) of agricultural products: it expresses the amount of energy (MJ) required to produce a unit of product, like 1 l of milk (Refsgaard et al., 1998, Meul et al., 2007). In other circumstances, when the indirect energy required to produce a unit of product can not be defined or estimated, the Energy Utilization Index (EUI) can be used: it defines the energy consumed for each animal breaded (kWh/head) or for each unit of finished product (kWh/l of milk or kWh/100 l), expressed on daily and annual basis (Edens et al., 2003; Ludington and Johnson, 2003). In the United States, the cooling tanks for cow's milk, usually characterized by high rated volume (several thousand litres), in good shape and with no energy saving devices (ECM, Milk Cooling Energy Conservation Measures), show a EUI ranging averagely between 0.8 and 1.2 kWh/100 l of cooled milk. If a device based on plate heat exchangers is installed for pre-cooling milk, the EUI decreased to 0.6-0.9 kWh/100 l. If in addition to the pre-cooler a variable frequency drive (VFD) is added, the rotation speed of the pump extractor can be decreased, reducing the milk flow on the heat exchanger, making the EUI further descending to 0.4-0.7 kWh/100 l (SCE, 2004). The EUI is related to milk cooled up to 7°C, while for milk cooling in Italy, storage temperatures are lower, about 4°C. This aspect shows that EUI for milk cooling in Italy and Europe are higher than the United States. There are also no data concerning neither the incidence of breeding sheep on the

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overall energy demand of national agriculture nor the energy consumption for cooling sheep milk. Furthermore the bibliography is scarce even for energy auditing studies in dairy sheep farms.

3. THE MILK COOLING SYSTEM

Milk is a good growth medium for many microorganisms commonly in the environment. The contamination could derive both from within the breast (endogenous flora) and from outside (exogenous flora). Some of these organisms may be harmful to human health, others may cause alteration of milk constituents (emphasising the storage and processing issues), and still others are a marker of insufficient hygienic conditions. The cooling, inducing stasis of the multiplicative effect at low temperature (microbial growth is inhibited completely at 4°C), is the best way to contain microbial proliferation, and its effectiveness is linked both to the performance of the tank and to the initial milk quality. Since cooling is not a sanitary treatment but a stabilisation method, both during breeding and milking, all procedures limiting the initial contamination of the milk should be applied (Pazzona, 1999). In fact the quality of raw milk and all dairy products is the result of activities related to the production process, from the farm to processing in the dairy company. The safety of the process is mainly ensured by preventive approaches, such as the application of good hygiene practices based on risk analysis and critical control points (HACCP, Hazard Analysis and Critical Control Point) (Vilar et al., 2012).

The cooling is not a sanitation process, because it does not kill microorganisms, but stops or slows down their growth. The milk derived from dirty animals, environments and milking systems, even if stored at 4°C, doubles its microbial charge after 24 hours, which becomes four times higher after 48 hours; on the contrary, storing the milk at 4°C produced by clean animals and environments, using clean utensils, maintains roughly unchanged the microbial charge even after 48 hours. During storage at low temperature, a psychrophile microflora can develop, which is able to produce enzymes

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The consequence is the taste of rancid (lipolysis), bitter (proteolysis and lipolysis), other unpleasant tastes and a deterioration of the rheologic properties of milk. Generally such defects appear only after 48 hours storage at 4-5°C and are more remarkable in the milk highly contaminated after two days storage (Unalat and CRPA, 1992).

Cooling the milk is meant as a decrease of its temperature without taking it to the freezing point. This involves the removal of a certain amount of heat that must be transferred and disposed in an external medium. When heat follows the gradient, the heat flow decreases temperature, which turns the body from hot to cold. A cooling machine allows to reverse the natural direction of heat flow, thus to transfer heat from a cold source to a hot body. All this happens, of course, consuming energy.

Among the practical refrigeration systems, the most common one in many applications (domestic cooling, industrial, air conditioning) is the compression refrigeration system, which uses substances with special thermodynamic properties as heat carriers, called Refrigerant Fluids. Nowadays the refrigerants commonly used are the R22 (chlorodifluoromethane), now replaced by R-507 or R-407C, and R-404a (a mixture of 1,1,1 trifluoroethane, and 1,1,1,2 tetrafluoroethane and pentafluoroethane) that, contrarily to CFCs used in the past, have an ozone depletion rate extremely low (Pazzona, 1999). Even the R12, used in the oldest cooling systems, will be withdrawn by 2031 and replaced by the refrigerant R-134a.

These fluids are subjected to changes of state (cooling cycle) that take place repeatedly and under control, within a refrigerating circuit composed of four main parts:

compressor

condenser

lamination device

evaporator.

The refrigeration cycle consists of a series of thermodynamic transformations charged to the refrigerant (Fig. 1).

