Lesson 14. Receiver, expansion valves and evaporators

Module 3. Refrigeration plant components

Lesson 14

14.1 Introduction

The total refrigerant charge required in a refrigeration system depends on operating loads, type of components and distance between the components etc. The quantity of refrigerant in the system must be adequate at all the times so that liquid refrigerant enters the expansion valve. Over charge in the refrigeration system may also cause reduction in the efficiency due to accumulation of liquid refrigerant in the condenser.

14.2 Receivers

The receiver has to play very important to evacuate the part of the system for maintenance and repair. The size of the receiver should be such that it can hold the entire charge of the refrigerant with 1/4 volume available for expansion and safety. It means that receiver should never be more than 80% full during pumping down of refrigerant in the receiver.

A simple sketch of a receiver is shown in the (Fig. 14.1). Receivers of large capacity plants are commonly made of steel with welded dished ends and are installed horizontally. Small receivers of small plant may be vertical as it is convenient for installation. The liquid pipe from the condenser to the receiver should be sufficiently sized and provided with slope to promote easy movement of liquid refrigerant. The outlet pipeline from the receiver is connected either from the bottom or by means of an internal standpipe. A liquid shut off valve is fitted at the outlet of the receiver. Ammonia receivers may have an oil drum pot, and it is mounted with little slope towards oil pot. Receiver is provided with safety pressure relief devices, level indicator etc. In practice, the receiver is one-sixth full during normal running of the system.

14.3 Expansion Values

This is one of the basic components of the vapour compression refrigeration system. The functions expansion devices are listed below.

1. It reduces the pressure of the refrigerant coming from the condenser as per the requirement of the system.

2. It regulates the flow of the refrigerant as per the load on the evaporator.

The different devices which are used to perform the above functions are listed below.

1. Hand expansion valve. 4. High side float valve

2. Automatic expansion valve 5. Low side float valve

3. Thermostatic expansion valve 6. Capillary tube

14.3.1. Hand expansion valve

The hand operated expansion valve is employed to manually regulate the flow of liquid refrigerant to the evaporator. However, its use is limited to systems operating under fairly constant loads for long periods of time. Where loads are subject to repeated and rapid fluctuations, a condition that prevails in many industrial and commercial systems, the need for greater flexibility in liquid flow control is required. Therefore, manually operated expansion valves are employed on by pass line. A sketch of a hand expansion valve is given in (Fig. 14.2)

14.3.2 Automatic expansion valve

The automatic expansion valve functions in response to pressure changes occurring within the evaporator to allow more or less liquid refrigerant to flow in the evaporator. It is often referred to as a constant pressure expansion valve. Its use is limited to systems operated dry expansion under fairly constant loads and generally confined to small units.

If the pressure of evaporator falls due to decrease in load, the spring pressure causes the valve to open more. The rate of evaporation increases due to increased quantity of refrigerant and builds up the pressure until the equilibrium is reached with the spring tension. Reverse action takes place when the pressure in evaporator increases. A sketch of a hand expansion valve is given in Fig.%2014.3%20%20Automatic%20Expansion%20valve.swf

14.3.3 Thermostatic expansion valve

The thermostatic expansion valve controls the flow of refrigerant through the evaporator in such a way that the quality of vapour leaving the evaporator will be always in superheated condition. If the quantity of liquid in the evaporator diminishes, more heat transfer surface is available for superheating the suction gas which raises the temperature of the feeler bulb and power fluid. The pressure of the power fluid thereby increases, which opens the valve wider and increases the flow of refrigerant into the evaporator. The increase in flow rate of refrigerant in the evaporator decreases the super heat of the suction gas which reduces valve opening. The degree of superheat of the vapor leaving the evaporator depends upon the initial setting of the spring tension. Once the valve is adjusted for a particular superheat, then that superheat will be maintained under all load conditions on the evaporator. The working principle of thermostatic expansion valve is explained in (Fig. 14.4) This is most widely used refrigerant control device for medium size refrigeration systems.

14.3.4 High side float valve

This type of float valve allows only a minimum amount of liquid to remain in the high pressure side of the system so that nearly all the refrigerant charge is in the evaporator. As the compressed vapour condenses in the condenser, it flows into the float chamber from which it is forced under his pressure into the evaporator when a sufficient amount has collected to raise the float and open the valve. The float is set to open at a given level in the high side float chamber, it follows that the liquid level in the evaporator is nearly constant at all times. As entire charge of refrigerant is in the evaporator, it is necessary that the system contain only enough liquid refrigerants in order to prevent its carry over to the compressor. A system working with high pressure float valve is shown in Fig.%2014.5%20%20Hign%20Pressure%20Float%20Valve.swf

The high side float control may be installed either above or below the evaporator as it is independent of the liquid level in the evaporator. The flow chamber must be very near to the evaporator.

The liquid level in the float chamber drops as the compressor is stopped and control valve is closed and remain closed until the compressor is restarted.

14.3.5 Low side float valve

Low side float valve maintains constant level of refrigerant in the evaporator by supplying quantity of liquid refrigerant required to take the load in the evaporator. It maintains the evaporator always filled with the liquid refrigerant under all conditions irrespective of evaporator temperature and pressure. This method of control is used only on flooded evaporator. This is used in multiple or in parallel working evaporators used in commercial or industrial applications. A low pressure float valve system is shown in (Fig. 14.6.)

