Commercial Refrigeration

Other examples of refrigeration are:

• Holding and displaying perishable food.
• Chilling liquid for process cooling (common in the food processing industry) and possibly to make hot water (to provide heat for air handler heating or reheat. Supermarkets routinely use this last concept.
• Chilling brine to freeze an ice sheet (e.g., a hockey arena).
• Heat pump systems (please see the separate section covering heat pumps in more detail.

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 Concepts Energy Use Characteristics of Refrigeration Refrigeration System Operating Characteristics CFC Issue Brine Systems Refrigeration System Selection Refrigerant Selection Compressor Selection Condenser Selection Typical Use Characteristics of Refrigeration Refrigerator Types Food Service Establishment Refrigeration Guidelines Ice Makers Energy Consumed in Ice Making Operation and Maintenance of Refrigeration Refrigeration Load Considerations Defrosting Refrigeration Systems Moisture in Refrigeration Systems

Refrigeration - Basic Cycle Concepts

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 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.

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.

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Energy Use Characteristics of Refrigeration

Typical Refrigeration Energy Use (KW/TON) and COPS Compressor Size and Type Operating Temperatures Large, over 25hp (19kW) Evaporator: -40F 0F 40F 45F Condenser: 105F 110F 100F 130F Open kW/Ton: 3.5 1.9 0.8 1 COP: 1 1.8 4.4 3.5 Hermetic kW/Ton: 3.8 2 0.9 1.2 COP: 1.7 3.9 2 9 Medium, 5 to 25hp (4-19 kW) kW/Ton: 3.9 2 0.9 1.1 COP: 0.9 1.7 3.9 3.2 kW/Ton: 4.2 2.1 1 1.3 COP: 0.8 1.7 3.5 2 Small, under 5hp (4 kW) kW/Ton: - - - - COP: - - - - kW/Ton: - 3.2 1.2 1.5 COP: - 1.1 2.9 2.3

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Refrigeration System Operating Characteristics

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.

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.

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.

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CFC Issue

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:

1. The direct effect of how much infrared radiation is absorbed by the offending CO2 and trace gases and its effect on the earth's climate; and
2. The indirect effect that relates to energy efficiency. If the use of a given refrigerant results in even a small increase in energy consumption over the 20 year or greater life of the equipment, the impact on global warming is of great concern. This is due to the added carbon dioxide that would be released from burning coal or other fossil fuels to supply that extra energy.

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.

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
Alternative refrigerants have been developed that can replace CFC refrigerants with only slight changes in equipment design and minimal effects on efficiency. The current principal refrigerant substitutes are shown in the following table. Several types of blends are being investigated in order to optimize performance while providing zero ozone depletion potential.

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.

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Brine Systems

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.

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Refrigeration System Selection

• Year round operation regardless of outdoor ambient (including low temperatures in winter), wide load functions in short time intervals, i.e., maintaining total refrigeration availability while the load varies from 0% to 100%,
• Frost control for continuous performance applications,
• Variations in the affinity of oil for refrigerant caused by large temperature changes, and oil migration outside compressor crankcase,
• Choice of cooling medium: (1) direct expansion refrigerant, (2) gravity or pump re-circulated or flooded refrigerant, or (3) secondary coolant (brines such as salt and glycol)
• System efficiency and maintainability,
• Type of condenser: air, water or evaporatively cooled,
• Compressor design: - open, hermetic, semi-hermetic motor drive; reciprocating, screw or rotary,
• System type: single stage, single economized, compound or cascade arrangement,
• Refrigerant choice: Type of CFC, HCFC, or HFC refrigerant is primarily selected based upon operating temperature and pressures. While ammonia is the most common refrigerant for large industrial applications, it is not used for most commercial refrigeration due to toxicity.

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Refrigerant Selection

The following characteristics are desirable:

• Nonflammable (to reduce the risk of fire hazard),
• Nontoxic (to reduce potential health hazards),
• Large heat of vaporization (to minimize equipment size and refrigerant quantity),
• Low specific volume in the vapor phase (to minimize reciprocating and screw type compressor size),
• Low liquid phase specific heat (to minimize heat transfer required when sub-cooling liquid below
  condensing temperature),
• Low saturation pressure required at desired condensing temperatures (to eliminate requirement for
  heavy duty or high pressure equipment),
• The low pressure portion of cycle should be above atmospheric pressure (to prevent inward leakage
  of air and water vapor into refrigerant piping), and
• High heat transfer coefficients.

Safety Classification of Refrigerants
All refrigerants have an allowable exposure limit (AEL) and threshold limit value (TLV). Heavier-than-air refrigerants can concentrate at floor levels and displace breathable oxygen. In ASHRAE Standard 15-1992, 7.4 System Application Requirements specifies the allowable limits on refrigerant amounts.

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
HCFC-123 is an environmentally acceptable alternative to CFC-11, having only a small fraction of the ODP and GWP of CFC-11. The B1 safety classification of this refrigerant is reflected in ANSI/ASHRAE Standard 34-1992. The B1 classification requires certain monitoring (ASHRAE Standard 15-1992) and this should be considered in connection with its use. In mid-1991 one manufacturing, duPont, reduced the allowable exposure limits from 100 to 10 parts per million (ppm), based on preliminary toxicology results. According to duPont's experience, typical worker exposure during servicing has not exceeded 8 ppm.

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.

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Compressor Selection

Reciprocating compressor designs include:

1. Single-stage (booster or high stage)
2. Internally Compounded

Condenser Selection

Water Cooled Evaporatively Cooled Air Cooled

• - shell and tube - Blow-through - Horizontal air-flow
• - shell and coil - Draw-through - Vertical air-flow
• - tube in tube - Static or forced air-flow

1. Size of the cooling load
2. Refrigeration used
3. Quality and temperature of available cooling water (if any)
4. Amount of water that can be circulated, if water use is acceptable

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Typical Use Characteristics of Refrigeration

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.

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Refrigerator Types

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.

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Food Service Establishment Refrigeration Guidelines

Foods requiring separate refrigeration are fish, bakery products, beverages and ice cream. Storage temperatures:

• Frozen Foods -20�C to 0�F
• Ice Cream -10 to -15�F
• Fish & Shellfish 23 to 30�F
• Meat & Poultry 30 to 38�F
• Dairy Products 38 to 46�F
• Fruits & Vegetables 44 to 50�F

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Ice Makers

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Energy Consumed in Ice Making

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Operation and Maintenance of Refrigeration

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Refrigeration Load Considerations

1. Heat leakage (in the form of latent and sensible heat) flowing into the space or product to be refrigerated,
2. Product load: the heat that must be extracted to change the product's initial temperature to the desired end temperature, including pull-down time allowable, and the
3. Internal sensible load: results from motors, lights and other heat-generating equipment in the conditioned space impacting the refrigeration 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).

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Defrosting Refrigeration Systems

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.

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Moisture in Refrigeration Systems

• Ice formation in expansion valves, capillary tubes or evaporators,
• Corrosion of metals,
• Copper plating,
• Chemical damage to insulation in hermetic compressors or other system materials.

• Faulty equipment drying in factories and service operations,
• Introduction of moisture during installation or service operations in the field,
• Low-side leaks (resulting in entrance of moisture-laden air),
• Leakage of water-cooled condenser,
• Oxidation of certain hydrocarbons of oil to produce moisture,
• Wet oil, refrigerant or both,
• Decomposing cellulose insulation in hermetically sealed units.