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The process can be described starting from the compressor, which compresses the refrigerant in gas state and low temperature coming from the evaporator (suction line), so that it can be condensed at high temperature. The compressed and high temperature refrigerant flows to the condenser (flow line), where it releases heat to the outside air, it condenses and cools down. The cooled and liquid state refrigerant still tends to be hotter than the milk. For this reason there is a thermostatic expansion valve (lamination device), where the refrigerant expands, cooling down to a very low temperature. The cold refrigerant flows through the evaporator (low pressure circuit), which is in contact with the milk (direct expansion systems) or with water (indirect expansion systems with ice builder). The evaporator performs a heat exchange from the milk to the refrigerant, which turns from liquid state to gas state. Then it comes back to the condenser, completing the cycle. In fact, when the refrigerant goes back to the condenser, the heat previously taken from the milk by the evaporator is transferred to the outside air (SCE, 2004).

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3.1 Compressor

The compressors commonly used in the milk cooling systems are hermetic reciprocating type, in which the compressor itself and the electric traction engine are mounted on the same shaft and sealed inside a case of welded steel. This type is characterised by a cylindrical metallic box in which the suction pipe, the delivery pressure and the electrical connections emerge outside (Fig. 2). Despite the open and semi-hermetic types used in refrigerators, the semi-hermetic

type has the advantage of being compact, low noisy, protected from the external environment; if a damage occurs, it can not be repaired, but this negative aspect is compensated by a high reliability and long life. The performance is influenced by the design characteristics of the compressor (displacement, valves type) and the working conditions (rotation speed, suction and discharge pressure, type of refrigerant used). According to this, manufacturers certify the rated performance as cooling capacity (kcal/h) and maximum power (kW). Some milk cooling systems use a new generation of chillers, also known as "Scroll compressors" (Fig. 3). They are cylindrical shape rotary compressors, characterized by two metal spirals: the first one is attached to the body of the compressor, while the second one is connected to the engine shaft and rotates eccentrically inside the first one. The orbital rotation of

the rotating scroll compresses the air in the spaces between the two spirals, until reaching the desired pressure at the center of the attached scroll, where it is expelled through the discharge line. The advantage of the scroll compressor is mainly due to the energy savings achieved, estimated about 20%, compared to reciprocating compressors of the same capacity (Emerson Climate, 2006).

Fig. 2. A hermetic reciprocating compressor.

Fig. 3. Scroll compressor with pattern of metal spirals.

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3.2 Condenser

The condenser is a heat exchanger that transfers heat to the external environment (Fig. 4). Milk cooling tanks are usually provided with air-cooled condensers. The cooling medium exchanges heat with the refrigerant only as sensible heat, thus the amount of heat that can be disposed of, at constant temperature drop and flow rate, depends on the specific weight and the specific heat of the refrigerant. Given the low specific heat (1 kJ/kg°C

against 4.18 kJ/kg°C of the water) and the limited coefficient of thermal adduction, the air condensers need high flow rates and large exchange areas. However they are simple under a designing and operating point of view, considering the availability of virtually unlimited air. The air-cooled condenser is the finned-pack type, formed by a series of thin diameter tubes, with the refrigerant flowing in, equipped with fins that extend the exchanging surface. An electric fan forces the passage of the air independently.

3.3 Lamination device

The lamination device provides the evaporator with liquid refrigerant at low pressure. This aim is achieved by pushing the fluid through a calibrated hole, causing a fast decrease of the condensing pressure, together with the cooling of the fluid through partial evaporation. Two parts characterises the lamination device: the capillary tube and the thermostatic expansion valve (Fig. 5). The capillary consists of a very thin tube, with diameter and length proportioned according to the power and the specific

operating conditions of the system. The thermostatic expansion valve injects a certain amount of refrigerant in the evaporator, according to the specific thermal load. It is a

Fig. 4. Air condenser.

Fig. 5. The lamination device: the capillary on the top and the thermostatic expansion valve on the bottom.

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3.4 Evaporator

The evaporator can remove heat directly from the substance to cool down, or indirectly when an intermediate fluid is used (pure water or alcoholic solution). The evaporators mounted in the milk tanks are often provided with the direct expansion type, characterized by neither fluid recycling, nor a clear separation between the vapor and the liquid phase of the refrigerant. In the direct expansion systems, the evaporator is welded on the outer side of the tub containing the milk. It can be formed by a pipe arranged in a spiral, or by two steel sheets welded among which a channelling of various shapes (mesh honeycomb, foam) is positioned, in which the refrigerant flows. The heat exchange surface, the conformation of the path and the flow rate of the refrigerant determine the heat exchange efficiency of the evaporator: the higher the flow rate, the higher the quantity of heat exchanged per surface unit.

3.5 Control devices and other system components

The control devices can start and stop the system (primary control), or regulate and protect it during operation (secondary control). Generally these devices consist of a sensor and an actuator that turns on the operation of the system. The most important ones are the thermostats and pressure switches that control the system, both primary and secondary type, as a function of temperature and pressure detected in different sections of the circuit. Moreover the complementary parts have the purpose of improving the performance of the refrigeration system, but they are not essential for its operation. There are components and parts commonly on both the high pressure circuit (oil separator, liquid receiver, filter drier, etc..) and low pressure circuit (suction accumulator).

