Commercial RefrigerationThe general purpose of a refrigeration system is to cool and store food and thereby preserve its shelf life. This is accomplished by removing heat energy from the low temperature storage condition (i.e. the heat source) and transferring this heat energy to a higher temperature medium (i.e., the heat sink) usually outdoor air.Other examples of refrigeration are:
Refrigeration is only a means to an end. In most cases, that end is the preservation of foods. Refrigeration is often a significant steady use of year-round electricity since this equipment runs even when the building is unoccupied. Therefore, it is usually cost effective to install the most efficient refrigeration practical. Consequently, utility representatives work closely with consumers during the early planning stages to help consumers understand their options. They will be alert to consumer expansion needs and the potential replacement of old inefficient equipment with new, improved units.
Refrigeration - Basic Cycle ConceptsHeat energy always flows naturally from a higher to a lower temperature level. That is, hot areas naturally cool off and cold areas naturally warm up. Therefore, moving heat from a lower to a higher temperature requires the input of work (or heat), usually to create a pressure differential in the cycle refrigerant.The refrigerant (acting as a heat transfer fluid) is used to transfer heat energy from a lower temperature to a higher temperature. The refrigerant is evaporated at a temperature lower than the desired temperature in the freezer or cooler. The condensing temperature of the refrigerant is increased by compression so that it can either be rejected to the environment or recovered as useful heat. The basic refrigeration cycle, with all steps combined, is shown: Step Two, Compression: The compressor raises the pressure of the refrigerant vapor, normally using an electric motor drive. This increases the temperature at which the vapors will condense to a temperature above the temperature of the heat sink. Most common compressors are reciprocating (piston and cylinder) or screw (looking much like an old meat grinder) compressor designs. Step Three, Condensing: The high-pressure refrigerant gas now carrying the heat energy absorbed at the evaporator plus the work energy from the compressor, enters the condenser. Since the refrigerant's condensing temperature is higher than that of the heat sink, heat transfer will take place, condensing the refrigerant from a high-pressure vapor to a high-pressure liquid. Step Four, Expansion: The condensed liquid's pressure is reduced (called "throttled") to the lower pressure evaporator using a valve, orifice plate or capillary tube device. In actual practice, the condenser cools the refrigerant a bit more, sub-cooling it below the condensing temperature. This is an important efficiency improving attribute to the cycle, since it reduces the amount of refrigerant liquid that has to evaporate (it is called flashing at this stage in the cycle) to a gas in the expansion valve to reduce the pressure and temperature of the liquid entering the evaporator. This reduction in flash gas is important to improve system performance.
Energy Use Characteristics of RefrigerationRefrigeration cycles transfer thermal energy continually from a region of low temperature to one of higher temperature. The higher temperature heat sink is usually ambient air or cooling water. This table lists some typical energy use data.
Refrigeration System Operating CharacteristicsRefrigeration systems must operate at all hours of the year, even when the building is unoccupied. Warmer weather tends to push refrigeration equipment to its capacity limit, thus creating a maximum operating kW and kWh.Evaporators - must be selected to provide the required cooling at all expected ambient conditions even with the maximum frost on the coils (i.e., just prior to defrosting). Evaporator coils used include two types of refrigeration systems: flooded evaporator and direct expansion. For direct expansion systems, two of the most commonly used refrigerant liquid metering devices are the capillary tube and the thermostatic expansion valve. In addition, proper provisions must be made for periodic defrosting of evaporator air-side surfaces. Defrosting may be accomplished using refrigerant compressor discharge hot-gas, water spray, or manually as selected to meet the user's objectives. Suitable drain connections should be provided to carry off the water resulting from defrost operations. Water-cooled condensers require cooling water from an external cooling tower, or from a lake, well, river or other similar source. Once-through use of city water for condensing purposes is prohibited in most locations. Air-cooled condensers are the most popular since they avoid other problems of water acquisition, treatment and disposal. The trade-off may be higher electrical consumption. As seen here, the evaporative condenser is a combination of a water cooled condenser and an air-cooled condenser that rejects heat through the evaporation of water into an airstream traveling across a condenser coil. Compressors - must be sized to meet the varying needs of each application. Provision must be made to protect the compressor from liquid carry over from the evaporator, in addition to the normal safety controls (high and low pressure cutout. oil pressure, etc.). The most common type of compressor used for commercial refrigeration systems is the reciprocating compressor. Reciprocating compressor types include single-stage (booster or high state), internally compounded, and open, hermetic or semi-hermetic.
