From the Field to the Supermarket

Transcription

1 From the Field to the Supermarket Post Harvest Cooling Part 4 of 4 By Andy Butler Retail Market Manager Star Refrigeration Ltd., Glasgow, Scotland This is the fourth and final part of a series of articles covering the cold chain from the field to the supermarket store. Each article looks at a cooling process that food undergoes before it reaches the consumer. Technical Editor. Recent Trends in European Supermarket Refrigeration Introduction Previous articles in this series have discussed post harvest cooling, processing plant cooling and temperature controlled storage/distribution. This fourth and final article in the series covers the chilling and freezing requirements, and the systems employed by supermarket chains in recent years. Display Case and Cold Room Operating Temperatures The range of perishable products available for purchase in today s supermarkets is extensive and varied with different products requiring different storage and display temperatures. These temperature requirements not only apply to the display cabinets, but also to the back of house storage rooms employed to accepted deliveries from the distribution centres. Table 1 shows the typical temperature requirements of various products and refrigerated storage rooms. Traditional Supermarket Systems and Refrigerants Since the phase out in Europe of chlorofluorocarbon (CFC) refrigerants and the imminent European phase out of hydrochloroflourocarbon (HCFC) refrigerants, the preferred refrigerant for the supermarket sector has been R404A a hydroflourocarbon (HFC). The systems designed for use with this refrigerant were initially all direct expansion vapour compression systems with no secondary refrigerant employed. (There are now a few exceptions to this in Europe that will About the Author Andy Butler started his career in engineering as an industrial electrician before joining J&E Hall as an industrial refrigeration engineer. In 1997 he was appointed Refrigeration Lecturer at City of Bath College. After four years he joined Space Engineering Services and headed the development of CO 2 refrigeration for the UK retail sector. Since 2009 he has been employed by Star Refrigeration, overseeing the roll out of CO 2 products to retail and food industry customers. He is a graduate in Building Services Engineering and is a member of The Institute of Refrigeration. Cold Chain d October - December 2012 C15 15

2 From the Field to the Supermarket... Table 1: Typical supermarket refrigerated fixture and product temperature ranges Fixture Product Temp. Frozen food well case with half glass door above Frozen food -18 C/-20 C Frozen food cold room Frozen food -23 C Meat and dairy display cabinet Fresh meat (wrapped), fresh poultry (wrapped), cheese(wrapped), butter etc. -1 C/+5 C Meat chill cold room Fresh meat (wrapped) -1 C Dairy cold room Fresh produce and fats +3 C Roll in milk chill cabinet Fresh milk +3/+5 Delicatessen serve over display cabinet Unwrapped fresh meat, cheese etc. Figure 1: High temperature R404A plant schematic +5 C be covered later in this article). Figure 1 is a simple schematic of a typical supermarket high temperature (HT) plant plant schematic for low temperature (LT) will be identical but with an evaporating temperature of -32 C or lower depending on the cabinet manufacturer. Typically, multiple packs are employed to service the refrigeration requirements of a supermarket, with either units in an internal plant room and remote condensers, or self contained units on the roof or in an outdoor plant area. This allows for the units to be of manageable dimensions and weight, and allows for single plant failure without the loss of all refrigeration service to the supermarket. Figure 2 shows a typical UK supermarket Figure 2: R404A packs on a UK supermarket roof roof with R404A packs providing refrigeration for the store the units with the larger condensers are the high temperature packs. Unit cooling duties vary, as do store refrigeration requirements, but common pack duties in the UK market are 100kW for high temperature plant and 30kW for low temperature plant. A typical 40,000 ft 2 supermarket will require 279 kw of high temperature refrigeration and 55 kw of low temperature refrigeration. This equates to three high temperature R404A units and two low temperature R404A units. Table 2 shows typical refrigeration requirements of different store sizes. Environmental Issues Refrigeration contributes both indirectly and directly