The oil separator is mounted on the pressure side of the compressor, and is characterised by a cylindrical box in which takes place the separation of a largest part of the lubricant contained in the gas; the oil accumulates at the bottom and is pulled back to the compressor crankcase, thus avoiding deposits on heat transfer surfaces of the evaporator and the condenser.

The liquid receiver is a container with cylindrical shape, placed upstream of the expansion valve (not present in plants in capillary), which temporarily collects the liquid from the condenser. It is a reserve of refrigerant that allows coping with rapid

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changes in heat load on the evaporator; during the shutdowns for maintenance of the system it hosts all the liquid.

The dehydrator filter has to eliminate the residual moisture in the refrigerant that may be originated from poor hermetic seal of the circuit or from lubricants and fluids not properly dehydrated. The humidity must be avoided in systems with halogenated hydrocarbons, thanks to their poor hygroscopicity. Moisture may cause the formation of oxides and metal deposits that would negatively affect the performance of components in the circuit.

The accumulator on the gas suction line preserves compressor from liquid slugging due to accidental income of refrigerant not evaporated. This may occur for mistakes in dimensioning or power system malfunctions of the evaporator, particularly when the thermal load is low during switching off, in systems with capillary tube.

3.6 Refrigeration systems

The refrigeration systems commonly used can be with direct or indirect expansion.

3.6.1 Direct expansion

This is the most common expansion type for sheep milk cooling systems. The evaporator of the refrigerating unit, honeycomb type, plaque or semi-tubular, is welded on the outer wall of the tub containing the milk which is in direct thermal contact with it. The heat exchange takes place directly on the refrigerant and the milk cooling is contemporary to the operation of the refrigerating unit (Fig. 6).

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3.6.1 Indirect expansion

With the indirect expansion system, heat is removed from the milk through an intermediate medium, usually water (Fig. 7). The evaporator, placed on the bottom of the interspace that surrounds the milk container, is immersed in water, which exchanges heat with the cooling fluid, producing an ice layer around the coil, generating a "chilled water storage".

In the most diffused models, called “instant cooling”, a second circuit, in addition to the refrigerator, allows the constant circulation of cold water during the milk cooling. The water with a temperature of 0,5-1 °C, is sucked by a pump and sprayed through nozzles against the outer walls of the milk container, removing heat from the entire surface; the warm water flows to the ice, fusing it and ensuring the continuous availability of cold water. In these systems there is no coincidence in time between the refrigerating unit work and milk cooling: the compressor operates in the interval between milkings to build ice; when the milk cooling is taking place, only the circulation system works.

Fig. 6. Scheme of a milk cooling system with direct refrigeration system (Pazzona, 1999). Picture on the right by Frigomilk.

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3.7 Milk cooling systems typologies

The milk cooling systems available on the market include tanks with capacities from 100 to 25,000 litres, whose external shape can be simplified to five main types: vertical cylindrical, horizontal cylindrical, semi-cylindrical, ellipsoidal (Fig. 8). The emptying takes place via a hole located at the bottom of the tank and equipped with a tap-tight. The chiller is equipped with one or more agitators to promote thermal exchange within the bulk milk and to prevent fat stratification. Rotation should not exceed 50 r/min, in order to avoid a strong and vigorous mechanical action, which may damage the constituents of the milk, primarily the fat globules, and cause the formation of air bubbles in the milk bulk, with potential oxidation and rancidity. The operation of the agitator is continuous during the milk cooling, while is cyclic during storage, with 2-3 minutes of stirring among 13-15 min break.

Fig. 7. Pattern and shape of a instant cooling tank. 1) evaporator, 2) reserve of ice water, and 3) water pump, 4) nozzles, 5) insulating layer, 6) agitator 7) manhole. Photo on the right by Packo.

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3.8 Standard regulation for milk cooling systems

The European standard reference for milk cooling systems is the EN 13732:2009, which specifies the characteristics for design, construction, operation and testing methods. This classification is a key element for the evaluation of the system during purchase, allowing immediate identification of the milk cooling system performance.

3.8.1 Milk cooling system manufacturing

The legislation requires that the tank and all accessories in contact with the milk must be designed and manufactured in order to prevent the contamination of milk, avoiding smells or flavours, and can be easily cleaned and disinfected without any corrosion or degradation of tank surfaces. Stainless steel is the most common material used for its chemical inertness and mechanical strength. The rated volume of the tank must be between 90 and 98% of its maximum capacity. The tank must be provided with covers

Fig. 8. Refrigeration systems with different tank shapes. From the top right: type vertical cylindrical tank with top opening, horizontal cylindrical, and ellipsoidal.

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for internal inspection and the drawing of the samples. The discharge duct must be made of stainless steel, with a minimum diameter of 50 mm and equipped with a stop tap. The outflow of the milk in the tank occurs thanks to the tilt angle that the bottom of the tank must be provided with, inclined toward the discharge duct.

