An Ultimate Guide to Secondary Refrigerant Systems

What is Secondary refrigerants?

Secondary refrigerants are usually liquids and are used to transfer heat from the substance being cooled to a heat exchanger where the heat is absorbed by a primary refrigerant. In an air conditioning system, it could be said that air is acting as a secondary refrigerant.

These fluids are cooled by a primary refrigerant then exposed to the source, being sensibly heated by that source, thus absorbing its energy, and the warmer fluid returned to the chiller, rejecting the heat to the primary refrigerant.

High-temperature applications such as air conditioning use chilled water as the secondary refrigerant while low-temperature applications use brines, glycols, and oils.

Large refrigeration plants often use secondary refrigerants to transport the cooling capacity from the plant room to the point of use.

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Benefits available from the use of secondary refrigerants:-

• The amount of primary refrigerant used can be minimised.
• Primary Refrigerant remains in the plant room under supervision (particularly important with ammonia).
• Pressure losses in the vapour compression cycle are minimised so increasing cycle efficiency - this gain is offset by the energy used to circulate the secondary refrigerant however.
• Hazardous primary refrigerants (e.g. NH3) can be used for applications such as air conditioning and food refrigeration.
• The mass of secondary refrigerant provides a holdover effect, hence no intermittent surging of thermal loads.
• Less Supervisory Staff needed as Refrigeration Plant can be switched on/off (automatically) even if a load occurs.
• Breakdowns can be repaired in the primary system while secondary cooling capability still maintained.

Disadvantages of Secondary Refrigerants:-

• When heat is absorbed into a secondary refrigerant, the fluid is sensibly heated rather than absorbed latent heat as is the case in a boiling process. As a result the fluid temperature rises across the length of the heat exchanger which in turn results in larger coolers being required for a given heat exchange duty.
• The mass flow rate of a secondary refrigerant is large in comparison to primary fluids because the specific heat capacity of liquids is small in comparison to their latent heat of vaporisation.

Types and Properties of Secondary Refrigerants

A good secondary refrigerant should have the following features:

• Non corrosive
• Inexpensive
• High specific heat
• Good heat transfer characteristics.
• Chemically stable
• Low viscosity

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