Danfoss VZH052 Manual page 36

Manage operating envelope
MOP (Max operating
pressure) control
Condensing pressure
control
Minimum pressure ratio
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In steady state, it is essential to prevent the
compressor running when evaporating T° is
higher than the specified envelope. Operating
the compressor higher than maximum
evaporating temperature will cause low viscosity
of lubricant and lead to high dilution. Eventually
the compressor will get damaged.
This protection can be achieved by using MOP
function on expansion device. MOP is a feature
added to EXV's (also to TXV's) that limit the
maximum suction pressure of the unit. The
customer would need to set this at the 27°C
In steady state, the condensing T° must be
maintained at a T° within envelope. This can be
done by using fan speed controller, or constant
pressure valve. Keep condensing pressure at a
minimum level is also important to maintain
the pressure differential across the thermostatic
expansion valve and prevent cut out on the LP
In steady state, the pressure ratio must be a
T° within envelope. 2 type of control can be
considered:
• Set the minimum condensing T° at 30°C (86°F)
together with MOP set at 27°C (80.6°F).
(80.6°F) limit we have on our VS operating
envelope.
Regardless of EXV or TXV, customer needs to
qualify the expansion device. Testing needs to be
done at both max and min operating conditions
to guarantee the valve closes enough on the min
and opens far enough on the max.
Complementary to MOP, the unit controller can
increase compressor speed to keep evaporating
T° lower than limit.
protection in cold ambient.
As an alternative the unit controller can increase
compressor speed to keep condensing T° lower
than limit.
• Unit controller monitors permanently
Condensing and Evaporating T°, and adjust
compressor speed or condensing T° to keep
running conditions within envelope.