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NAME-409 Marine Engineering -II Cdre M Muzibur Rahman, (E), psc, PhD, BN Refrigeration and Air Conditioning The subject of refrigeration and air conditioning has evolved out of human need for food and comfort. What is Refrigeration? Refrigeration may be defined as the process of achieving and maintaining a temperature below that of the surroundings, the aim being to cool some product or space to the required temperature. What is Air Conditioning: Air Conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odor and circulation, as required by occupants, a process, or products in the space. 1 Cdre Muzib, psc, PhD
Second Law of Thermodynamics It is impossible to extract an amount of heat (QH)from a hot reservoir and use it all to do work (W). Some amount of heat (QC) must be exhausted to a cold reservoir. This precludes engine. This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law. Cdre Muzib, psc, PhD 2
Second Law of Thermodynamics It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statement is also applicable to air conditioners and heat pumps, which embody the same principles. This is the "second form" or Clausius statement of the second law. Cdre Muzib, psc, PhD 3
Second Postulate: 2nd Law of Thermodynamics Cdre Muzib, psc, PhD 4
Reversed Carnot cycle Cdre Muzib, psc, PhD 5
Performance of Refrigeration System The performance of refrigerators is expressed in terms of coefficient of performance (COP): Under the same operating condition: The COP improves by 2 to 4% for each °C rise of evaporating temperature or each °C fall of condensing temperature. Cdre Muzib, psc, PhD 6
Type of refrigeration process: a. Vapour compression refrigeration: Here moving part is the compressor which sucks the refrigerant (vapour) from evaporator and compresses it to the high pressure to provide work input to have cooling effect. b. Vapour absorption refrigeration: compression are carried out by two different devices called as the absorber and the generator. Thus the absorber and the generator replace the compressor in the vapor absorption cycle. The absorbent enables the flow of the refrigerant from the absorber to the generator by absorbing it. In this case the flow is maintained by a pump. Here, the process of suction and Cdre Muzib, psc, PhD 7
Vapour Compression Refrigeration Systems The basis of modern refrigeration is the ability of liquids to absorb enormous quantities of heat as they boil and evaporate. Professor William Cullen of the University of Edinburgh demonstrated this in 1755 by placing some water in thermal contact with ether under a receiver of a vacuum pump. The evaporation rate of ether increased due to the vacuum pump and water could be frozen. This process involves two thermodynamic concepts, the vapour pressure and the latent heat. A liquid is in thermal equilibrium with its own vapor at a pressure called the saturation pressure, which depends on the temperature alone. If the pressure is increased for example in a pressure cooker, the water boils at higher temperature. The second concept is that the evaporation of liquid requires latent heat during evaporation. If latent heat is extracted from the liquid, the liquid gets cooled. The temperature of ether will remain constant as long as the vacuum pump maintains a pressure equal to saturation pressure at the desired temperature. This requires the removal of all the vapors formed due to vaporization. If a lower temperature is desired, then a lower saturation pressure will have to be maintained. Cdre Muzib, psc, PhD 8
Vapour compression cycle: Assumptions Cdre Muzib, psc, PhD 9
Vapour Compression Refrigeration cycle Cdre Muzib, psc, PhD 10
P-h Diagram Cdre Muzib, psc, PhD 15
A flash tank is a pressure vessel, wherein the refrigerant liquid and vapour are separated at an intermediate pressure. The refrigerant from condenser is first expanded to an intermediate pressure corresponding to the pressure of flash tank, Pi using a low side float valve. The float valve also maintains a constant liquid level in the flash tank. In the flash tank, the refrigerant liquid and vapour are separated. The saturated liquid is fed to the evaporator after throttling it to the required evaporator pressure, Peusing an expansion valve. Depending upon the type of the system, the saturated vapour in the flash tank is either compressed to the condenser pressure or throttled to the evaporator pressure. In the absence of flash tank, the refrigerant condition at the inlet to the evaporator would have been considerably with high vapour quality which would reduce the refrigeration effect. Cdre Muzib, psc, PhD 22
