Refrigeration is cooling of food and beverage stores and wine cellars. Air-cooling and food storage depend on adequate and economical refrigeration systems. Refrigeration is necessary for complete, year-round air-conditioned comfort for hotels
Specific HeatSpecific heat is the amount of heat required to raise one pound of a substance 1°F at atmospheric pressure. (Notice the difference with the definition of the BTU: the BTU is the heat required to raise the temperature of water, whereas specific heat is for any substance.) British Thermal Unit (BTU): The amount of heat needed to raise one pound of water 1°F at atmospheric pressure.
Sensible HeatSensible heat is that heat given off or absorbed by a substance which does not cause the substance to change phase. Sensible heat changes are observed as changes in temperature and are measured by a thermometer.
Latent HeatLatent heat is given off or absorbed by a substance that is changing phase. The temperature and pressure remain constant during the phase change until all the substance has been transformed. These temperature and pressure conditions are unique and are called saturation conditions. The latent heat of vaporization (LHV) "is the heat required to transform a liquid to a gas at constant temperature and pressure. The latent heat of condensation (LHC) is equivalent in magnitude to the LHV for the substance, but now we are going in the opposite direction in transforming the gas to a liquid. When a liquid is transformed to a solid as in the ice-making process, the liquid gives off its latent heat of fusion (LHF) to form the solid. The LHF to transform a pint of water to 1 lb. of ice at 32°F is 144 BTUs. We would have to add 144 BTUs to every pound of ice to melt it.
RefrigerationCooling of an object and the maintenance of its temperature below that of surroundings.
BASIC SCIENTIFIC PRINCIPLES
Vapour-Compression Refrigeration Cycle• Refrigerant
• Thermostatic expansion valve (TXV)
The most common refrigeration system is the vapour compression refrigeration cycle. It is cooling cycle that has almost universal application and use. It can be used for food preservation and air-cooling. It is efficient, with a moderate operating cost, and easy to control. Installation costs are usually moderate and maintenance is generally high.
Refrigeration CycleThe operation of the cycle depends on a product that has a low boiling point. The product is called refrigerant.
It must have the following properties
— High latent heat of vaporization - maximum cooling
— non-toxicity - not a health hazard
— Desirable saturation temperature - for operating pressures
— Stability - nonflammable/nonexplosive
— Ease of leak detection
— Low cost
— Readily available
The refrigerant's boiling point must be low enough to absorb heat at low temperature required for food chillers and freezers. As the refrigerant boils at a low temperature, it is absorbing heat (latent heat of vaporization). For example, if a food freezer is to be maintained at a temperature of 0°F (-17.8°C), the boiling point of the refrigerant must be lower than 0°F; if a food chiller is to be maintained at 40°F (4.4°C), the boiling point of the refrigerant must be lower than 40°F.
Varying its pressure can change the boiling point of a refrigerant. Increasing refrigerant pressure results in a higher boiling point; reducing refrigerant pressure causes it to boil at a lower temperature. The purpose of the vapour compression refrigeration cycle is to regulate pressure, allowing the refrigerant to absorb at one place and release it at another.
The vapour compression refrigeration cycle consists of five basic components: expansion valve, evaporator, compressor, condenser and receiver.
ExpansionLiquid refrigerant enters the expansion valve at high pressure. To use in the evaporator, its pressure must be reduced, which will reduce its boiling point. Expansion valve reduces pressure and controls the flow of refrigerant to the evaporator. Expansion valves can be adjusted to regulate the temperature of the evaporator.
Refrigerant leaves TXV (Thermo Expansion Valve) at a much lower pressure. As pressure drops, vapour formation begins as refrigerant enters saturation region. The expansion valve is the primary system control. It can also activate the compressor. When the valve allows refrigerant to flow into the evaporator, it permits the compressor to operate; when the valve closes, it stops the compressor. Temperature sensing devices can also activate the expansion valve and the compressor.
EvaporationFrom TXV, refrigerant is a saturated mixture of liquid and vapour. The evaporator absorbs heat and must be located in the space that is to be cooled. It can maintain temperatures as low as the boiling point of the refrigerant. Hence, if you regulate the refrigerant pressure in the evaporator, you will regulate the temperature of the cooled space. Products that are warmer than the boiling point of the refrigerant are placed in a space, such as a freezer or chiller, which is to be cooled. Cooling coil in the evaporator acts as a heat exchanger and refrigerator absorbs its latent heat of vaporization from the surrounding. The heat gain of the refrigerant is equal to the heat loss of the cooled space and its contents. Now, the refrigerant is slightly superheated at 10°F. The evaporator ensures no liquid goes to compressor.
The primary function of a refrigeration cycle is to absorb heat; this is done in the evaporator. The secondary purpose is to recycle the refrigerant. That is why the remaining portions of the cycle are required.
The purpose of the compressor is two fold i.e., to pump the refrigerant gas out of the evaporator and to increase refrigerant pressure. In low pressure, superheated refrigerant vapour enters the suction side of the compressor. Refrigerant is compressed from a low-pressure vapour to high-pressure vapour. Increase in energy provides the driving force to allow the refrigerant to flow through the system. The compressor is activated by the evaporator temperature rise.