CFC IssueCFCs and the ozone layer - The ozone layer in the upper atmosphere absorbs much of the ultraviolet light from the sun and, in so doing, shields us from its harmful effects. Ozone in the lower atmosphere (formed by the sun's action on air pollutants resulting in smog) is harmful. Ozone in the earth's upper atmosphere acts as a protective shield while ozone at the surface is undesirable.Some scientists believe CFCs are contributing to deterioration of the ozone layer. The theory is this: CFCs are extraordinarily stable compounds that they do not break down in the lower atmosphere. Although heavier than air traces of CFCs have been found in the upper atmosphere where they have been predicted to last for 100 years or more. Affected by ultraviolet radiation, these CFC traces slowly decompose and release chlorine. Chlorine (Cl2) with the presence of sunlight is known to catalytically decompose ozone, an oxidizer. The catalyzation of ozone forms chlorine oxide (which is unstable) and produces oxygen. The unstable chlorine oxide then breaks down to again form chlorine, and produce oxygen. This process keeps repeatedly attacking the ozone. With the looping process described it is believed that a single chlorine atom may be able to destroy as many as 100,000 ozone molecules. Volcanic eruptions and other (such as lightning) massive releases of chlorine by Mother Nature play a role in the observed fluctuations in the ozone layer. Scientists are in general agreement, however, that CFCs have a negative effect on the ozone layer. Because the potential detrimental effects of chlorine containing refrigerants on the ozone layer, a world body was convened, with several follow-on reviews. As a result these steps have been taken to reduce consumption (production plus imports minus exports) and implement phase out. Refrigerant Phase out Schedule
Recycled CFCs and HCFCs are not included in the consumption ban and will continue to be used beyond the ban dates. Although these dramatic steps pose serious challenges, the purchasers of commercial air conditioning and refrigeration equipment need not be excessively concerned. Albeit HCFCs will be phased out, HCFC22, R500 and 502, and HCFC-123 will be available for sufficient time to permit orderly transition to acceptable alternate refrigerants. In addition to the phase out. the Clean Air Act Amendments required the Environmental Protection Agency (EPA) to issue mandatory regulations for the recapture, recycling, and safe disposal of refrigerants. The amended Act also prohibits venting of CFCs. HCFCs or any other alternative refrigerants during service, repair, and disposal. HCFCs (and HFCs with no chlorine at all and no interaction with ozone) are recognized as absolutely critical to the transition away from the phased out CFCs. Because they do contribute to ozone depletion although on a very small scale HCFC refrigerants are included in the phase out timetable. The eventual production phase out of HCFC123, 22 and other HCFC refrigerants should not be a factor in deciding which refrigeration equipment to purchase during the 1990s. These refrigerants should be available during the life of the equipment. Ozone Depletion Potentials - The relative effect of the chemicals on the ozone layer is measured by assigning relative factors, using CFC-11 as the reference. Those without an atom of chlorine are known as HFCs and have zero ODP. Current average values of the ODP, as well as the Global Warming Potential (GWP) and atmospheric lifetimes, for a number of refrigerants are shown in the following table. These values are averages of measurements from several different sources. Future additional scientific data may result in revisions to some of these values. CFCs and global warming the greenhouse effect - CFCs have also been implicated in the potential for global warming, due to their ability to trap heat in the atmosphere. This effect is another atmospheric phenomenon that is under scrutiny: whether temperatures on this planet are gradually rising due to incoming sunlight being trapped by gases, much as cloud cover reduces radiation cooling at night. Although there are many uncertainties and conflicting views, there is growing concern in the scientific community that global warming may be occurring. Carbon dioxide (CO2) is known to be the main contributor to the Greenhouse Effect, in which upper atmospheric gases absorb the sun's infrared radiation possibly causing global warming. Other upper atmospheric trace "greenhouse" gases (including methane, nitrous oxides [NOx], along with some CFCs and HCFCs) are also perceived to be linked to the global warming issue. The "Greenhouse Effect" has two parts:
Most alternative refrigerants are not "drop in" substitutes. Research and development are resulting in some additional substitutes, such as R507 and R404A as replacements for R502. HCFC22 (which is widely used in the United States and elsewhere) is the predominant refrigerant used in screw, scroll and reciprocating equipments (and in virtually all unitary equipment). Potential replacements include R134a, R407C and R410A. Currently, there is no clear substitute for HC1:4C123. the replacement for CFCl l. Conventional Refrigerants - Although CFC11 and 12 production has been phased out, commercial refrigeration and air conditioners can continue to use them for many years. As new servicing and leak repair practices reduce CFC losses, less refrigerant will be needed to keep present equipment in operation. Also, the recovery, recycling and reuse of CFC refrigerants from operating and retiring equipment has become routine practice, extending the supply of these refrigerants well beyond the 1990s. Environmentally Acceptable Refrigerants Are Now Available
