The Vapor Compression Refrigeration Cycle, Step By Step

THE VAPOR COMPRESSION REFRIGERATION CYCLE, STEP BY STEP The Vapor Compression Refrigeration Cycle is nearly 200 years old, but it does not seem ready to leave the scene any time soon. While some people have viewed this method as environmentally harmful and inefficient, the cycle is still applicable in the industrial sphere. Natural gas plants, petroleum refineries, and petrochemical plants and most of the food and beverage processes are some of the industrial plants that utilize vapor compression refrigeration systems. What is its defining feature of these systems? The simplest explanation of this system is a heat engine working in reverse, technically referred to as reverse Carnot engine. In other words, it is the transfer of heat from a cold reservoir to a hot one. Clausius Statement of the Second Law of thermodynamics states: “It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body”. Since the vapor compression cycle is against the Second Law of Thermodynamics, some work is necessary for the transfer to take place. WHY DO WE USE THE TERM “COMPRESSION”? The Vapor Compression Refrigeration Cycle involves four components: compressor, condenser, expansion valve/throttle valve and evaporator. It is a compression process, whose aim is to raise the refrigerant pressure, as it flows from an evaporator. The high-pressure refrigerant flows through a condenser/heat exchanger before attaining the initial low pressure and going back to the evaporator. A more detailed explanation of the steps is as explained below. STEP 1: COMPRESSION The refrigerant (for example R-717) enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters to the condenser. Since

this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types. STEP 2: CONDENSATION The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to a flow of water. This water goes to a cooling tower for cooling in the case of water-cooled condensation. Note that seawater and air-cooling methods may also play this role. As the refrigerant flows through the condenser, it is in a constant pressure. One cannot afford to ignore condenser safety and performance. Specifically, pressure control is paramount for safety and efficiency reasons. There are several pressure-controlling devices to take care of this requirement STEP 3: THROTTLING AND EXPANSION When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture, typically in proportions of around 75 % and 25 % respectively. Throttling valves play two crucial roles in the vapor compression cycle. First, they maintain a pressure differential between low- and high-pressure sides. Second, they control the amount of liquid refrigerant entering the evaporator. STEP 4: EVAPORATION At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it evaporates and absorbs latent heat of vaporization. Heat extraction from the refrigerant happens at low pressure and temperature. Compressor suction effect helps maintain the low pressure.

There are different evaporator versions in the market, but the major classifications are liquid cooling and air cooling, depending whether they cool liquid or air respectively.

Main Parts Of Vapor Compression Refrigeration Cycles: 1. Evaporator Its function is to provide a heat transfer surface through which heat can pass from the refrigerated space into the vaporizing refrigerant. This is generally a Fin & Tube (Hair-Pin type) heat exchanger, similar to Air-Cooled Condensers. 2. Suction Line It carries the low-pressure vapor from the evaporator to suction inlet of the compressor. 3. Compressor The function of the compressor is to draw refrigerant vapor from the evaporator and to raise Its temperature and pressure to such a print to that it may be easily condensed with normally available condensing media. It also maintains a continuous flow of the refrigerant through the system.

Compression Ratio Absolute Discharge Pressure / Absolute Suction Pressure The Capacity of a Compressor is determined by its Mass Flow rate (Lb/Min) and not by Volume Flow(CFM). The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues. 4. Discharge Line It conveys the high pressure and high-temperature refrigerant from the compressor to the condenser. 5. Condenser The function of the condenser is to provide a heat transfer surface through which heat passes from the refrigerant to the condensing medium which is either water or air. Types of Condensers: Air-Cooled Water-Cooled 6. Liquid Receiver It acts as, a reservoir that stores the liquid refrigerant coming from the condenser and supplies it to the evaporator according to the requirement. 7. Liquid Line It carries the liquid refrigerant from the receiver and conveys it to the expansion valve. 8. Expansion valve Function Of This valve is to Supply a proper amount of refrigerant to the evaporator after reducing its pressure considerably so that the refrigerant may take sufficient amount of heat from the refrigerating space during evaporation