Ton Refrigeration It is defined as the heat of fusion absorbed by melting 1 ton (1000 kg) of pure ice at 0 °C (32 °F) in 24 hours. It is equivalent to the consumption of one ton of ice per day and originated during the transition from stored natural ice to mechanical refrigeration. A refrigeration ton is approximately equivalent to 12,000 BTU/h or 3.51685 kW or 211 kJ/min. Air-conditioning and refrigeration equipment capacity in the U.S. is often refrigeration). Many manufacturers capacity in BTU/h, especially when specifying the performance of smaller equipment. specified in "tons" also (of specify Cdre Muzib, psc, PhD 25
Condensers: Based on the external fluid, condensers can be classified as: a) Air cooled condensers b) Water cooled condensers, and c) Evaporative condensers Air-cooled condensers: In air-cooled condensers air is the external fluid, i.e., the refrigerant rejects heat to air flowing over the condenser. Air-cooled condensers can be again of natural convection type or forced convection type. Natural convection type Forced convection type Cdre Muzib, psc, PhD 26
Water Cooled Condensers: In water cooled condensers water is the external fluid. Depending upon the construction, water cooled condensers can be further classified into: 1. Double pipe or tube-in-tube type 2. Shell-and-coil type 3. Shell-and-tube type Double Pipe or tube-in-tube type: Double pipe condensers are normally used up to 10 TR capacity. Figure aside shows the schematic of a double pipe type condenser. In these condensers the cold water flows through the inner tube, while the refrigerant flows through the annulus in counter flow. Headers are used at both the ends to make the length of the condenser small and reduce pressure drop. The refrigerant in the annulus rejects a part of its heat to the surroundings by free convection and radiation. coefficient is usually low refrigerant drainage if the tubes are long. The heat of transfer liquid because poor 27 Cdre Muzib, psc, PhD
Shell-and-coil type: These condensers are used in systems up to 50 TR capacity. The water flows through multiple coils, which may have fins to increase the heat transfer coefficient. The refrigerant flows through the shell. In smaller capacity condensers, refrigerant flows through coils while water flows through the shell. When water flows through the coils, cleaning is done by circulating suitable chemicals through the coils. Cdre Muzib, psc, PhD 28
Shell-and-tube type: This is the most common type of condenser used in systems from 2 TR upto thousands of TR capacity. Here, the refrigerant flows through the shell while water flows through the tubes in single to four passes. The condensed refrigerant collects at the bottom of the shell. The coldest water contacts the liquid refrigerant so that some subcooling can also be obtained. The liquid refrigerant is drained from the bottom to the receiver. There might be a vent connecting the receiver to the condenser for smooth drainage of liquid refrigerant. The shell also acts as a receiver. Further the refrigerant also rejects heat to the surroundings from the shell. The most common type is horizontal shell type. Vertical shell-and-tube type condensers are usually used with ammonia in large capacity systems so that cleaning of the tubes is possible from top while the plant is running. Cdre Muzib, psc, PhD 29
Evaporative condensers Here, both air and water are used to extract heat from the condensing refrigerant. It combines the features of a cooling tower and water-cooled condenser in a single unit as follows: Used in medium to large capacity systems Normally cheaper compared to water cooled condensers Used in places where water is scarce. Since water is used in a closed loop, only a small part of the water evaporates. Make-up water is supplied to take care of the evaporative loss. The water consumption is typically very low, about 5 percent of an equivalent water cooled condenser with a cooling tower. Since condenser has to be kept outside, this type of condenser requires a longer length of refrigerant tubing, which calls for larger refrigerant inventory and higher pressure drops. Since the condenser is kept outside, to prevent the water from freezing, when outside temperatures are very low, a heater is placed in the water tank. When outside temperatures are very low it is possible to switch-off the water pump and run only the blowers, so that the condenser acts as an air cooled condenser. Another simple form of condenser used normally in older type cold storages is called as atmospheric condenser. The principle of the atmospheric condenser is similar to evaporative condenser, with a difference that the air flow over the condenser takes place by natural means as no fans or blowers are used. A spray system sprays water over condenser tubes. Heat transfer outside the tubes takes by both sensible cooling and evaporation, as a result the external heat transfer coefficient is relatively large. The condenser pipes are normally large, and they can be either horizontal or vertical. Though these condensers are effective and economical they are being replaced with other types of condensers due to the problems such as algae formation on condenser tubes, uncertainty due to external air circulation etc. Cdre Muzib, psc, PhD 30