HCFC22 - This HCFC has only about one-third the GWP and only a small fraction of the ODP of CFC 11, as shown in the table. Much of the existing pool of knowledge supports the position that HCFC22 is part of the near term (transition) solution. However HCFC22 is currently included in the HCFC phase out provisions. No health or safety issues have been identified and buyers of HCFC22 equipment can be assured of its continuing availability for the expected life of the equipment as HCFC22 will be produced in declining amounts from 1996 until 2030. Refrigerant Prices - Rising prices for the present CFC refrigerants can be expected, considering their decreasing availability and increasing tax rate. Because of their environmental impact, the U.S. government has imposed federal taxes on fully halogenated CFCs. The excise tax on CFCs and halons is ODP weighted. The tax is applicable only to Class I compounds, which include the five CFCs listed. No one is sure how expensive substitute refrigerants will be over time. With the incremental taxes, the price of the CFC alternatives HFC134a and HCFC123 is now equal to or less than the rising CFC-12 and 11 prices. To put this issue in perspective, consider this example. Assuming a 4% loss of charge per year, and 2 pounds of refrigerant per ton capacity at a cost of $7.00/lb, the refrigerant cost is $0.56 ton/year of refrigeration design capacity. When compared with installed costs, and annual maintenance and energy costs, the refrigerant cost represents a small part of refrigeration equipment ownership costs. In any event it can be expected that whatever refrigerant is used the price will be higher than it has been in the past.
Brine SystemsBrine systems are typically used where temperatures below 32°F are required and it is not desirable to circulate a direct refrigerant (R-22, R-502, ammonia, etc.).A brine system is rarely used in commercial refrigeration applications. Brine systems use a high concentration of salt water or other anti-freeze solution which is chilled, then pumped around to do the required cooling. The common brines used for refrigeration are sodium chloride (common salt), calcium chloride and various glycol solutions. A brine system's advantages are that all refrigeration equipment is in the engine room directly under the supervision of the engineer, and that a leak in any other part of the building will leak only brine (causing considerably less damage and repair costs than a refrigerant leak). Its biggest disadvantages are that it usually consumes more energy to maintain a required temperature, and the brine may be corrosive.
Refrigeration System SelectionRefrigeration system engineering factors include:
Refrigerant SelectionAs the vital working fluid in a refrigeration system, the refrigerant is selected to provide the best refrigeration effect at a reasonable cost.The following characteristics are desirable:
Safety Classification of Refrigerants ANSI/ASHRAE Standard 34-1992R defines the safety groups. The table below illustrates the grouping of a number of popular refrigerants.
ASHRAE Standard 15-1992 - Safety Code for Mechanical Refrigeration - specifies maximum permissible quantities of refrigerants. From a safety-monitoring standpoint, the refrigerant amount is unlimited when, along with other requirements, detectors are located in areas where refrigerant vapor from a leak is likely to concentrate to provide an alarm at the following levels: Safety Group A1 (>400 ppm TLV low toxicity) refrigerants, such as HFC-134a and HCFC-22, alarms should provide warning at below 19.5% volume oxygen. Safety Group B1 (higher toxicity) refrigerants, such as HCFC-123, alarms should provide warning at no higher than their TLV (or toxicity measure consistent therewith). With HCFC-123, due to its lower AEL and TLV (currently 10 ppm AEL and TLV) than conventional refrigerants, adequate equipment room ventilation must be verified. HCFC-123 Refrigerants that are highly toxic or flammable are not recommended for commercial use where people may be present. Note: R-500 and R-502 are azeotropic mixtures of two refrigerants, one of which is a CFC. As the manufacturing of all CFCs is now phased out, these two refrigerants are in increasingly short supply.
Compressor SelectionThe most common type of compressor used for commercial refrigeration systems is the reciprocating compressor of either the open, hermetic or semi-hermetic type.Reciprocating compressor designs include:
Condenser SelectionCommon types of refrigerant condensers for commercial refrigeration use are:Water Cooled Evaporatively Cooled Air Cooled
Typical Use Characteristics of RefrigerationThe food sales and food service consumers represent the majority of the refrigeration applications: Food sales consumers are typically grocery stores, meat markets, delicatessens, supermarkets, and food lockers. Food service situations are restaurants, cafes, drug stores serving food, taverns, grills, tea rooms, cafeterias, dining rooms, carry-out's, delicatessens, and stadium concession stands.Other common refrigeration uses are for beverage cooling at taverns, bars, service stations, offices, employee break rooms, water cooling in offices, stores, service establishments, public buildings recreation area, and theaters, and ice making such as cube and block ice for retail sales, fresh food display and as a hotel/motel amenity. In addition, refrigeration is commonly used in institutions, hotels, hospitals, schools, and in special applications such as cold storage for such products and applications as flowers, medicines, candies, fresh fruits and vegetables, photo processing, laboratory supplies, fishing bait, and morgues.