The agitator must be constructed to avoid any contamination from the outside; it must be able to be easily cleaned and disinfected, and there must be adequate protection so that the operator does not come in contact with moving parts.

In indirect expansion systems, the water tank must be of adequate size for the proper functioning of the control systems and the accumulation of ice-cold water circulation; the ice formation must be smooth on the whole surface of the evaporator; there must be the possibility to inspect the ice reserve and the water replacement. The accumulation of the ice must be sufficient enough to cool down from 35 °C to 4 °C, without any further actuation of the chilling unit, 60%, 30% or 20% of the rated volume of a tank respectively for two, four or six milkings.

3.8.2 Equipment for regulation and control

The control switch of the chilling unit must include the following functions: 0, off; automatic milk cooling; manual milk cooling; harvest; washing. A timer manages the agitator for predetermined time intervals, independently from other setup. Selecting the harvest function, a time switch must operate the agitator for not less than two minutes.

The thermostat must ensure that the cooling process begins as soon as the second or subsequent milkings are introduced; it must operate adequately with a milk quantity between 10 and 100% of the rated volume, with a milk temperature between 0 and 35 °C and at an ambient temperature between -20 °C and the maximum operating temperature, which is related to the milk tank temperature class. The thermostat automatically controls the milk agitator and the simultaneous operation of the condensing unit in direct expansion systems, or the agitator and the water freezing circulation devices in indirect expansion systems. The indirect expansion system has an independent adjustment device for each condensing unit, which automatically controls

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10% and 100% of the rated volume, there is enough ice to meet the required performance.

The cooling and the electric system must meet the standards set by international and national specifications. Each cooling tank must be equipped with a thermometer, which detects the temperature of the milk bulk between 10% and 100% of the rated volume. The graduated rod for measuring the milk quantity stored must be able to measure volumes between 10 and 100% of the rated volume and must be scaled with a millimeter definition.

3.8.3 Performance

The operating characteristics of a milk cooling system are defined according to the number of milkings that the tank may store, the ambient temperature during operation, the time required for cooling the milk from 35 to 4 °C. According to EN 13732:2003 the performance must be specified using the following classification:

number of milkings. It is the number of milkings that can be stored in the tank

before the collection. The number of milkings can be 2, 4 or 6, depending on whether the milk collection is daily, alternate days or every two days;

temperature class. This is the maximum operating ambient temperature

guaranteed by the manufacturer for optimal performance of the tank. It is indicated by a capital letter. The safe operating temperature is the maximum ambient temperature for the cooling plant to operate;

cooling time class. It indicates the maximum time necessary for cooling the milk from 35 to 4 °C. It is indicated with a roman number (Table 3).

Table 3. Performance classes of milk cooling systems.

Temperature class Maximum ambient temperature (°C) Safe operating

temperature (°C) Milk coolingtime class Cooling time from 35to 4°C (hours)

A 38 43 0 2,0

B 32 38 I 2,5

C 25 32 II 3,0

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Thus if a performance class 2BII is reported on the identification plate attached on the tank (Fig. 9), that means a two milkings tank, with temperature class B (the cooling process takes place properly only below an ambient temperature of 32 °C) and a cooling time less than 3 hours.

The cooling time class defines the maximum time within which a tank of a particular performance class must ensure the cooling of all the milk from 35 to 4 °C, with an ambient temperature between 5 °C and the one corresponding to the temperature class of the cooling tank.

The milk rate is referred to:

a two milkings system, empty or containing 50% of its rated volume with milk at 4 °C, in which a quantity of milk at 35 °C equal to 50% of the rated volume is spilled at once;

a four milkings system, empty or containing 25, 50 or 75% of its rated volume with milk at 4 °C, in which a quantity of milk at 35 °C equal to 25% of the rated volume is spilled at once;

a six milkings system, empty or containing 16.6, 33.3, 50, 66.6 or 83.2% of its rated volume with milk at 4 °C, in which a quantity of milk at 35 °C equal to 16.6% of the rated volume is spilled at once.

The operating characteristics and technical specifications of the system shall be reported on a metallic identification plate, easily visible and permanently attached to the tank (Fig. 9). The information should include at least:

manufacturer's name or trade name;

b) type and serial number;

rated volume in litres;

class of service expressed by three symbols indicating respectively the number of milkings, the temperature class, the cooling time class and optionally in brackets the fat content of the milk used during the performance test. For example 2BII (4.5%) indicates a milk cooling system for two milkings, with a temperature class B corresponding to a maximum operating ambient temperature of 32 °C and belonging to the II class, corresponding to a cooling

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The regulation states that there must be also the indication about the manufacture year, the amount of refrigerant in the refrigerating circuit, the compressor power and the maximum cooling capacity.

