Walk-in refrigerators and freezers commonly used in fast-food and other restaurants, institutional kitchens, convenience stores and other businesses are room-sized insulated compartments, typically between 7 and 23 m2 of floor area, 2.4 m high, and refrigerated by a self-contained system.
Much larger refrigerated rooms are used in large supermarkets and food processing and packaging plants, but these are usually supplied by large central refrigeration systems. This discussion focuses on smaller, self-contained walk-in refrigerators and freezers, although many of the energy-saving tips found here can be applied to larger facilities.
There are an estimated 96 000 walk-in refrigerators, freezers and combination refrigerator/freezers in Canada, and each year about 3300 new or replacement units are sold.
As shown in Table 1, walk-in refrigeration equipment consumes a substantial amount of electricity, ranging from 16 200 kWh per year for a typical refrigerator to 30 200 kWh per year for a refrigerator-freezer. In comparison, a small house powered entirely by electricity consumes about 30 000 kWh per year.
|Walk-in refrigerator (15 m2)||16 200||$1,300|
|Walk-in freezer (15 m2)||21 400||$1,700|
|Walk-in refrigerator-freezer (31 m2)||30 200||$2,400|
*Assumed cost of electricity: $0.08 per kWh
Refrigeration capacity can range from 1½ to 5 tons. The most common sizes for restaurants are 2 tons for walk-in refrigerators and 3 tons for walk-in freezers.
Commercial walk-in refrigeration systems are typically made up of pre-fabricated walls and ceilings insulated to R 27 (8 or 10 cm of polyurethane foam), with an insulated floor.
A typical system is shown in Figure 1. The room is cooled by a packaged unitary or split refrigeration system consisting of:
- an evaporator fan-coil (usually mounted near the ceiling in the room)
- a compressor (usually mounted outside on the roof of the room or on the floor near the insulated room)
- a condensing coil, which may be water-cooled or air-cooled
Figure 1. A Typical Walk-In Refrigerator or Freezer
Larger systems are also assembled with prefabricated insulated panels and a large single refrigeration machine or multiple smaller split or packaged refrigeration systems. In supermarkets and food processing plants, where there may be other refrigerated cabinets or display cases, walk-in refrigerated rooms may also be supplied by large central refrigeration machines, with a refrigerant or coolant distribution system that supplies most of the refrigerated equipment in the facility.
The efficiency of the system is sometimes a matter of trade-offs. For example, locating the air-cooled condensing coil outdoors can improve the efficiency of the refrigeration system during winter because of the lower outdoor air temperature. Locating the air-cooled coil indoors allows the reject heat to contribute to heating the building in winter, but this can overheat the building during mild or warm weather and increase the air-conditioning load in summer.
All walk-in refrigeration systems use a working fluid called a refrigerant. The refrigerant absorbs heat from the room and rejects the heat through evaporation and condensation within the closed refrigeration system. Heat may be rejected to any of three places: to the building interior, to the cooling water (which may be discarded) or to the outdoors.
Refrigerant selection involves balancing conflicting requirements such as:
- ability to transfer heat
- chemical stability
- compatibility with compressor lubricants
Safety codes require a non-flammable refrigerant of low toxicity for some applications.
The performance of refrigeration components and systems depends strongly on the properties of the selected refrigerant, and the design must be optimized to achieve the highest possible efficiency.
Refrigerant Environmental Concerns
The environmental consequences of a refrigerant leak from a system must be considered because of the impacts on ozone depletion and climate change. From the 1940s until recently, the most popular refrigerants were chlorofluorocarbons (CFCs), such as R11, R12 and R502, and hydrochlorofluorocarbons (HCFCs), such as R22 and R123.
CFCs cause more atmospheric damage than HCFCs. The molecules of CFCs such as R11 and R12 contain only carbon and the halogens chlorine and fluorine. Because of their great stability, CFCs persist in the atmosphere for many years and eventually diffuse into the stratosphere. There, CFC molecules break down and release chlorine, which destroys ozone (causing ozone depletion). The ozone layer protects the earth from harmful ultraviolet radiation from the sun. In the lower atmosphere, these molecules absorb infrared radiation, which may contribute to climate change.
Substitution of a hydrogen atom for one or more of the halogens in a CFC molecule greatly reduces its atmospheric lifetime and lessens its environmental impact. These compounds are called HCFCs and have less potential for ozone depletion and climate change.