The pressure of the refrigerant is increased to raise its boiling point so that it can condense (change back to a liquid for recycling) at a high temperature. Since the thermal properties of the refrigerant vary, the pressure requirements for this process vary.
Energy in form of electricity to operate a motor, is used to drive the compressor. This represents major energy consumption component of the system. The compressor also adds some energy to the refrigerant. This results in a higher refrigerant temperature.
CondensationRefrigerant returned to starting point as a high pressure and temperature sub-cooled liquid. The purpose of the condenser is to release the refrigerant heat that was absorbed in the evaporator and during compression. Condensers are usually placed in an air or water environment. The environment temperature must be lower than the refrigerant temperature and boiling point at the higher pressure as the refrigerant leaves the compressor. Hence, the refrigerant is cooled in the condenser, losing its latent heat of condensation, so that it returns to a liquid state. Seawater heat exchanger is used to absorb the LHC and discharge it overboard. If environment temperature is high, refrigerant pressures must also be high. Compressor energy requirements depend on the condenser environment temperature. Higher condenser environment temperatures require larger amounts of compressor energy, which reduces the efficiency of the system. The efficiency of the compression process can be increased when cool water is used to absorb heat at the condenser.
A condensed refrigerant, with its high temperature and pressure, leaves the condenser and flows into the receiver - the temporary storage space and surge tank for the sub-cooled refrigerant, for future use. If serves as a vapour seal to prevent vapor from entering the expansion valve. Frequently, the lower portion of the condenser is used as a receiver, especially in small reach-in chillers and freezers and with some types of window air conditioners.
In summary, the expansion valve reduces the pressure of the liquid refrigerant as it flows to the evaporator; a low-pressure liquid refrigerant absorbs heat in the evaporator; its pressure is increased at the compressor; it loses its heat in the condenser; a high pressure liquid refrigerant flows to the receiver a temporary storage tank and moves to the expansion valve.
ABSORPTION REFRIGERATION CYCLE
The primary absorption refrigeration cycle used for air conditioning in the hotel industry is lithium bromide absorption cycle. Water is the refrigerant, which limits the minimum cold temperature of the cycle.
Cold water produced by the cycle is used to absorb heat from the building. Air, which cools the building, can be moved over a series of cold-water pipes with zone temperature controls, or cold water can be piped to each room and individual room thermostats used to regulate air flow. The latter system provides maximum room response but has a higher installation cost.
|absorption refrigeration cycle|
The operation of complete cycle can be analyzed by referring to figure. Heat is absorbed at the evaporator by the water refrigerant. The water refrigerant is at an extremely low pressure and therefore has a low boiling point. Water changes to a low-pressure steam (water vapor) as it absorbs heat.
Water vapour from the evaporator is attracted to and is absorbed by a salt solution contained in the absorber (salt attracts and absorbs water). A common salt solution is lithium bromide brine. The latent heat of condensation is released by the water vapour refrigerant when it is absorbed by the lithium bromide brine and changes to a liquid (water). The absorber gets hot and must be cooled, just like the condenser in the condenser in the vapour compression refrigeration cycle. When lithium bromide absorbs water, the brine solution is weakened and its attraction for additional water is reduced.
The weakened brine solution is pumped from the absorber and sprayed into the generator, driving off the excess water that was absorbed as water vapour. Since the pump increases the pressure of the weak brine solution, water now has a higher boiling point that it had in the evaporator or in the absorber.
Heat is normally supplied as steam in the generator, although any source of heat will allow the system to operate. The brine that is left has a high salt ratio - excess water has been driven off - and it is called strong brine. The strong brine flows by gravity back toward the absorber and is sprayed into the absorber. The spraying process reduces brine pressure, so absorber and evaporator pressure remains low and in equilibrium.
The water vapour produced in the generator at a higher pressure proceeds to the condenser. Because of its higher pressure and resulting higher boiling point, water vapour can now be condensed, releasing the latent heat of vaporization that was absorbed in the generator at a relatively high environment temperature. The higher-pressure liquid leaves the condenser and flows back to the evaporator.
The water is sprayed into the evaporator, which reduces pressure on the water refrigerant. The water cycle is now complete.
WATER COOLING TOWERS
A water-cooling tower is used in conjunction with water-cooled condensers and /or absorbers. It cools the warm water flowing from the condenser or absorber so that it can be recycled back to the condenser and/or absorber. Figure shows how a water-cooling tower can be used with an absorption refrigeration system. Condensers in the vapour compression refrigeration cycle can also be connected to water-cooling towers. The cooling tower operation is relatively simple and requires minimal energy for efficient operation. Water is pumped and sprayed into a confined air space. When water is sprayed into air, some of it must absorb heat, the latent heat of vaporization. The latent heat comes either from air or from the remaining water being pumped into the cooling tower; thus, the non-vaporized water is cooled. This remaining cool water is fed back to the condenser for reuse. The water absorbed by the air must be replaced. Replaced water is called makeup water.
|water cooling system|
The efficiency of the cooling tower is greatly increased if any or all of the following conditions exist: a high outside air temperature, a low outside relative humidity and increased air flow through the tower. The absorbed heat (warm water) can also be used to partially heat (preheat) domestic hot water in buildings.