During storage of the milk between two cooling sessions, the average milk temperature must not exceed 4 °C. Furthermore, the thermal insulation of the tank must be dimensioned in order to ensure that (for the specific temperature class and rated volume of the tank) the temperature increase of the stored milk is less than 3 °C in 12 hours. During cooling or storage there should be no ice in the milk. The agitator must not cause overflow of milk from the tank, even when filled to its rated volume. The HACCP control plans the hygiene and safety equipment for milk cooling, identifies several critical points to be monitored during cooling. The risk is due mainly to an increase of the microbial charge and can be prevented through the choice of a proper initial sizing of the milk tank during purchase, the periodic inspection and maintenance of the tank and the thermometer (Vilar et al. 2012).

Fig. 9. A identification plate attached to a 430 l tank, 2 milkings, with temperature class B and II class cooling time.

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4. DIMENSIONING A MILK COOLING SYSTEM AND MANAGEMENT COSTS

The milk produced daily, the collection interval (daily or alternate days) and the maximum cooling time are the parameters for dimensioning the cooling tank, both in terms of capacity and performance of the refrigerating unit. The choice between the direct or indirect expansion system depends on the need for getting a quick cooling and containing the electricity bills of the farm. In fact, the electric power installed in indirect systems is 40-50% lower than the direct expansion systems, since the operating time of the refrigerating unit for producing the ice reserve does not coincide with the cooling time of the milk, and usually requires a long time (8-10 hours), with a consequent higher energy consumption, relatively to a cooling session.

The rated volume of the tank (Vn) is calculated according to the daily milk production and the frequency of collection: in a daily delivery, the tank should be large enough to contain two milkings, or four milkings in the case of collected on alternate days. The daily amount of milk must be calculated referring to the period of maximum production of the cattle or flock (Lmax), increased by 10% to take into account any peaks, and multiplied by the collection interval, expressed as number of days (n):

n L

Vn  max 1,10

Whereas in the farm, especially sheep and goats, the milk production is subjected to strong seasonal variations, the daily average milk bulk refrigerated (L), as the ratio between the total annual production and the length of the milking season, is equal to 73-75% of the daily maximum milk production. Therefore the capacity of the tank, dimensioned as function of Lmax, is usually under-utilised.

The ratio between the amount of milk cooled between two consecutive deliveries and the rated volume of the tank defines the utilization coefficient (CU):

V n L CU  

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For an optimal use of the system, CU should be close to 1, even if its real annual value is averagely 0.70. A lower coefficient suggests an excessive over-sizing of the tank, which lead to an increase of fixed and variable costs. Even the collection interval must be taken into account when choosing the cooling tank type. It is important to use the milk cooling system compatibly to its design features: using a two milkings tanks for alternate days collection means to under-utilise it, increasing the cost for one milk litre cooling. On the other side, using a four milkings tank as two milkings does not guarantee, due to the lower power of the compressor, the performance indicated by the cooling time class.

The cooling time, thus the performance of the cooling tank, depends on the efficiency of the refrigerating unit and in particular by the power of the compressor. The performance class plays a very important role during the choice of the system, since there is a positive correlation between the cooling time and the bacterial growth, which heavily affects the milk quality. A quick cooling corresponds to a significant decrease of the microbial growth factor, while the influence of the initial bacterial charge is statistically less significant. In fact the microbial growth factor is not influenced by the initial microbial charge of the milk, but from the performance class of the cooling tank: storing milk with a very high microbial charge (3.65 million cells/ml) for 20 hours in a I class tank, causes an increase in the microbial growth factor only 11% higher than what found using a tank with the same performance class, using milk with initial microbial charge five times lower (Pazzona and Murgia, 1992).

As for the temperature class (A, B or C), it is necessary to consider the climatic condition in which the cooling tank is supposed to work. A high ambient temperature decreases the efficiency of heat dissipation by the air-cooled condenser, causing the cooling time to be longer, increasing the specific electric energy consumption. In the mediterranean climate the class B is usually the most appropriate, since the corresponding ambient temperature coincides roughly with average maximum temperatures in summer.

The milk cooling system price, which is important to determine the annual depreciation and interest rates based on a lifetime of 10 years, is the most relevant element of assessment when purchasing the system. Excluding the differences in materials and standard features offered by the manufacturers on the market, the price of the system increases with the capacity of the tank, but generally is not proportional, thus the price

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unit (€/100 litres) is reduced in larger tanks than those with smaller capacity. Within the same rated volume and performance, the price changes according to the type of plant and the refrigeration system. The price is higher in two milking tanks compared to four milkings, due to the higher power of the refrigerating unit installed. Furthermore the price is higher in indirect expansion systems, compared to the direct expansion, due to the higher complexity of the refrigerating unit. The presence of auxiliary equipments, such the automatic washing devices or pre-coolers, can significantly affect the price of the system and should be assessed on the basis of qualitative and economic advantages, which must widely cover the higher investment (Pazzona, 2009). The technical and economic analysis about the purchase of milk cooling systems suggests that the economic convenience increases together with the quantity of milk produced. On the other hand the need for initial capital reduces purchases by small livestock farms (Sant'Anna et al., 2003). The variable costs, meaning by this term those strictly linked to utilization of the cooling tank, include electricity consumption, detergents for washing, ordinary and extraordinary maintenance operations. The energy consumption for cooling is higher in indirect expansion systems, compared to the direct expansion. The cost for washing increases in tanks with larger rated volumes, and is halved in four milkings tanks, compared for two milkings with the same capacity. The operating costs, if referred to the one litre of milk, vary depending on the quantity actually refrigerated and the CU of the tank. A high CU reduces the unitary cost because the fixed cost (depreciation, interest, cleaning, maintenance) is spread over a larger milk production (Pazzona et al., 2009).

