Lesson 4. Theoretical and actual cycle, performance of refrigeration cycle

Module 1. Fundamentals of refrigeration

Lesson 4
THEORETICAL AND ACTUAL CYCLE, PERFORMANCE OF REFRIGERATION CYCLE

4.1 Introduction

A working cycle of VCR system is represented in P-H diagram in .The processes of evaporation (4-1) and condensation (2-3) are treated as isothermal processes. The expansion process (3-4) is a constant enthalpy process; while compression process (1-2) is an isentropic process. Based on the operating condition evaporating cycle can be presented on the P-H diagram to obtain values of refrigerating effect, work of compression to calculate the theoretical process of the plant. The operating condition of the plant varies depending upon the temperature requirement as well the temperature of cooling medium available at condenser. The cycle described bellow is a simple saturated cycle in which the liquid after condensation and vapour after evaporation are saturated and lie on the saturated liquid and vapour curves respectively in P-H chart.

4.2 Actual Cycle

The actual cycle deviates slightly due to pressure drop caused by friction in piping and valves. In addition to this there will be heat loss or gain depending on the temperature difference between the refrigerant and the surrounding. Further compression will be polytropic due to friction and heat transfer instead of isentropic. The actual VCR cycle is depicted in fig. 4.2. It is clear from the both actual and theoretical cycles that little pressure drop takes place when refrigerant pass through the evaporator (From 4-a). The processes a-b and b-c are depicting superheating of suction vapour inside the evaporator and outside the evaporator respectively; where as the processes c-d and d-1 are showing the pressure drop in line and wire drawing effect pressure drop inside the compressor valve respectively. The processes 2-e and e-f are the pressure drop in compressor discharge valve and delivery line respectively. Processes f-h is desuperheating of gas in condenser and h-3 is sub cooling of liquid in condenser.

The operating cycle deviates as discussed above however, the saturated cycle is used to determine the COP of the cycle for all practical purpose. It is well observed that the pressure drop in the evaporator due to frictional pressure drop and momentum pressure drop is larger than that of the condenser.

Fig. 4.1 Actual vapor compression refrigeration cycle on P-H diagram

4.3 Performance of Refrigeration System

The performance of refrigeration plant is expressed as "Co-efficient of Performance (COP)" which is defined as the ratio of refrigerating effect produced to the work of compression. It is obvious that higher COP is desirable to reduce the operating cost of the system. The COP is greatly influenced by several factors like operating variables, part load performance, design of plant components, maintenance of the system etc. Many research workers have emphasized the importance of matching design of plant components and efficient heat transfer at evaporator and condenser in order to achieve optimum level of performance.

Refrigeration systems have been a subject of continuous modification and development to meet the specific demand of the industry. Systems for large-scale air conditioning for business complexes, cold storages, hospitals, library, etc. are very common in US and in many other developed countries. There are number of technical and global issues which include energy conservation, alternative refrigerant due to depletion caused by CFC refrigerants and technical problems of air quality. It is necessary to implement energy management strategy which involves total commitment at all levels, education/awareness campaign, training, energy audit, cost­ benefit analysis, maintenance programme etc.

4.4 Theoretical and Actual Cop

The operating cycle of the plant can be plotted on P-H diagram corresponding to suction and the discharge pressure of the system to calculate theoretical COP of the plant. The measurement of actual COP, which is the ratio of actual cooling effect produced to the actual power consumption, is difficult under practical conditions. The work of compression can be easily obtained by installing energy meters but the rate of cooling effect produced is difficult to estimate, as the refrigerating effect is continuously in use. The measurement of actual COP is more important to know the performance of the plant.

The estimation of actual refrigerating effect produced is practically difficult; however under certain conditions, it is possible to calculate the rate of refrigerating effect produced by indirect method. For any vapour compression refrigeration plant, Q = R + W; where Q = heat removed at condenser, kJ/h; R = refrigerating effect produced, kJ/h; W = work of compression, kJ/h. If it is possible to estimate the value of heat removed at condenser by measuring the change in enthalpy of the cooling medium, the refrigerating effect can be estimated as the work of compression is measured by energy meters. The COP of vapour compression refrigeration plant varies from 2.5 to 4.5 depending on the operating conditions of the plant.

4.5 Factors Affecting the Performance of Refrigeration Plant

The COP of a vapour compression refrigeration plant is mainly affected by operating conditions of the plant as well as maintenance aspects of the plant. The operating conditions of the refrigeration plant play very important role on the performance of the refrigeration system. The important factors, which are affecting the performance of vapour compression refrigeration system, are listed below.