to global warming. Indirectly, through the consumption of electricity produced from the burning of fossil fuels. Directly, greenhouse gas emissions occur through the leakage of hydrofluorocarbon (HFC) refrigerants used in refrigeration systems for display and storage of food. As the most commonly used refrigerant in supermarket applications is R404A, an HFC with a global warming potential (GWP) of 3860, supermarkets are finding it difficult to achieve their pledged carbon emissions targets. In recent years natural refrigerants have been proposed as an environmentally friendly solution for supermarket refrigeration. These refrigerants including ammonia, hydrocarbons and CO 2 do not contribute to ozone depletion and have low GWP. Supermarkets, however, also indirectly produce significant emissions a s t h e y a r e l a r g e consumers of electricity and as much as 50% of this is consumed by refrigeration equipment. Energy efficienc y is now a priority for all in Figure 3: Annual electricity consumption F60 supermarket order to reduce energy bills and environmental impact. Table 3 shows a Table 3: Comparison of refrigerant properties REFRIGERANT R134a R404A R717 CO 2 Natural Substance NO NO YES YES Ozone Depletion Potential (ODP) Global Warming Potential (GWP) Critical Point Triple Point 40.7 Bar C -103 C Bar Table 2: Refrigeration requirements for different supermarket floor areas Formats x 1000ft² 37.3 Bar 72 C c.-100 C <0.05 Bar HT Total kw 113 Bar C C 0.06 Bar LT Total kw F F F F F F F F F Bar 31.1 C C 5.2 Bar Flammable or Explosive NO NO YES NO Toxic NO NO YES NO C16 Cold Chain d October - December

3 comparison of refrigerant properties and GWPs and Figure 3 shows the annual electricity consumption in kwh of a 60,000ft 2 supermarket. Legislation Affecting Retail Refrigeration Over the last 20 years, legislation has prohibited the use of ozone depleting CFC and HCFC refrigerants but the use of the HFC refrigerants has remained legal and commonplace throughout most of Europe. These refrigerants have a very high global warming potential (GWP) and their use is now subject to legislation under EC Regulation No. 842/2006 on Certain Fluorinated Greenhouse Gases (F-Gas Regulations). This legislation came into force in July 2007 and obligates the supermarket operators to register and record HFC usage, employ only trained personnel and carry out leak testing at specific time intervals. The F-Gas Regulations are due for review in December 2012 and recently leaked documents reveal that it is intended to phase out high GWP refrigerants such as R404A and eventually phase out all HFCs. The proposed time scale calls for a ban on new commercial plants operating HFC refrigerants with a GWP in excess of 2150 by 1st January Also a ban on use of HFCs with a GWP in excess of 2150 for servicing plant with a charge equal to, or more than, the equivalent of 5 tonnes of CO 2. Retail Refrigera t i o n s Response to Le g i s l a t i o n a n d Environmental Issues For the most part, retailers have replaced all HCFC plant in their estates with plant operating on R404A. Unfortunately, this choice of refrigerant has now become high profile due to its very high global warming potential. For this reason environmental groups have targeted large corporations (this includes supermarket chains) and applied pressure for reductions in carbon emissions. As well as calling for reductions in energy consumption, the use of (particularly in terms of leakage to atmosphere) HFC refrigerants has been under scrutiny - especially since the introduction of the F-Gas Regulations. It has now become part of a company s corporate responsibility to reduce its carbon emissions and the supermarket chains have made public statements pledging to reduce these. Time scales and quantities differ from company to company but reduction in the use of (and/or leakage of) HFC refrigerant is included without exception. Various fluids have been investigated and trialled but the most prolific of the emerging technologies employed to replace R404A is carbon dioxide (R744). In the UK alone there are some 200 supermarkets operating CO 2 refrigeration plant. CO 2 as a Refrigerant CO 2 has several noteworthy characteristics: Non-toxic Non-flammable ODP 0 GWP 1 Higher Triple Point Pressure than traditional refrigerants Lower Critical Point Temperature than traditional refrigerants Higher operating pressures High Refrigeration Volumetric Capacity High heat transfer characteristics Inexpensive Figure 4: CO 2 refrigerant properties