Refrigerants Refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications halogenated hydrocarbons such as propane. are ammonia, sulfur dioxide, and non- The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including free from toxicity and flammability. It would not cause ozone depletion or climate change. Since different fluids have the desired traits in different degree, choice is a matter of trade-off. The desired thermodynamic properties are a boiling point below the target temperature, a high heat of vaporization, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature. Cdre Muzib, psc, PhD 32
Refrigerants may be divided into three classes according to their manner of absorption or extraction of heat from the substances to be refrigerated: Class 1: This class includes refrigerants that cool by phase change (typically boiling), using the refrigerant's latent heat. Class 2: These refrigerants cool by temperature change or 'sensible heat', the quantity of heat being the specific heat capacity x the temperature change. They are air, calcium chloride brine, sodium chloride brine, alcohol, and similar nonfreezing solutions. The purpose of Class 2 refrigerants is to receive a reduction of temperature from Class 1 refrigerants and convey this lower temperature to the area to be air- conditioned. Class 3: This group consists of solutions that contain absorbed vapors of liquefiable agents or refrigerating media. These solutions function by nature of their ability to carry liquefiable vapors, which produce a cooling effect by the absorption of their heat of solution. They can also be classified into many categories. Cdre Muzib, psc, PhD 33
Desired properties of a Refrigerant: 1. Vapor density: To enable use of smaller compressors and other equipment the refrigerant should have smaller vapor density. 2. Enthalpy of vaporization: To ensure maximum heat absorption during refrigeration, a refrigerant should have high enthalpy of vaporization. 3. Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation. 4. Dielectric strength: In hermetic arrangements, the motor windings are cooled by refrigerants vapor on its way to the suction valve of the compressor. Therefore, dielectric strength of refrigerant is important property in hermetically sealed compressor units. 5. Critical temperature: In order to have large range of isothermal energy transfer, the refrigerant should have critical temperature above the condensing temperature. 6. Specific heat: To have minimum change in entropy during the throttling process, the specific heat should be minimum. For this, liquid saturation line should be almost vertical. 7. Leak tendency: The problems with leakage are wearing out of joint or the material used for the fabrication of the system. A denser refrigerant will have fewer tendencies to leak as compared to higher density refrigerant. The detection of leaks should be easy to loss of refrigerant. Leakage can be identified quickly if the refrigerant has distinct color or odour. 8. Toxicity: The refrigerant used in air conditioning, food preservation etc. should not be toxic in nature as they will come into contact with human beings. Refrigerants will affect human health if they are toxic. 9. Cost of refrigerants: The quantity of refrigerant used in industries is very less. The cost of the refrigerants is generally high when compared to other chemicals in the industry. Cost Per kg R22 Tk 800-1000, R134A Tk 1200-1800, R503 Tk 3000 – 4000. 10. Availability: Refrigerants should be available near the usage point. It must be sourced and procured within a short period to enable the user in case of leaks, maintenance schedules etc. Cdre Muzib, psc, PhD 35
Primary and secondary refrigerants Primary refrigerants are those fluids, which are used directly as working fluids. When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator. Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Commonly used secondary refrigerants are the solutions of water and ethylene glycol, propylene glycol or calcium chloride. These solutions are generally called brines. If the operating temperatures are above 0oC, then pure water can also be used as secondary refrigerant. Brines are used at sub- zero temperatures. Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other. An important property of a secondary refrigerant is its freezing point. The temperature at which freezing of a brine takes place depends on its concentration. The concentration at which a lowest temperature can be reached without solidification is called as eutectic point. Cdre Muzib, psc, PhD 36