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5. ENERGY SAVING IN MILK COOLING

As already mentioned, the main electric users in a dairy farm are characterised by milk cooling and heating of the water required washing the milking plant. Improving the efficiency of these operations saves energy and thus reduces the economic impact. Some accessory devices, such as the pre-cooler and the heat recovery unit, allow a 40-50% decrease in energy consumption for milk cooling.

5.1 Pre-cooling

The pre-cooler is a device that allows a fast heat exchange in countercurrent between the milk and water coming from a well or the public water supply. The milk is partially cooled down, depending on the temperature of the water.

The design adopted for milk pre-coolers is usually characterised by a plate or a tubular heat exchanger. The first is formed by a variable number of stainless steel plates with a shaped surface, generally in a herringbone pattern, arranged in series and strictly connected to a supporting structure, using a pressure plate and the threaded tie-rods (Fig. 10).

The milk and the water flowing in countercurrent on contiguous plates and along the ducts formed by the shaping increasing the turbulence of the flow, improving the heat

Fig. 10. Plate heat exchanger (Techno System) and a single plate chiller with herringbone pattern (Packo).

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exchange efficiency between the two fluids. The seal between plates is ensured by a seal of nitrile rubber disposed along the perimeter of each plate.

The second type of pre-cooler is the coaxial tubular heat exchanger, formed by a coil of variable length, comprising an inner tube of stainless steel in which the milk flows, wrapped by a second outer tube that creates a cavity in which the water flows. The system is located inside a metallic container with cylindrical shape (Fig. 11). The insertion of the pre-cooler does not require any

modification of the milking plant and does not interfere with the duration or the milking technique. The heat exchanger is mounted on the transfer tube from the milking plant to the cooling tank, downstream of the extractor pump and the filter. The movement of the milk inside the exchanger is controlled by the milk pump, thus no auxiliary pump is required, while the circulating water is controlled by a solenoid valve whose opening time is adjusted according to the required flow rate. The performance of the

pre-cooler, meant as the efficiency in moving heat from the milk to the cooling medium, depend on many factors such as the contact surface, the flow rate, the retention time of the milk inside the pre-cooler and the temperature difference between the two fluids. The effectiveness of cooling is expressed as the real temperature decrease of the milk, compared to the maximum potential decrease. The potential decrease occurs when the temperature of the milk in output equals the cooling medium temperature at the entrance of the heat exchanger, and therefore the efficiency is equal to 1:

wi mi mo mi T T T T E    Where:

Tmi= Inlet milk temperature

Tmo= Outlet milk temperature

Fig. 11. Tubular pre-cooler (Packo).

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The increase of the contact surface between the two fluids, which depends on the number and the area of the plates in plate heat exchangers, and on the length, diameter and number of tubes in tubular ones, determines a higher effectiveness of the heat exchanger. A high flow rate, with a consequent higher speed of the fluids, increases the turbulence of the heat exchange. The ratio between the water and milk flow rate and ranges commonly between 1:1 and 2.5:1; the higher the ratio, the better the efficiency of the pre-cooler, because of its higher temperature difference between the two fluids. However improving the flow rate can be difficult due to the limited availability of water, thus a ratio of 2:1 is generally used. The average temperature difference between the milk and the cooling medium should be as large as possible. This is achieved by ensuring an efficient countercurrent movement of the two fluids.

The main advantages related to pre-cooling are the reduction of the cooling costs and the improvement of the milk quality. The economic benefit is due to the reduction in energy costs, since the use of pre-cooler saves about 40-50% of the electricity needed for cooling milk in ordinary conditions. In fact, after the pre-cooler the milk temperature can decrease up to 15-20 °C, depending on the water temperature. The pre-cooler allows the milk cooling to be completed with a considerably shortened cooling time, or to use a cooling tank with a less powerful compressor, so that energy consumption is lower.

5.2 Instant cooling

The technology is characterised by heat exchangers, in which iced water circulates, produced by an appropriate refrigeration system that generates and accumulates ice (Fig. 12). The temperature of the milk can rapidly decrease to 4 °C. The instant cooling has the purpose to limit the development of bacterial microflora. Furthermore it prevents any temperature rise of the milk already in the tank, when adding successive milkings.