The most significant characteristics of CO 2 as a refrigerant are the critical and triple points. The critical point is at a relatively low temperature at 31 C at the relatively high pressure of 73 bar and the triple point occurs at C at a pressure of 5.2 bar (see Figure 4). CO 2 is the only common refrigerant to have a triple point above atmospheric pressure. With an air cooled plant it is not possible to remain below the critical point at higher ambient temperatures. This results in plant operating in transcritical mode in higher ambient temperatures. For a correctly designed plant at design loading transcritical operation will begin at approximately 23 C to 26 C. It is worth noting that for the majority of the year in the UK, CO 2 plant will operate in subcritical mode. Transcritical operation will only occur on the hottest days of the year and, even then, only for the hottest few hours of the day. This is not the case for southern Europe where long periods of transcritical operation will occur, resulting in lower efficiency. Currently Employed Supermarket CO 2 Refrigeration Systems Although CO 2 has been employed in the retail sector since the early nineteen nineties, these installations were rare and bespoke. Since approximately 2006, however, there has been a drive to develop mass produced CO 2 refrigeration packs led by the Scandinavian states due to their unilateral HFC restriction legislation. Now in 2012 there are several companies throughout Europe manufacturing and supplying CO 2 refrigeration packs for supermarket applications. A difference in customer requirements and development teams goals has meant that there are several different types of pack available on the European market. Single Temperature Flash Gas Bypass Plant This type of plant is employed where there is little or no requirement for frozen food storage and display. In this situation the small frozen food requirement will be catered for with the use of integral cabinets. A typical application is a convenience store. Plant Operation See Figure 5. High pressure (<73 bar), high temperature compressed subcritical CO 2 vapour leaves the discharge of the compressors and passes to the oil separator. This is a typically coalescing unit that will remove 99%+ of oil droplets from the Cold Chain d October - December 2012 C17 17

4 From the Field to the Supermarket... Figure 5: Typical single temperature flash gas bypass schematic discharge vapour and return these to the compressor crankcases. The discharge vapour then passes to the gas cooler which, when the CO 2 is subcritical, condenses and subcools the CO 2. The subcooled liquid CO 2 passes through the Industrial Control Motor (Transcritical)* (ICM(T)) valve where approximately 3 to 5 C of subcooling (at gas cooler outlet) is maintained by throttling of the valve to achieve optimum gas cooler pressure. From here the subcooled liquid CO 2 passes to the intermediate pressure receiver. As pressure rises in the receiver flash gas bleeds to the suction line through the flash gas bypass valve, thus maintaining an intermediate pressure of approximately 38 bar (gauge). This allows for the use of off the shelf brazed copper pipe and components with relatively low design pressures throughout the remote portion of the installation. From the receiver the saturated CO 2 liquid passes to the cabinet expansion valves. Here, further expansion occurs and refrigerant flow control is provided by reference to superheat at the evaporator outlet (setting will be approximately 6K). Superheated low pressure CO 2 vapour then passes from the display cabinets to the compressor suction for compression. For transcritical operation, high pressure (approximately 90 bar), high temperature compressed transcritical CO 2 vapour leaves the discharge of the compressors and passes through the oil separator to the gas cooler where the transcritical CO 2 is cooled but no condensing occurs. The now cooled transcritical CO 2 vapour passes through the ICM (T) valve where expansion occurs. This pressure reduction produces a mixture of saturated liquid and vapour CO 2 that passes to the intermediate receiver. The remainder of the cycle is identical to plant operation in subcritical mode. *Industrial Control Motor (Transcritical) (ICM(T)) Valve: This valve is common to all transcritical CO 2 systems. It is a direct operated motorised valve operated by a 4-20mA or 0-10V signal from a controller. The controller continually monitors gas cooler outlet temperature and pressure and