Vapor Absorption Refrigeration Cdre Muzib, psc, PhD 37
There are several common combinations of absorbent-refrigerants: • Water and Ammonia • Lithium Nitrate and Ammonia • Lithium Bromide and Water • Lithium Chloride and Water Cdre Muzib, psc, PhD 39
Simple Absorption System: 1) Condenser: Just like the vapor compression cycle, the refrigerant enters the condenser at high pressure and temperature and gets condensed. The condenser is of water cooled type. 2) Expansion valve or restriction: When the refrigerant passes through the expansion valve, its pressure and temperature reduces suddenly. This refrigerant then enters the evaporator. 3) Evaporator: The refrigerant at very low pressure and temperature enters the evaporator and produces the cooling effect. In the vapor compression cycle this refrigerant is sucked by the compressor, but in the vapor absorption cycle, this refrigerant flows to the absorber that acts as the suction part of the refrigeration cycle. 4) Absorber: The absorber is a sort of vessel consisting of water that acts as the absorbent, and the previous absorbed refrigerant. Thus the absorber consists of the weak solution of the refrigerant (for example: ammonia) and absorbent (e.g. water). When ammonia from the evaporator enters the absorber, it is absorbed by the absorbent due to which the pressure inside the absorber reduces further leading to more flow of the refrigerant from the evaporator to the absorber. At high temperature water absorbs lesser ammonia, hence it is cooled by the external coolant to increase its ammonia absorption capacity. 5) Pump: When the absorbent absorbs the refrigerant strong solution of refrigerant-absorbent is formed. This solution is pumped by the pump at high pressure to the generator. Thus pump increases the pressure of the solution to about 10bar. 6) Generator: The refrigerant-ammonia solution in the generator is heated by the external source of heat. This is can be steam, hot water or any other suitable source. Due to heating the temperature of the solution increases. The refrigerant in the solution gets vaporized and it leaves the solution at high pressure. The high pressure and the high temperature refrigerant then enters the condenser, where it is cooled by the coolant, and it then enters the expansion valve and then finally into the evaporator where it produces the cooling effect. This refrigerant is then again absorbed by the weak solution in the absorber. Cdre Muzib, psc, PhD 41
Ammonia-Water Absorption Refrigeration Absorption refrigeration systems have important commercial and industrial applications. The principal components of an ammonia-water absorption system are shown in the figure. Absorber coolant Cdre Muzib, psc, PhD 45
Ammonia-Water Absorption Refrigeration The left-side of the schematic includes components familiar from the discussion of the vapor-compression system: evaporator, condenser, and expansion valve. Only ammonia flows through these components. Absorber coolant Cdre Muzib, psc, PhD 46
Ammonia-Water Absorption Refrigeration ►The right-side of the schematic includes components that replace the compressor of the vapor-compression refrigeration system: absorber, pump, and generator. These components involve liquid ammonia-water solutions. Absorber coolant Cdre Muzib, psc, PhD 47
Ammonia-Water Absorption Refrigeration A principal advantage of the absorption system is that – for comparable refrigeration duty – the pump work input required is intrinsically much less than for the compressor of a vapor-compression system. Absorber coolant Cdre Muzib, psc, PhD 48
Ammonia-Water Absorption Refrigeration ►Specifically, in the absorption system ammonia vapor coming from the evaporator is absorbed in liquid water to form a liquid ammonia-water solution. ►The liquid solution is then pumped to the higher operating pressure. For the same pressure range, significantly less work is required to pump a liquid solution than to compress a vapor (see discussion of Eq. 6.51b). Absorber coolant Cdre Muzib, psc, PhD 49
Ammonia-Water Absorption Refrigeration ►However, since only ammonia vapor is allowed to enter the condenser, a means must be provided to retrieve ammonia vapor from the liquid solution. ►This is accomplished by the generator using heat transfer from a relatively high-temperature source. Absorber coolant Cdre Muzib, psc, PhD 50
