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.