SOLAR AIR COOLING
Using one of three systems can effectively use solar cooling for building air-cooling: radiative cooling, solar vapour compression, and solar absorption.
Each system has an auxiliary backup energy system, which increases its maintenance, repair, and installation costs.
A simple solar-cooling system is radiative cooling. Water is used to absorb building heat during the day and is circulated through pipes, plates or other devices and allowed to radiate heat at night outside the building.
Solar Vapour Compression
A second, potentially feasible solar cooling system operates like a vapour compression refrigeration cycle. The solar-energy system drives a compressor, so electrical energy is not required to operate an electric motor. A solar collector is used to heat a liquid or to change a liquid to vapour. The heated liquid or vapour proceeds to a heat exchanger equipped with an auxiliary heater (a back up system) to heat a second fluid, changing it to vapour. The heated second vapour drives an engine and a low temperature vapour discharged from the engine is condensed in a condenser and continues back to the auxiliary heater for recycling.
Solar Absorption Cooling
A third solar-cooling system is an absorption system. Like lithium bromide absorption cycle, the cycle has four components. An evaporator absorbs heat from the area to be cooled. A liquid refrigerant absorbs its latent heat of vaporization, and its vapour is attracted to a chemical solution in the absorber. The refrigerant gas is absorbed by a chemical solution, and its latent heat of condensation is released. The liquid-refrigerant and chemical mixture flows to a generator, where solar heat is used to drive off the refrigerant as a vapour. The separated refrigerant gas proceeds to condenser. The absorbing chemical flows back to the absorber for recycling from the generator, sometimes called separator. The refrigerant gas is condensed in the condenser, and an expansion device reduces its pressure before it flows back to the evaporator. If sufficient solar energy is not available at the generator, an auxiliary form of heat has to be used as a backup system. The system is fairly efficient but requires a large, solar-collection area.
There are several additional techniques that can be used for cooling. The feasibility and potential use of these techniques generally depend on a low cost energy source. One such system is absorption system that can be used for food chillers and freezers. It is very similar to the solar absorption cycle, except that a low-cost energy source is used instead of solar energy, at the generator. The most common source of energy for these units has been either natural gas or LPG. These are frequently used as portable cooling units and are generally found in recreational vehicles.
Air can be cooled by a compression and expansion process. Air is compressed to a relatively high pressure. The compressed air is quickly released to a space through an expansion valve. As compressed air is released, its pressure is suddenly released and it expands. The expansion process requires energy. This energy comes from sensible heat contained in air in the space to be cooled, thus reducing the air temperature in the space.
Swamp or Evaporative Coolers
Swamp or evaporative coolers operate like cooling towers and produce very cool, humid air. If the relative humidity of the air to be cooled is high, the process is not very efficient, which may restrict its use. Energy is required in the form of water and electricity to operate water pumps and an air fan.
REFRIGERANTSThe halocarbon family of refrigerants is generally used with vapour compression refrigeration cycles because of its excellent safety records. Halocarbons refrigerants are essentially non-toxic, non-corrosive, non-explosive and non-flammable. In addition, they have relatively low specific volume this means that the mechanical system can be physically small for a specific heat removal rate and have moderate to low condensing pressures, i.e. lower compression energy requirements. One disadvantage is that special equipment and chemical analyzers are necessary to locate a refrigerant leak in the system.
There are three primary refrigerant types, namely R-11, R-12, and R-502 and two secondary refrigerant types, R-13 and R-503, used m the hotel industry for vapour compression refrigeration cycles for air cooling, food chillers, and food freezers that are chlorofluorocarbons (CFCs). As CFCs destroy the ozone layer around the earth when they are released to environment: they are no more used these days. New vapour compression refrigeration cycles cannot use CFCs as refrigerants.
The substitute refrigerants are blends of halocarbon refrigerants, namely R-22, R-152a, R-124, R-125, R-218 and R-290.
Walk-In ChillersWalk-in chillers are normally custom designed. Custom – design includes prefabricated components that can be intermixed to satisfy several design variables. For example, one compressor may be used with a group of condensers and/or evaporators, or one condenser may be used with one of several compressors and/or expansion valve.
Walk-in units are available in almost any size, from a small unit of 20 square feet of floor to very large rooms, such as refrigerated warehouses used in institutional complexes.
CARE AND MAINTENANCE
Safety Associated Refrigeration
• Phosgene gas hazard (created when refrigerant is exposed to high temperatures)
• Handling procedures: Wear goggles and gloves to avoid eye irritation and frostbite
• Asphyxiation hazard in non-ventilated spaces (bilges since heavier than air)
While repairing air conditioning unit employers should:
• Require that all electrical equipments be de-energized before any repairs are performed;
• Provide employees with education and training in the recognition and avoidance of electrical hazard
• Ensure that electrical equipment is installed to meet the manufacturers, specifications, and ensure that equipment is restored to the manufacturers, specifications before any work is begun.