The Metering Device converts the High Pressure and High-Temperature Liquid from Condenser to Low Pressure and Low-Temperature Liquid-Vapor mixture, which will be fed to the Evaporator. An expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheat at the outlet of the evaporator. Types of Expansion devices Thermostatic EV Capillary tube Hand operated EV Automatic or Constant Pressure EV Float expansion Types of Vapor Compression Cycles : We have already disused that vapor compression cycle essentially consists of compression, condensation, throttling and evaporation. Many scientists have focused their attention to increase the coefficient of performance of the cycle. Through there are many cycles, yet the following are important from the subject point of view: 1. 2. 3. 4. 5. Cycle Cycle Cycle Cycle Cycle with with with with with dry saturated vapor after compression, wet vapor after compression, superheated vapor after compression, superheated vapor before compression, and under cooling or sub cooling of refrigerant, Advantages of Vapour Compression System : It has a smaller size for the given capacity of refrigeration. It has less running cost. It can be employed over a large range of temperatures. The coefficient of performance is quite high. Less time required to produce refrigerant effects.

Disadvantages of Vapour Compression System : The initial cost is high. The prevention of leakage of the refrigerant is the major problem in the vapor compression system. More wear and tear and noise due to Moving Parts Liquid droplets in suction line may damages. Saturated Vapour after Compression A vapour compression cycle with dry saturated vapour after compression is shown on T-s diagrams in Figures (a) and (b) respectively. At point 1, let T1, p1 and s1 be the temperature, pressure and entropy of the vapour refrigerant respectively. The four processes of the cycle are as follows : Compression Process The vapour refrigerant at low pressure p1 and temperatureT1 is compressed isentropically to dry saturated vapour as shown by the vertical line 1-2 on the T-s diagram and by the curve 1-2 on p-h diagram. The pressure and temperature rise from p1 to p2 and T1 to T2 respectively. The work done during isentropic compression per kg of refrigerant is given by

w h2 – h1 where h1 Enthalpy of vapour refrigerant at temperature T1, i.e. at suction of the compressor, and h2 Enthalpy of the vapour refrigerant at temperature T2. i.e. at discharge of the compressor. Condensing Process The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure p2 and temperature T2 as shown by the horizontal line 23 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing through the condenser, gives its latent heat to the surrounding condensing medium. Expansion Process The liquid refrigerant at pressure p3 p2 and temperature T3 T2, is expanded by throttling process through the expansion valve to a low pressure p4 p1 and Temperature T4 T1 as shown by the curve 3-4 on Ts diagram and by the vertical line 3-4 on p-h diagram. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporized in the evaporator. We know that during the throttling process, no heat is absorbed or rejected by the liquid refrigerant. Vaporizing Process The liquid-vapour mixture of the refrigerant at pressure p4 p1 and temperature T4 T1 is evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which, is to be cooled, This heat which is absorbed by the refrigerant is called refrigerating effect and it is briefly written as RE. The process of vaporization continues up to point 1 which is the starting point and thus the cycle is completed. We know that the refrigerating effect or the heat absorbed or extracted by the liquid-vapour refrigerant during evaporation per kg of refrigerant is given by RE h1 – h4 h1 – hf3

where hf3 Sensible heat at temperature T3, i.e. enthalpy of liquid refrigerant leaving the condenser. It may be noticed from the cycle that the liquid-vapour refrigerant has extracted heat during evaporation and the work will be done by the compressor for isentropic compression of the high pressure and temperature vapour refrigerant. Coefficient of performance, C.O.P. (Refrigerating effect)/( Work done)

2. Cycle with wet vapor after compression, 3. Cycle with superheated vapor after compression, 4. Cycle with superheated vapor before compression, and 5. Cycle with under cooling or sub cooling of refrigerant, Advantages of Vapour Compression System : It has a smaller size for the given capacity of refrigeration. It has less running cost.