1. Evaporating temperature

2. Condensing temperature

3. Sub-cooling of liquid refrigerant

4. Super heating of suction gas

5. Heat transfer at evaporator condenser

6. Presence of non-condensable gases in the system

7. Volumetric efficiency of compressor

8. Multi- stage compression and throttling system

9. Design of plant components

10. Maintenance of the plant

4.5.1 Effect of evaporating temperature

The evaporating temperature of the plant is maintained depending on the temperature requirement for the given application. Lower evaporating temperature reduces the refrigerating effect per kg of refrigerant circulated in the system and increases the work of compression leading to reduction in COP of the plant. Therefore, it is desirable to operate a refrigeration plant with highest possible evaporating temperature and undue lower evaporating temperature should be avoided.

4.5.2 Effect of condensing temperature

The condensing temperature is fixed by the temperature of cooling medium available as well as the efficiency of heat transfer at the condenser. The rate of heat rejected at the condenser is function of overall heat transfer co-efficient, heat transfer area and the temperature difference between the refrigerant and the cooling medium. Lower condensing temperature is desirable to get higher COP of the system. The efficiency of cooling tower is very important to get lower temperature of water for water-cooled condenser of the plant. In case of evaporative condenser, dry bulb temperature, wet bulb temperature and velocity of air play important role in heat transfer at the condenser. It is possible to save energy by operating the refrigeration plant during colder hours of the day.

4.5.3 Sub-cooling of liquid refrigerant

Sub-cooling of liquid refrigerant is desirable as it increases the refrigerating effect without any change in work of compression. The sub-cooling achieved in the condenser using low temperature water is maximum advantageous as compared to sub- cooling using liquid refrigerant from the receiver.

4.5.4 Effect of super heating of suction gas

For trouble free operation of refrigeration system, a little super heating of suction gas is desirable to eliminate the chances of wet compression. However, excessive super heating of suction gas is not desirable as it increases the work of compression and piston displacement of the compressor. It also causes compressor head cooling difficulties in tropical countries like ours. The power consumption for compression of refrigerant under superheated condition will increase due to diverging nature of isentropic lines.

4.5.5 Heat transfer at evaporator and condenser

The efficiency of heat transfer at condenser and evaporator is greatly influencing the performance of the plant. The design of these components of the plant to achieve maximum rate of heat transfer under the practical conditions is very important consideration. The rate of heat transfer is function of overall heat transfer coefficient (U-value), heat transfer area and the temperature difference. Efficient cleaning of condenser and defrosting of evaporator at regular interval is important to achieve higher U-value. It is necessary to examine the factors affecting the rate of heat transfer depending on the type of evaporator and condenser to optimize the rate of heat transfer

4.5.6 Multi-stage compression system

It is usual practice to manufacture single stage compressor with compression ratio around 7 to 8. However, for large capacity plants even in this pressure ratio, the multi stage compression is employed. The compressed refrigerant vapour after first stage compression is passed through flash chamber where it becomes saturated by cooling at intermediate pressure. Multi-compression system reduces the work of compression as well as improves the volumetric efficiency of the compressor and decreases the temperature of refrigerant gas leaving the compressor. The volumetric efficiency plays significant role on the capacity of the refrigeration plant. Optimum inter-stage pressure is necessary to maintain in order to achieve maximum advantage of two-stage compression system.

4.6 Estimation of Refrigeration Plant Capacity

The total refrigeration requirement of a dairy plant can be estimated by calculating the cooling load of various operations and based on the hourly refrigeration load requirement. The capacity is expressed as ton of refrigeration which is equivalent to 12600 kJ/h (3000 kcal/h) heat removed by the plant from the evaporator. The refrigeration load of chillers, pasteurizers, cold storages etc. is calculated to arrive at the total cooling requirement of the plant. A simple way of estimating a capacity of refrigeration plant of a bulk cooler having 5000 liters milk storage capacity is given below. (Density of milk=1.032 g/cm3; Specific heat of milk = 3.9 kJ/kg K; Initial temperature of milk=35°C; Final cooling temperature=2 °C)

Heat to be removed from the milk = 5000 x 1.032 x 3.9 x (35-2)

= 664092 kJ

The bulk coolers are designed to cool the entire quantity of milk in about 3.5 hours. Therefore,

Heat to be removed from the milk per hour = 189741 kJ/h

Capacity of the refrigeration plant in ton = 442723/12600

= 15 ton

4.7 Efficient Use of Refrigeration

It is essential to make efficient use of refrigeration produced to achieve over all advantage of efficient operation of the plant. In dairy plants usually ice-bank refrigeration system (thermal energy storage system- TES) is widely used for chilling and processing of milk. TES system is widely recognized as a demand side management technology for shifting cooling electric demand from peak day-time period to off peak night time. In many countries TES has enabled users to significantly reduce their electricity cost by reducing peak demand and taking advantage of lower off peak usage rates often with large utility incentive payment.

The following aspects are important for conservation of refrigeration in dairy and food plants.

1. Efficient design of cold storage and ice-bank refrigeration system.

2. Selection of energy efficient refrigeration plant and its components.

3. Minimize air change load in cold storages

4. Proper insulation to chilled water pipelines.