adjusts valve position to maintain the desired conditions. Dual Temperature Flash Gas Bypass Booster Plant The unit shown in Figure 6 and 7 is the most common type Figure 6: Typical dual temperature flash gas bypass booster schematic Figure 7: Typical dual temperature flash gas bypass booster (example shown 50kW HT/10kW LT) installed in European supermarkets. The package unit in Figure 7 shows a 50kW HT/10kW LT housed unit with gas cooler mounted above the compressor housing. Manufacturers offer these units in a range of duties for internal or external use with remote or close couple gas coolers. Plant Operation Plant operation is very similar to the single temperature unit in Figure 5 but with the addition of frozen food display cabinets fed from the same liquid line as the dairy cabinets. In this example the superheated CO 2 vapour returning from the frozen food evaporators is compressed and then desuperheated by a coil within the receiver prior to mixing with the suction vapour returning from the dairy cabinets. Other desuperheating methods, with heat exchangers external to the receiver, are employed by the various manufactures but plant operation is similar in all cases. Transcritical Low Pressure Receiver Plant This type of plant was designed for the UK s largest retailer to their specific criteria. The two most significant criteria were fade out protection during power cuts and flooded evaporator operation for increased efficiency. Flash gas bypass systems do not meet either of these criteria due to vessel/component design pressures of as low as 42 bar (even lower for booster packs) and C18 Cold Chain d October - December

5 Figure 9: Transcritical LPR pack with close coupled gas cooler Figure 8: Low pressure receiver plant for cascade application schematic direct expansion operation. A low pressure receiver pack has a lowest design pressure of 75 bar. This combined with correct vessel sizing ensures that relief valves will not lift should power be lost to the plant. Approximately fifty supermarkets of varying sizes are operating with these units connected via cascade condensers to subcritical direct expansion CO 2 packs. Plant Operation See Figure 8. High pressure (<73 bar), high temperature compressed subcritical CO 2 vapour leaves the discharge of the compressors and passes to the oil separator. This is a typically coalescing unit that will remove 99%+ of oil droplets from the discharge vapour and return these to the compressor crankcases. The discharge vapour then passes to the gas cooler which, when the CO 2 is subcritical, condenses and subcools the CO 2. The subcooled liquid CO 2 passes through the ICM(T) valve where approximately 3 O to 5 O C of subcooling (at gas cooler outlet) is maintained by throttling of the valve to achieve optimum gas cooler pressure. The now subcooled CO 2 passes to the liquid separator and into the coil of the low pressure receiver where it is further subcooled. It is reasonable to expect up to 15 O C of subcooling to occur in the coil of the low pressure receiver. From here the subcooled liquid CO 2 passes to the expansion valves at the display cabinets, cold rooms and cascade condensers of the subcritical CO 2 LT packs. Here reexpansion occurs and control is provided by reference to superheat at the evaporator outlet. For this system no superheat is required as the low pressure receiver prevents liquid refrigerant returning to compressor suction. For this reason the CO 2 is returned to the low pressure receiver wet as a mixture of saturated liquid and vapour. Saturated low pressure CO 2 vapour then passes from the low pressure receiver to the oil coolers. Here oil pumped from the compressor crankcases is cooled by giving up heat to the saturated CO 2 vapour, hence the vapour is superheated. The now superheated CO 2 passes to compressor suction for recompression. For transcritical operation, high pressure (approximately 90 bar), high temperature compressed transcritical CO 2 vapour leaves the discharge of the compressors and passes through the Figure 10: CO 2 pump station schematic oil separator to the gas cooler where the transcritical CO 2 is cooled but no condensing occurs. The now cooled transcritical CO 2 vapour passes through the ICM(T) valve where expansion occurs. This pressure reduction produces a mixture of saturated liquid and vapour CO 2 that passes to the liquid separator. (Should the pressure in the liquid separator reach 65 bar, CO 2 vapour is released to the low pressure receiver via the back pressure regulator). The remainder of the cycle is identical to plant operation in subcritical mode. CO 2 Pump Stations These CO 2 units were the specified solution for a high end UK supermarket chain, and the first to be installed at multiple stores in the UK. The company s estate operated on R404A direct expansion packs that were relatively new and well maintained. This solution allowed the existing plant to be retained with a 90% reduction in R404A charge achieved. Typical pump station duty is 100kW cooling at the conditions shown in Figure 10. Figure 11: 100kW CO 2 pump station For Cold Chain d October - December 2012 C19 19

