Components of a Cold Storage

i. Cold Room

It is an insulated room, cabin or compartment with an insulated door for loading and unloading the products to be stored. Size of the cold room varies considerably depending on the variety and amount of products to be stored. There may be more than one cold room in a dairy plant operating at different temperatures. Generally separate cold rooms are designed for different dairy products of varying temperature requirements.

ii. Cooling Coil/Diffuser

Cooling coil. also called as evaporator/ evaporator coil, is the only component of refrigeration machine which is fixed inside the cold room. As the name suggests,it cools the products kept inside the cold room. Depending on the size of cold room/store, one or more cooling coil (evaporator coil) with air diffusing system is fixed inside the cold store. In air diffusing system a fan or blower pulls the air over the cooling coil and diffuses this cold air in whole space.It is fixed at heat height on the suitable wall of cold store. However, height may vary as per the requirement. Maximum distance is maintained between the cooling coil and cold store door. Sometimes air ducts hanging from roof may also be used for proper distribution of cold air in the whole space. The cooling coils may be of two types as ‘direct expansion evaporator coils’ or ‘chilled water coils’.

a) Direct Expansion Cooling Coil

In this type of cooling coil, the refrigerant is directly expanded and evaporated.That’s why it is also called as evaporator. During evaporation, the refrigerant absorbs heat of the surrounding air and produce cooling effect. It is more efficient and cheap method of refrigeration. But the drawback is that any leakage of refrigerant like ammonia can spoil the food products stored.

b) Chilled Water Coil

In this system chilled water from Ice bank is circulated through the cooling coil. It is preferred where the purity of air is highly important i.e. where there is a risk of spoilage of all the foodstuffs because of the presence of traces of refrigerant in cold air due to any leakage.

iii. Compressor Room

In a large sized cold storage of a dairy plant, a separate room for compressors is provided. This room is situated as near to cold room as possible to minimize the piping and insulation cost and also to enhance the overall performance of the plant.Generally there are more than one reciprocating type compressor. Separate compressors are there for cold rooms working at different temperatures because of difference in suction pressure. However, the discharge pressure and temperature of all the compressors is same. So the discharge line of all the compressors is merged into a single line and connected to condenser.

iv. Condenser and Receiver

As the discharge temperature of the all the compressors is same, a single condensing unit is employed. Either a water- cooled condenser with cooling tower or an evaporative condenser is used. The condenser is situated in an open space nearest to condenser room/cold room to minimize the piping cost. The receiver is also situated near to condenser to collect the compressed and condensed refrigerant gas and then to supply it to cooling coil through control/ expansion valves.

v. Expansion/Control Valves

Separate expansion valves and control valves are fixed for different cold rooms.These are fixed on the outside of that wall of cold room on which the cooling coil is fixed. In this way these are connected to cooling coil across the wall. With dry expansion evaporator, thermostatic expansion valve is used and with flooded evaporator, float control valve is used.

vi. Refrigeration Piping and Refrigerant

All the major components of refrigeration system are connected to each other through pipes. Steel pipes are used with ammonia as a refrigerant and copper pipes are used with freon refrigerants. The pipe connecting the cooling coil with compressor,also called as suction pipe, carry the low temperature refrigerant gas. So insulation is provided on it. All other pipes are bare. As generally the compressor, condenserLayout and receiver are situated at some distance away from the cold room, the length of connecting pipes carrying refrigerant is also very large. Because of this, the quantity of refrigerant is large. So, the cost of refrigerant is important, as costly refrigerant cannot be used in large quantity. Generally ammonia gas is most accepted refrigerant in the cold storage of a dairy plant. Ammonia is cheap, easily available and also very good refrigerant in performance.

vii. Safety Devices

In addition to the major components, many safety devices are also installed in a cold storage plant. Some of the safety devices and their function are as discussed below:

1. Low pressure cut-out

2. High pressure cut-out

3. Temperature Indicator/Controller

(a) Low Pressure Cut-out

In a refrigeration plant, when sometime the cooling load decreases or evaporator becomes ineffective due to some other reason, the evaporation of refrigerant liquid in the evaporator coil decreases. Due to decrease in evaporation of refrigerant, the pressure of refrigerant vapours also decrease in the suction side of compressor.This low pressure also called high vacuum may cause damage to the equipment,i.e., any pipe or joint can burst. It also puts heavy load on compressor. This high vacuum is avoided by the use of low-pressure cut-out, which switch off the compressor automatically as soon as the pressure in evaporator decreases below a certain value.

(b) High Pressure Cut-out

In a refrigeration plant, the condenser effectiveness may decrease sometimes due to failure of cooling water supply to condenser or by deposit of dust or sludge etc. on its surface or by the presence of air with refrigerant vapour in the condenser.When condenser effectiveness decreases, the vapours are less able to condense and due to more vapours continuously coming from compressor, their pressure may increase than the normal value. This high pressure is dangerous to the equipment i.e. compressor and condenser. Hence to avoid high pressure, a high-pressure cutout is connected to discharge line, which switch off the compressor automatically in case the pressure in condenser exceeds to an upper limit.

(c) Temperature Controller

In a cold storage, once the food/dairy products stored come at a low storage temperature, there is no more heat to be extracted from the foodstuffs kept in cold storage. However, some heat may come inside from the hot surrounding but its rate is very less due to effective insulation all around the cold room. It is also very less as compared to heat extraction rate of the refrigeration plant. Thus, there is no further need to keep on running the refrigeration plant until the inside temperature again increases above a certain limit. So, for that purpose some automatic methodis required which switch on and switch off the compressor depending on the temperature inside the cold room. An electronic temperature controller is generally used for this purpose. When the refrigeration plant is operated, it senses the temperature of cold room continuously through a probe hanged inside. As soon as the required low temperature is reached inside the cold room, it automatically activates a relay switch, which switches off the compressor. This temperature, at which the compressor is switched off, is called cut-out temperature. Now again,when inside temperature starts increasing due to inflow of some heat from surrounding and also through operating the door of cold room, and when it crosses an upper limit, temperature controller starts the compressor. This upper limit temperature is called cut-in temperature. In this way, temperature controller is the most important control device in a refrigeration plant.

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