Cold Storage Refrigeration System : An overview

cold storage plants are large warehouses equipped with a vapour compression system for refrigeration, traditionally using ammonia as the refrigerant. The system functions through a network consisting of a compressor, condenser, expansion valve and an evaporator (referred to as bunker coils when placed in a bunker). A brief description of the refrigeration process is outlined in the next few paragraphs.

The compressor sucks in saturated and dry ammonia vapour at design evaporating temperature and pressure and compresses the vapour into a pre-determined higher pressure, called the condensing pressure,with corresponding temperature. The actual temperature at the condensing pressure is considerably higher due to superheating it the compression process.

The low pressure liquid ammonia inside the bunker coils evaporates into vapour being sucked by the compressor and in the process cools down, absorbing heat from the surroundings, thus lowering the temperature of the store. The evaporator coils are kept flooded with low pressure and low temperature liquid ammonia with the help of an accumulator vessel inside which boiling or evaporation of liquid takes place. The evaporating ammonia is sucked by the compressor and the cycle is established.

If the performance of the refrigeration system is to be optimized, it is necessary to ensure that pipes of right dimensions and quality are used across the system. Apart from the intricate pipe network in condensers and evaporators, designated as 'heat exchangers', the vapour compression refrigeration system has to include interconnecting pipes for these heat exchangers.

Given below are bried descriptions about the design & application of pipes at various stages of the refrigeration process, along with their sizes & specifications.

Discharge pipes transmit the highly superheated ammonia vapour under high pressure, from the compressor to the condenser pipes. To ensure minimal pressure drop during transmission of the ammonia vapour, the pipe sizes should be determined based on the refrigeration tonnage of the system.

Pipe sizes normally used : 15mm NB to 200mm NB Black Heavy Plain Ended (BHPE) Steel Pipes.

The vitally important cooling process depends on these heat exchangers which incorporate a network of piping with flooding accumulator, served by expansion valve at the inlet and suction valve at the outlet.

Pipe Size normally used : 40mm NB Black Heavy Plain Ended (BHPE) Steel Pipes

The pipe surface is a function of the cooling to be achieved in the warehouse, and the heat transfer coefficient of pipes. For instance every 1 TR heat load for the flooded evaporator and a temperature difference of 30°F during the peak loading period, approx 240 rft of 40mm NB BHPE evaporator coils would be required under air agitation of ceiling fans.

Basis of pipe selection : Each set of bunker cooling coils normally comprises 40mm NB black heavy pipes and are usually between 50 to 60' long x 8 rows x 6 pipes high, using 40mm NB U-bend from 2 ft. long pipe with 5 ½" tube centers. The clearance between consecutive rows is kept about 6" to 8".

Fruits and vegetables other than potatoes are more susceptible to temperature and humidity, which must be closely controlled for freshness. However, we have appended a generic chart for vegetables and fruits.