Because CFCs and HCFCs have undesirable environmental impacts, these compounds are being phased out to meet the conditions of the Montreal Protocol, an international agreement to control the production of ozone-depleting substances, including refrigerants that contain chlorine and bromine. The original Protocol was signed in 1987 by the European Economic Community (now the European Union) and 24 other nations, including Canada and the United States. The Protocol came into force on January 1, 1989.
National regulations in Canada and the United States require the phasing out of R22 and R142b in new equipment by 2010 and of all HCFCs by 2020. These regulations allow limited production for service and stockpiling for service until 2030, consistent with the Montreal Protocol. Some European countries already ban HCFC use in new equipment and even for servicing existing equipment.
For converting or replacing equipment designed for CFCs, two alternative kinds of refrigerants are now available: HCFCs and HFCs (hydrofluorocarbons). Refrigerants currently (in 2005) used for conversions and in new equipment are HFC-134a, HFC blends and HCFC-22. HCFC blends are generally used only in conversions. Ammonia is also an option for new equipment where safety codes permit. The appropriate substitute for your equipment depends on the type of equipment and on your operational requirements.
Refrigeration Equipment Selection
Refrigerated room design and the refrigeration equipment are selected according to the temperature to be maintained, the functions required and the need for access and display doors. Another key consideration is the cooling or freezing capability required – the need (for health reasons) to cool the product from room temperature to the required storage temperature within a given time. This requirement is met by installing sufficient overcapacity beyond what would be needed to only maintain the room and the pre-cooled products it houses at the target temperature. The temperature control system normally cycles the compressor to match the cooling requirement under variable loading of the refrigerated space.
Attention to parasitic loads, operating conditions and heat recovery can all improve efficiency.
Parasitic loads generate heat that must be removed by the compressor, thus adding to the cooling load and increasing the energy consumed. Parasitic loads come from heat sources inside the refrigeration room, for example, interior lights, fans, defrost systems and heaters that prevent surface condensation.
Operating conditions can be improved by refrigeration components that can lower the compressor load.
- Heat recovery lowers overall energy use by recovering heat expelled by the refrigeration system and using it for domestic water heating and space heating.
The amount of energy consumed by a refrigeration system can be reduced by specifying or retrofitting some of the measures described below.
Reducing Parasitic Refrigeration Loads
The effects of the following measures are additive. Each one incrementally and independently reduces the load on the compressor.
High-efficiency refrigeration compressors: High-efficiency refrigeration compressors use more-efficient electric motors and have lower compressor losses. These losses (in the form of heat) have to be removed by the refrigeration system. The use of high-efficiency compressors can save from 5 to 10 percent in energy costs.
High-efficiency evaporator fan motors: High-efficiency motors that drive fans release less heat into the refrigerated room than conventional induction motors. This reduces the energy draw by the fan motor and the compressor. System energy savings can be from 5 to 10 percent.
High-efficiency condenser fan motors: High-efficiency condenser fan motors can also reduce energy requirements. System energy savings are 3 to 5 percent for these motors because they operate outside the refrigerated compartment.
Energy-efficient lighting: High-efficiency lighting can reduce energy use and reduce the cooling load on the compressor:
incandescent light bulbs can be replaced by compact fluorescent lights
- T 12 fluorescent lamps with magnetic ballasts can be replaced by T 8 fluorescent lamps and electronic ballasts
Energy savings range from 2 to 10 percent, depending on the size of the refrigerated space and how the lighting system is designed and used.
- incandescent light bulbs can be replaced by compact fluorescent lights
Anti-sweat heater controls: Display doors have anti-sweat heaters around them to keep external surfaces free of condensation during high humidity conditions. The heaters are usually on all the time, but they may not be needed in drier weather. Anti-sweat heater controls that sense humidity can be added and set to turn heaters off when not needed. Savings can be from 2 to 4 percent of the system's total energy consumption.
- Defrost controls: Ice builds up on the evaporator coil during compressor operation, creating an insulating layer that reduces heat transfer through the evaporator coil and increasing the load on the compressor. Frost must be removed by heating, and this consumes energy.