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5.3 Heat recovery

The thermal energy extracted from the milk by the cooling tank, together with the heat produced by the refrigerating unit for compression of the refrigerant, is normally dispersed into the atmosphere through the air-cooled condenser. The systems for heat recovery are characterised by heat exchangers, which can use the thermal energy for heating water required in several other operations of the farm. The heat recovery unit does not interfere with the refrigeration cycle, but simply changes the type or the combination of the condensers used to dissipate heat from the refrigerant. One solution consists on replacing the air-cooled condenser with a water condenser, which allows moving all the heat from the refrigerant to the water. To prevent the continuous raising of the temperature inside the recovery system from reducing the efficiency of the system and overloading the compressor, the hot water must be removed continuously, or a source of permanent cooling should be installed. Even the increase of the condensing temperature for recovering more heat decreases the efficiency of the refrigerating circuit, overcharging the compressor and causing the energy consumption by 40-50%. In fact the larger amount of energy required with high condensation temperatures is used only to raise the pressure of the same condensation circuit. Therefore, in order to preserve the life of the refrigeration system and to contain the

Fig. 12. Ice storage system for instant cooling. The machine is connected to a tubular heat exchanger (Packo).

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The technology usually involves plate heat exchangers, placed between the compressor and the air-cooled condenser. The plates exchange heat with the refrigerant, recovering thermal energy before being dissipated by the condenser. Other solutions use the energy recovered from the refrigerant to heat the water, which is stored in a boiler and distributed, after further heating, to cover other thermal energy demands in the farm, such as the washing of the milking plant (Fig. 13 and 14).

Fig. 13. Diagram of heat recovery unit in the hot water storage tank. The heat is recovered from the refrigerant before reaching the condenser, through the plate heat exchangers. The water heated through the heat exchangers is stored inside a boiler in which an electrical resistance is installed to reach the final temperature desired (Ecolacteo).

Fig. 14. Heat recovery storage tank, for a cooling tank with high rated volume. To the right a detail of the control panel, from where hot water temperature can be adjusted, by regulating the electrical resistance inside the boiler (source Ecolacteo).

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6. STANDARD REGULATION FOR PERFORMANCE TESTS

The performance of milk cooling systems is evaluated through the milk cooling time. According to the official methods described in the standard EN 13732/2009 regulation, tests can be performed both with milk and water, and can be either complete or simplified. Manufacturers usually carry out performance tests in order to establish the performance class to be reported on the tank identification plate. These tests are performed in the laboratory, using the following test and environment conditions, also known as Standard Test Conditions (SC):

milk/water rate of 50% of the rated volume for a two milkings tank, or 25% for a four milkings tank;

ambient temperature constant, equal to the value established by the

temperature class for which the manufacturer guarantees the optimal performance of the cooling tank;

initial milk/water temperature of 35 °C; brand new cooling tank to be tested.

The result of the test is the Standard Cooling Time (SCT), thus the cooling time required to cool down the milk during SC. The SCT establish the milk cooling time class. However when testing a milk cooling system in the farm, the Operating Test Conditions (OC) are often different from the SC in the laboratory. Thus the variables affecting the performance of the tank, in terms of cooling time, must be taken into account.

6.1 Complete test

The complete test is carried out by monitoring the temperature trend over time of the milk/water bulk, for the entire cooling session from 35 °C (although the temperature of the milk is always inferior) to 4 °C. Unfortunately the method requires some hours to be completed. The main variables influencing the cooling time are:

ambient temperature;

milk/water tank stage during the test;

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After the test, each variable is corrected to the value assumed in SC by using equations. Therefore all milk cooling systems can be compared each other, even when monitored in OC different from SC.

6.1.1 Ambient temperature

This parameter affects the efficiency of the refrigeration cycle and the cooling time. With an ambient temperature higher than the one defined by the temperature class reported on the plate, the manufacturer does not guarantee the optimal performance of milk cooling system, while at a lower temperature, the tank works in favourable conditions. To classify the performance of a milk cooling system, the test should be conducted with the ambient temperature indicated by the manufacturer, thus the temperature provided by the temperature class (25 °C for a tank in class C, 32 °C for tanks in class B or 38 °C for class A tanks). This is possible through the application of experimental equations (Pazzona, Murgia, 1997). After the test, by estimating the influence of ambient temperature (Ta) on the cooling time, the following correction factors can be calculated:

hat= 1.3925 – 0.0203 x + 1.846 · 10-4x2

hatI= 1.4958 – 0.0219 x + 1.988 · 10-4x2

Where:

hat = ambient temperature correction coefficient for Ta = 25°C (C class tanks)

hatI= ambient temperature correction coefficient for Ta = 32°C (B class tanks)

x = ambient temperature during test (°C)

These correction coefficients, which are applied to the average ambient temperature observed during the test, allow to balance the effect of temperature on the cooling time, when it is equal to the value indicated by the temperature class reported on the cooling tank.

