Lesson 16. Defrosting, refrigeration piping and balancing of different components of the system.

Module 4. Refrigeration plant controls

Lesson 16

16.1 Introduction

The condensation of water vapour of the room/cold storage causes formation of frost over the evaporator. Formation of ice takes place in all the evaporators which are operating below the freeing point of water (0 ÂșC). The accumulation of ice over the heat transfer surface reduces the heat transfer rate as the ice is poor conductor of heat. Therefore, it is necessary to remove the ice deposited over the evaporator at periodic time interval. The operation of removing frosted ice from the evaporator is known as defrosting of evaporator. The period of defrosting depends on type of evaporator, relative humidity of the cold room, evaporation temperature etc.

16.2 Methods of Defrosting

16.2.1 Manual defrosting

The simplest of defrosting is to shut down the plant manually and restart it when the accumulated ice is melted away from the cooling coil. This method is not suitable for big capacity evaporators working at very low temperature as it takes long time and causes warming of the product stored in the cold storage.

16.2.2 Automatic periodic defrosting

The starting and stopping of refrigeration plant is automatic as per the change of evaporating pressure/temperature and defrosting is completed naturally. The frosting of evaporator coil causes reduction of suction pressure due to reduced heat transfer between the coil and the air. When the suction pressure falls below the predetermined value due to reduction of heat transfer on account of excessive frosting, pressure operated control stops the plant. (Fig. 16.1)

16.2.3 Water defrosting

In this method, Water is sprayed in ample quantity over the ice accumulated on the evaporator to washout the ice from the coil. This method is used in many commercial cold storages. During defrosting cycle, the supply of refrigerant is stopped and water is poured over the cooling coil. The water together with the melted ice is removed through pipelines. The time required for defrosting varies from 10-20 minutes depending on the amount of ice deposited and the temperature of water used for defrosting.

16.2.4 Hot gas defrosting

The defrosting carried out by using hot refrigerant gas from the compressor is called hot gas defrosting of evaporator. Hot gas defrosting is shown in the Fig.%2016.2%20%20Hot%20gas%20defrosting%20of%20evaporator.swf . The process of defrosting is performed at regular interval (6-10 hours) by the action of solenoid valve which supplies hot refrigerant gas after compression to the evaporator. Hot gas supply for few minutes melts the ice accumulated on the evaporator. The condensed refrigerant is re-evaporated in the re-evaporator before it goes to compressor.

16.2.5 Defrosting by reversing the cycle

When hot refrigerant gas is from the compressor is passed to the evaporator, it melts the frost accumulated on the evaporator coil. The normal operating conditions of the cycle and defrosting cycle are shown in (Fig. 16.3). This method is not used in commercial systems.

16.2.6 Electric defrosting

This method of defrosting is employed for fined coil evaporator. A bank of electric heaters is located near the coil. During defrosting, the system remains closed and heater starts to melt the frost accumulated on the evaporator. The time required for defrosting varies from few minutes to 30 minutes depending on the size of the evaporated and level of frost deposited on the evaporator. This method is now widely used in household refrigerator.

In dairy and food cold storages, water defrosting and hot gas defrosting are commonly employed for defrosting of evaporators.

16.3 Refrigeration Piping

It is necessary to connect all the component of the refrigeration plant using well designed pipelines. The quality of refrigerant and the flow rate of refrigerant are different in the system and accordingly size of pipelines are required in the system. For example, after the compressor the refrigerant is hot vapour to be supplied to condenser while liquid refrigerant flows in the pipeline between receiver and expansion valve. The distance of various components of the refrigeration system is one of the important considerations in selection of pipelines in order to minimize the pressure drop. The material of pipeline depends on the type of refrigerant used in the system. Small capacity systems using R-22 or R-134a requires copper pipelines to connect various components with flare fittings and brazing work. Copper pipelines can not be used in ammonia plant as ammonia is corrosive to copper and its alloys. Mild steel pipelines are commonly used in ammonia plant.

16.4 Balancing of Different Components of the System

Balancing of various components of the refrigeration plant is very important design aspect of the system. Misbalancing of any component may greatly affect the performance of the system. Under size expansion valve may result in to lower evaporating pressure and over size expansion valve supplies higher flow rate of refrigerant leading liquid pumping of refrigerant. The rate of the heat transfer plays very important role in the design of evaporator and condenser. It is necessary to pump the vapour from the evaporator at the rate it is produced in the evaporator. If compressor is not pumping the vapour produced in the evaporator, then pressure of the evaporator increases and it will not be possible to achieve desirable temperature of cold storage. It is also obvious that evaporation of refrigerant in the evaporator takes place depending on the load and therefore capacity control of compressor is one of the essential requirements in economical working of refrigeration plant.

Last modified: Friday, 19 October 2012, 5:25 AM