Refrigerator TypesThe reach-in refrigerator is found in most establishments that serve food. Size varies from 10-75 cubic feet; (avg. 50 cubic feet). In order to satisfy demand for more refrigerator space, the walk-in refrigerator is often specified. The size of a walk-in refrigerator is from 175-2500 cubic feet (avg. 500 cu ft).Bulk dispensing of refrigerated products represents an accepted use of specialized refrigeration equipment. These units are desired and accepted by the consumer and owner (e.g., the milk dispenser). At the same time the owner receives a greater profit on each glass dispensed than by carton or bottle method.
Food Service Establishment Refrigeration GuidelinesThe average person consumes 2 pounds of food at one sitting (solid and liquid items). About 1-1/2 cubic feet of refrigerator volume per person served is a good starting point when determining refrigeration requirements. A reach-in refrigerator using wire shelves should normally be loaded to 60% volume capacity. Accessories such as pan and tray slides can increase usable space to 90%.Foods requiring separate refrigeration are fish, bakery products, beverages and ice cream. Storage temperatures:
Ice MakersIce makers produce ice in various shapes and forms. Advantages of on-site automatic ice makers are in savings (an ice maker can pay for itself out of savings from cost of purchase and delivery), convenience (no waiting for deliveries or time wasted chipping and cracking), sanitation (since ice is normally untouched by human hands), and being automatic (unattended units shut off when bin is full).
Energy Consumed in Ice MakingElectric consumption of an ice maker depends on water supply temperatures. Contact the water company for the average temperature of water in your area.
Operation and Maintenance of RefrigerationProper operation of the refrigeration plant maintains required temperatures and moisture levels. This is automatically controlled in commercial systems. Some attention is occasionally required by the user to readjust temperatures slightly to defrost coils, and to start and stop units when required. Safety controls are set to react as appropriate and protect the equipment from damage in the event of malfunction or component failure. Since system failure can be both expensive and dangerous, particularly where food or other product spoilage is involved, there should also be an on-going, well planned and executed preventive maintenance program.
Refrigeration Load ConsiderationsThe designer must consider all aspects of the load, including all items contributing to it, to properly design the refrigeration system. The following factors must be considered (even though they may not be involved in the final load):
Process refrigeration loads, while influenced to a certain degree by these same factors, are dictated mainly by production requirements (unless thermal storage techniques are used).
Defrosting Refrigeration SystemsAny evaporator surface operating below 32°F collects frost. Unless some provision is made to remove it, frost builds up and acts as an insulator, thereby increasing operating cost. In fact, frost has an insulating value up to 50 percent of that of cork! Prevention, defrosting, or periodic removal are common methods used to solve the frost problem. Defrost cycles are also common in situations above freezing temperatures where the coil operates below freezing at some point during the running cycle, and thereby collects frost. One common defrost method here is to blow the room air (which is above 32°F) over the coil to melt the frost when the compression cycle is off.Commercial and domestic equipment use some form of automatic control to operate the defrost cycle. Common frost prevention and defrosting methods used with commercial systems are hot gas and electric heater systems. Hot gas defrost uses the hot discharge (high pressure) gas directly from the compressor piped to the evaporator with a control valve to begin and end the defrost cycle . Electric heaters are commonly used to defrost domestic and commercial evaporators. Although the cost of the electricity to operate these heaters may be appreciable, defrosting is sure and rapid. Always check the scheduled operating time for these heaters to be sure they only operate as long as necessary.
Moisture in Refrigeration SystemsRefrigeration systems are very sensitive to moisture in the refrigerant side of the system.If moisture gets into the system, failure may occur due to:
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Step
One, Evaporation: Liquid refrigerant at a sufficiently low pressure
is brought into contact with the heat source (the medium to be cooled).
The refrigerant absorbs heat and boils, producing a low-pressure vapor.
The heat exchanger used for this process is called the evaporator. 
Condensers
- must be selected to operate at all outdoor weather conditions in the
area. Air-cooled condensers must be supplied with the proper controls
to permit operation at low outdoor ambient conditions. Water-cooled
condensers may require water regulating valves to keep condensing pressure
high enough to enable the thermal expansion valves to function. The
type of condenser selected depends largely on the size of the cooling
load, refrigerant used, quality and temperature of available cooling
water (if any), and noise considerations.