The defrost cycle is usually initiated by a timer, whether needed or not. Energy-efficient defrost control systems eliminate unnecessary defrost cycles. The most effective controls are demand defrost controls, which initiate defrosting in a variety of ways, such as by measuring the temperature or pressure drop across the evaporator, by measuring frost accumulation and by sensing humidity. All of these methods are more effective than using a simple timer to initiate defrosting. Estimated energy savings range from 1 to 6 percent.
Improving Operating Conditions
The effect of applying the following measures in combination may not be additive, since some become less effective once others have been implemented.
Floating-head pressure controls: In outdoor air-cooled condensers, floating-head pressure controls take advantage of low air temperatures to reduce the amount of work for the compressor by allowing the head pressure to vary with outdoor conditions. This reduces compressor load and energy consumption and can extend compressor life. Floating-head pressure controls are standard on many new systems and can be retrofitted for existing systems. Estimated savings range from 3 to 10 percent.
Naturally sub-cooling the liquid refrigerant: An oversized condenser - or an additional heat exchanger that increases the heat exchange area to the liquid-filled portion of a condenser - can provide additional natural cooling to the condensed refrigerant. The result of a lower-temperature refrigerant liquid in the system is a lower evaporator temperature and reduced load on the compressor. Estimated savings range from 5 to 9 percent.
Mechanically sub-cooling the liquid refrigerant: By further cooling the refrigerant liquid, the temperature of the evaporator will be reduced, resulting in a significant increase in cooling capacity of the refrigeration system and an increase in overall efficiency. The liquid refrigerant can be cooled with a relatively small-capacity mechanical-cooling system or with a refrigerant line from a central system. Estimated savings are as much as 25 percent for grocery store refrigeration systems.
Liquid pressure amplifiers: Liquid pressure amplifiers are small refrigerant pumps that raise the liquid line pressure to reduce capacity loss at low head pressures when outdoor temperatures are cool. This increases efficiency and ensures lubricant circulation through the compressor. With air-cooled condensers, the efficiency gains increase as the outdoor air temperature drops. Energy savings can be as high as 20 percent.
- Evaporative condensers: Refrigeration systems that use air-cooled condensers to reject heat can be equipped with a wetted filter to cool ambient air as it enters the condenser. This increases the cooling condenser capacity and cools the liquid refrigerant, thus reducing compressor load. Estimated savings are from 3 to 9 percent.
Walk-in refrigerators and freezers are normally equipped with their own compressor/condenser package, which is cooled to remove the heat generated by the vapour compression refrigeration cycle. Typically, this heat is released into the environment (to the ambient air or to water). Where the equipment is water-cooled (once-through or re-circulating type), some of that heat can be recaptured for other purposes.
Recaptured heat is commonly used for domestic water heating because hot water is needed year-round. Restaurants, for example, use large volumes of hot water for food preparation and dishwashing. Heat recovered from the condenser cooling water can be used to preheat water in a tank that supplies the normal water heater. Supplying a water heater with warm water reduces the amount of fuel required to bring the water up to the useful temperature. Where there is a good match between hot water use and condenser heat availability, the refrigeration system can replace about 25 to 40 percent of the water-heating energy. For once-through systems, heat recovery can also significantly reduce the amount and cost of municipal water that would normally be required for cooling.
An informative case study on the Homewood Health Centre in Guelph, Ontario, is available from the Office of Energy Efficiency's Web site. The Centre recovers heat from 10 rack-mounted, water-cooled kitchen refrigeration compressors by preheating cold water from 10°C to 30°C. This saves about 40 percent of the gas that would otherwise heat water and also saves 9000 m3 of cooling water per year.
Heat recovery systems are custom applications that use readily available plumbing and heating components, but they must be sized properly and designed in a way that ensures that the condenser is cooled within the required specifications under all possible operating conditions of the heat recovery system. To ensure this, the normal once-through cooling system is left in operation as a backup.
When the refrigeration condenser is air-cooled, heat may be rejected to the interior of the building or to the outdoors, depending on the location of the condenser. Although rejecting the condenser heat indoors has the advantage of helping to heat the building during winter, during summer the heat must be rejected to the outside to avoid overheating the building and increasing the need for air conditioning. Consult a qualified refrigeration contractor and heating contractor to determine the suitability of your application for heat recovery for building heating.
There are currently no Canadian regulations on minimum energy efficiency for walk-in refrigerators and freezers. However, high-efficiency options can be specified when ordering new units or when retrofitting existing units.
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