Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Page 42

TECHNICAL
INCORPORATING COLD CHAIN

capacity staging and/or a reduction in
liquid enthalpy. An analogy here would be
boiling water, the greater the rate of heat
input the greater the degree of boiling.
Greater boiling is merely a greater rate of
evaporation and tends to increase the
water’s specific volume effectively raising
the water’s wet level. The water’s wet level
then of course drops with reduced boiling.
If a TEV was used to maintain a wet level
then with reduced boiling, more refrigerant
mass would be required to maintain that
wet level.
By floating the head pressure i.e.
allowing the head pressure to drop with
falling ambient temperatures, we affect
a reduction in both compressor capacity
staging and liquid enthalpy. The resulting
increased evaporator operating charge,
in most cases, is greater than any increase
in liquid line mass from reduced condenser
vapour density.
Overall system refrigerant mass
redistribution by reduced evaporator
refrigerant quality most often affects a
reduction in liquid line mass to the point
where expansion devices are liquid
starved. Any expansion valve, EEV or TEV,
has its capacity considerably reduced
when starved of refrigerant, when not
being fed by a solid liquid seal.

inclusion of a control characteristic to cycle
condenser fans in the event of low load
and/or low ambient valve liquid starvation
occurring as a result of liquid line vapour.
These manufacturers hereby recognise
the system refrigerant redistribution, which
occurs with floating heads.
Sub-cooling to eliminate liquid line vapour
Liquid saturated ∆T/∆P ratios increase
considerably at the lower 20oC conditions

such that for any given liquid line static or
component pressure drop an increase in subcool by some 56% is required. Even with the
reduced mass flows resulting from incr