18
DGT protection is required if the high
and low pressure switch settings
do not protect the compressor
against operating beyond its specific
application envelope. Please refer to
the examples on page 18, which show
(1) where DGT protection is required
(example 1) and (2) where it is not
(example 2).
Every heat pump must have a
discharge temperature protection
device. In reversible air-to-air and air-
to-water heat pumps the discharge
temperature must be monitored
during development tests by the
equipment manufacturer.
A flooded start occurs when there is
a large quantity of liquid refrigerant
in the compressor. Liquid finds its
way into a compressor by means of
off-cycle migration. Liquid floodback
occurs when liquid refrigerant returns
to a compressor while it is running.
Excessive liquid refrigerant in the
compressor will cause oil dilution,
washout of oil from the bearings, or
loss of oil from the sump. The crankcase
heater will not be effective if the
system design allows for uncontrolled
floodback to the compressor during
running or starting.
Performer® scroll compressors can
tolerate occasional flooded starts
as long as the system charge does
not exceed that given in the table in
the following section. Performer®
scrolls can also handle intermittent
floodback, but system design must
be such that repeated and excessive
floodback is not possible.
Off-cycle refrigerant migration occurs
when the compressor is located at
the coldest part of the installation,
when the system uses a bleed-type
expansion device, or if liquid can
migrate from the evaporator into
the compressor sump by gravity. If
the system charge exceeds the limit
shown in the table, a crankcase heater
must be used.
During operation, liquid floodback
may be detected by measuring either
the oil sump temperature, which
should be at least 18
°
F above the
saturated suction temperature, or the
discharge gas temperature, which
should be at least 54
°
F above the
saturated discharge temperature.
If at any time during operation the
oil sump temperature drops below
18
o
F above the saturated suction
temperature, or if discharge gas
temperature falls below 54
°
F above
the saturated discharge temperature,
the lubricant will become excessively
diluted and tests will have to be
performed in order to select an
appropriate method for compressor
protection. Repetitive liquid floodback
testing must be carried out under
TXV threshold operating conditions:
high pressure ratio and minimum
load, with the measurement of both
suction super heat and discharge gas
temperature.
liquid receiver
protection against flooded
starts and liquid floodback
A liquid receiver is strongly recom-
mended on split systems and remote
condenser systems with a total
refrigerant charge in excess of the
recommended maximum for the
compressor. Because of their long
refrigerant lines, such systems have
a relatively high system charge that
is hard to define with accuracy. Also,
these systems quite often tend to be
overcharged in the field. By installing
a liquid receiver, a pump-down cycle
can be introduced in order to safely
store the refrigerant charge during
off-cycles, greatly reducing the chance
of refrigerant migration back to the
compressor.
On unitary or close-coupled systems,
where the system refrigerant charge
is expected to be both correct and
definable, the entire system charge
may be stored in the condenser during
pump-down if all components have
been properly sized.
discharge gas temperature
protection (dgt)
sYsteM design RecoMMendations
Summary of Contents for Performer SM 084
Page 39: ......