6 From the Field to the Supermarket... the long term due to the upcoming European legislation changes on the use of HFC refrigerants. The drive to reduce carbon emissions from supermarket refrigeration has also resulted in efforts to reduce energy consumption. This target has resulted in several technologies becoming commonplace on refrigeration equipment. These include inverter driven compressors, electronically commutated condenser (and gas cooler) fans and improved maintenance regimes. Many of the supermarket chains now have continuous energy monitoring of equipment and are seeing reduced energy consumption, and a reduction in fuel bills. Conclusion Figure 12: Hydrocarbon cooled pump station schematic new supermarket applications the hydrocarbon cooled pump station shown in Figure 12 is currently under trial at several stores. Other Carbon Emission Reduction Initiatives Although CO 2 has emerged as the front runner as the replacement for R404A in European supermarket refrigeration, there are trials of other HFC refrigerants underway. R407A and R407F have been employed due to their reduced GWP when compared to R404A. It is likely that these trials will be of little use in Ultimately, there will be legislation or a total ban on HFC refrigerants in Europe at some point in the future. Retailers are already implementing changes to the way they operate their refrigeration, and the European refrigeration industry is still trying to catch up in terms of training personnel on the new technology. This is being addressed, but is more difficult than it might have been due to the different types of system that have been installed to date. Developments will continue and within the next few years the various CO2 solutions employed today will be allocated their place in the market by default. Energy costs and efficiency are very high on the retailers agenda and the choice of solution is likely to be dictated by these two factors, as capital costs for the various v CO2 options are similar. A Huge Success India Cold Chain Show 2012 held from 22nd to 24th November at HITEX Exhibition Centre in Hyderabad was a grand success with 3,437 trade visitors and industry experts from service and user sectors attending the exhibition and conference. The concurrently held India Cold Chain Summit was attended by 219 delegates. More than 125 companies were represented at the event which hosted 101 exhibitors. The 3-day exhibition and conference was organized by Reed Manch Exhibitions in association with The Federation of Cold Storages, Andhra Pradesh & Tamil Nadu, and positioned as the most important event for the Indian cold chain industry. The Federation of Cold Storages Andhra Pradesh held its AGM on 24th November alongside ICCS 12. The exhibition had participants from countries such as Israel, Germany, Australia, China, United Arab Emirates, Spain, United States, and Italy. In the opening ceremony, Mr. Anuj Mathur, Managing Director of Reed Manch Exhibitions, thanked the joint work of exhibitors, sponsors, and appreciated their support to make India Cold Chain Show a good platform for the cold chain industry s progress. The next edition of ICCS is planned to be held in 2013 on September at the Bombay Exhibition Centre, Goregoan, Mumbai, with even more participation. The concurrent India Cold Chain Summit which was held on 22nd and 23rd November focused on the theme Cold Chain Challenges in Distribution Process & Developing a Sustainable & Profitable Cold Chain Business. Industry leaders gathered during the summit and discussed various important subjects including importance of efficient cold chain distribution system, integrating energy efficiency into cold storage design and many more relevant topics. For more information on the event, please visit com or contact Siddharth Narain at siddharth.narain@reedmanch.com C20 Cold Chain d 20 October - December 2012