When the compressor is stopped, the liquid in the evaporator continues to vaporize until the pressure reaches a point corresponding to the temperature. At that point, evaporation ceases and the valve closes to isolate the evaporator.

14.3.6 Capillary tube

This device is only used for small capacity units like domestic refrigerators, water coolers and small commercial freezers. It is a small diameter tube connected between condenser and evaporator. The required pressure drop is caused due to heavy frictional resistance offered by a small diameter tube. The resistance is directly proportional to the length inversely proportional to the diameter. Different length and diameter combinations are recommended for the required pressure drop and flow quantity. The use of these expansion devices is limited to small units.

14.4 Evapotators

The evaporator is a heat exchanger where the actual cooling effect is produced. The evaporator receives the low pressure refrigerant from the expansion valve and brings the material to be cooled in contact with the surface of the evaporator. The refrigerant absorbs the heat from the materials to be cooled (air/water/milk/any other material). The refrigerant takes up its latent heat from the load and becomes vapour. The refrigerant vapour produced in the evaporator is pumped by the compressor and low pressure is maintained to maintained low evaporating temperature. The evaporators are fabricated from different materials having various designs depending on the requirement of cooling the product or material. The following factors are to be considered in the design of the evaporators.

14.4.1 Heat transfer

The heat transfer capacity of the evaporator is given by

Heat Flow rate (kJ/h), Q = U · A · (Tf – Ts )

Where, U = Overall Heat Transfer Co-efficient, kJ/(m2·h·K)

A = Heat transfer surface area of evaporator, m2

Tf = Temperature of the fluid to be cooled in the evaporator, K

Ts = Evaporating temperature of refrigerant, K

The requirement of surface area of the evaporator depends on value of U as well as temperature difference between the material to be cooled and the temperature of refrigerant.

14.4.2 Material of construction

The selection of material to be used for construction of evaporator is based on several factors such as type of refrigerant, thermal conductivity of the metal, cost, ease of fabrication, product to be cooled, etc. Copper is commonly used in small capacity plants due to its higher conductivity and ease of fabrication but it can not be used in ammonia plant as ammonia is corrosive to copper. Steel tubes/pipelines are commonly used in large capacity ammonia plant.

14.4.3 Velocity of refrigerant

Heat transfer co-efficient increases with increase in velocity of refrigerant in the evaporator. But increased velocity causes more pressure loss. It is very important point to be considered while selecting liquid over feed system.

14.4.4 Cooling requirement

The selection of metal for fabrication is mainly decided by the product to be cooled. The evaporator is fabricated from S. S. for ice-cream freezer, milk cooling equipment etc., while evaporator of ice-bank system is made from steel tubes. Air cooling evaporators for cold rooms, blast freezers, air-conditioning, etc. have finned pipe coils with fans to blow air over the coil.

14.5 Types of Evaporators

  • Flooded Evaporators
  • Dry type Evaporators

14.5.1 Flooded evaporators

The evaporator which is always filled with the liquid refrigerant during operation of the system is called flooded evaporator (Fig. 14.7). This type of evaporator gives higher rate of heat transfer and entire surface area of the evaporator is utilized for the heat transfer. The refrigerant boils and the saturated vapour refrigerant leaves the evaporator from the top. This is widely used in large ammonia systems. The refrigerant enters a surge drum through a float type expansion valve. The vapour produced due to expansion of liquid refrigerant is directly pumped by the compressor of the system. This vapour does not take part in refrigeration hence its removal makes the evaporator more compact and reduces pressure drop. The liquid refrigerant enters the evaporator from the bottom of the surge drum. The mixture of liquid and vapour bubbles rises up along the evaporator tubes and is separated as it enters the surge drum. The unevaporated liquid refrigerant circulates again in the tubes along with the constant supply of liquid refrigerant from the expansion valve. The lubricating oil tends to accumulate in the flooded evaporator hence an effective oil separator must be employed after the compressor.

14.5.2 Dry type evaporators

The liquid refrigerant is fed into the evaporator through the expansion valve and the quantity of refrigerant is so controlled that super heating of refrigerant vapour takes place at the end of evaporator (Fig. 14.8). Any increase or decrease in super heat at the end of evaporator due to change in evaporator load, alters the opening of expansion valve. The increase in load increases super heat at the end of evaporator which increases the flow rate of refrigerant to bring the super heat as it was earlier. The reverse action takes place when load decreases. These evaporators are provided with thermostatic expansion valve. In this type of evaporators, the oil keeps on, until it gets back to the compressor suction.

These evaporators are further classified as natural convection evaporator or forced convection evaporators. Forced convection evaporators used in cold storages of dairy and food plants. In ammonia plant, any oil present with liquid refrigerant will fall to the bottom of the evaporator which is drawn off from the oil drain connection provided in the evaporator.

Evaporators employed for air cooling have fined tube as where as liquid cooling evaporators may be shell and coil type or any form suitable for cooling of liquid. Plate type evaporator can be used for cooling of packaged product by conduction.

Last modified: Friday, 19 October 2012, 4:05 AM