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View Subscription Details: Letter ( 07/12/2005 ) - HTML Format

Minimum allowable condensing pressure

Liquid supply

Sometimes refrigeration operators maintain artificially high condensing pressure for the sole purpose of moving liquid throughout the plant. Usually, 30- 40 psig pressure difference is required for moving liquid from one part of the plant to another part. The excessive pressure drops can be the result of poor design or undersizing of valves and liquid lines.

Only thermostatic expansion valves (TXV) require higher pressure difference. In industrial ammonia refrigeration we are using TXVs for direct expansion (DX) evaporator coils and for liquid injection oil cooling on rotary screw compressors. DX evaporator coils usually have application for high temperature refrigeration load. These coils are used very often for dock cooling and sometimes for high temperature coolers.

Many processes in industrial refrigeration require a balance between demand and supply. Regarding DX coils, we have to balance the refrigeration load (demand) and liquid supply of refrigerant to the coil.

TXVs usually have liquid supply from high pressure receiver. Lower condensing pressure will reduce capacity of these valves. All properly chosen TXVs have reserve capacity (around 20%) to adjust for an increase of refrigeration load and periods of lower condensing pressure. So the first step of condensing pressure reduction can be compensated by TXV reserve capacity.

Do we need the maximum refrigeration capacity of DX coils all year around? No. A major part of refrigeration load for high temperature coolers and docks is heat that is transferred from ambient air.

During periods of cool weather we can reduce capacity of DX coils by reducing condensing pressure and this reduced capacity will match reduced refrigeration load from ambient surrounding.

For example, during winter operation the refrigeration load for high temperature coolers and docks will be less than 50% of maximum refrigeration load. Therefore, we can reduce capacity of DX coils down to 50% by reducing condensing pressure lower than 100 psig. Thus we have a balance of demand and supply.

Sometimes condensing pressure should be increased for short period of time to make sure proper oil return to the compressors.

In next newsletter we will have a look at liquid injection oil cooling for screw compressors.

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