Airedale OptiChill FreeCool 500kW - 1365kW - Manual (Page 48)

OptiChill Freecool
Active Cabinet Exhaust
Chillers
48
Chillers
Technical Manual : 7197700 v1.2.0. 02_2013
Design Features & Information
Electronic expansion valves (EEV)
Thermostatic Expansion
Valves (TEV) Whilst offering versatile control at full design duty of the unit, TEVs do not automatically
optimise themselves to all operating conditions. Therefore, if the unit is operating at 40%
or 50% of full load, especially at a lower ambient temperature than that for which the
valve was sized, the conventional TEV must have the design head pressure available to
ensure good refrigerant control. Maintaining an artificially high condensing pressure is
normal in conventional systems.
Electronic Expansion
Valves (EEV)
Using an EEV allows for good refrigeration control whilst operating at part load and lower
ambient conditions with a reduced condensing pressure. By fitting an EEV and adjusting
the head pressure control setting an increase in the system EER (Energy Efficiency
Ratio) of up to 30% can typically be seen. The Mollier diagram shown below helps to
illustrate how this increase in efficiency is achieved.
EEV’s differ to normal thermostatic expansion valves in their ability to maintain control of
refrigerant flow and the suction superheat at reduced head pressures. The turn-down rate
of a typical EEV is superior to that of its thermostatic equivalent, such that a reduced
optimum condensing pressure can be maintained at low compressor load. However low
the load is on the compressor, from zero to 100%, there will not be a problem with turn
down, even down to 30% of the valves rated capacity.
B
u
b
b
le
D
e
w
1
1 3
4
2
1 Compression
2 Condensation
3 Expansion
4 Evaporation
Reduced Condensing
Pressure
Pres
s
ure
(Ba
ra)
Evaporating Pressure
In
c
re
a
s
e
in
C
o
o
lin
g
D
u
ty Enthalpy (kJ/kg)
R
e
d
u
c
tion
in
C
o
mpr
e
s
s
o
r
In
p
u
t
P
o
w
e
r
Key
Cooling Cycle @ 22°C ambient with a conventional TEV fitted.
Cooling cycle @ 22°C ambient, demonstrating a typical EEV condensing
temperature taking full advantage of lower ambient air temperatures (below 30°C).