6.1.2 Milk rate

The performance of the tank should be measured for a milk rate equal to 50% of the rated volume for two milkings tanks, 25% in four milkings tanks or to 16.6% in a 6

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milkings tank. These milk volumes are difficult to be found inside the tank in OC, so that even in this case the correction coefficients are necessary. Thus the effect of the milk rate on the cooling time can be measured, assuming the milk volume exactly equal to the milk rate of the number of milkings:

hmr= 2.432 – 3.114 x + 0.5086 x2

hmrI= 3.011 – 10.847 x + 11.629 x2

Where:

hmr = correction coefficient for milk rate of two milkings cooling tanks

hmrI= correction coefficient for milk rate of four milkings cooling tanks

x = milk stage as fraction of 1

6.1.3 Initial milk temperature

The milk entering the tank is always below 35 °C, so that the cooling time measured could be significantly lower than what indicated by the manufacturer with the cooling time class. The correction coefficient for the initial milk temperature is obtained using the equation:

hmt= 4.8606 – 0.2055 x + 2.7244 · 10-3x2

Where:

hmt= correction coefficient for initial milk temperature

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6.1.4 Age of the milk cooling system

The efficiency of the refrigerating circuit decays during the working life of a tank, having a negative influence on the cooling time, compared to a new tank. The influence of age on performance can be estimated through the following equation:

ha= 1.0005 – 9.119 · 10-3x – 2.727 · 10-4x4

Where:

ha= correction coefficient for the cooling tank age

x = age of the cooling tank (years)

By multiplying together all the correction factors, the total correction coefficient (ht) can be obtained:

ht= hat· hmr· hmt· ha

This coefficient, multiplied by the Total Cooling Time (TCT), thus the cooling time measured after the complete test in OC, allows the calculation of the SCT, thus the total cooling time required to cool down the milk in SC, which are:

ambient temperature equal to the temperature class of the tank;

milk rate equal to 50%, 25 or 16.66% respectively for 2,4 or 6 milkings tank; initial temperature of milk/water: 35 °C;

new milk cooling system.

The SCT allows comparing the performance of milk cooling systems tested in several different OC, because the SCT is always referred to SC.

The factors mainly influencing the performance of the cooling tank are the milk rate and the initial milk temperature, from which derives a wide variation (Δh) of the corresponding correction coefficient. The other two factors (ambient temperature and age), having tighter variations, have a limited influence on the SCT (Fig. 15).

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At the end of the test, the complete cooling curve can be plotted, derived from the OC during the test (Fig. 16).

In the present study, the age coefficient was not considered in the calculation of SCT, as 4 8 12 16 20 24 28 32 36 40 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170

Cooling time (min)

T em p er at u re (° C)

Fig. 16. Milk cooling curve after a complete test over a 800 l cooling tank, 2BII.

Fig. 15. Correction factors and maximum variation (Δh) for the variability range factor plotted in abscissa. From the left: trend of the correction coefficient relative to the ambient temperature (from 5 to 38 ° C), milk rate (0.15 to 1.00), initial milk temperature (from 5 ° C to 35 ° C) and age of the cooling system (from 0 to 30 years).

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higher age, so that the coefficient is outside its age intervals, inducing an over-estimation of the influence of age on the SCT, giving unreliable results.

6.2 Simplified test

Compared to the complete test, the simplified test requires only 60-90 minutes to be completed, considering that the monitoring is carried out over a tighter temperature interval. In fact the complete milk cooling curve can be divided into three segments, within which the cooling rate is approximately constant (Fig. 17).

Since the milk temperature in the tank is always less than 35 °C and, in some cases, the cooling ends when the milk has not reached 4 °C yet, the test is performed only for a reference segment which lies between 24 and 14 °C, and through an appropriate correction coefficient applied on the cooling time measured, the entire cooling curve, thus the SCT, is calculated. Also the segments from 35-24 °C or 14-4 °C can be used for performing a simplified test. Obviously the SCT calculated with these coefficients is affected by an error (about 4.5%), which leads to an approximation with a magnitude of 7-8 minutes. However this approximation is acceptable for simply checking the milk cooling system performance for the estimation of the energy consumption.

Fig. 17. Experimental milk cooling curve divided into its three segments, on a 340 l cooling tank, two milkings, BII class, with a milk rate of 0.550 and average ambient temperature of 16 °C.

4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 0 10 20 30 40 50 60 70 80 90 100 110 120 130

Cooling time (min)

T em p er at u re (° C)

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In the simplified test, in order to calculate the entire cooling curve and the TCT, the cooling time measured on the specific segment from 24 to 14 °C (Tr) must be multiplied for the coefficient of the simplified control (hsc), which is equal to 3.2388:

TCT = Trx hsc(24-14 °C)

The other two correction factors, relating to performance tests carried out by monitoring the other two portions of the curve are:

hsc(35 – 24°C) = 3.4657

hsc(14 – 4°C) = 2.4310

These coefficients were developed examining the cooling curves acquired during several performance tests on sheep and cow milk cooling systems (Pazzona and Murgia, 1997).

Energy consumption analysis of sheep milk cooling systems

Ciclo XXV

ENERGY CONSUMPTION ANALYSIS OF SHEEP MILK

COOLING SYSTEMS

ABSTRACT

RIASSUNTO

TABLE OF CONTENTS

I.

INTRODUCTION