Page 470 - Mechanical Engineers' Handbook (Volume 4)

8 Defrost Methods 459

13 mm, with a length-to-diameter ratio from 3 to 20. Current applications for short-tube
1
restrictors are primarily in air conditioners and heat pumps.
Like a capillary tube, short-tube restrictors operate with choked or near-choked flow in
most applications. 22 The mass flow through the orifice is nearly independent of conditions
downstream of the orifice. The flow rate does vary with changes in the condenser subcooling
and pressure.
In applying short-tube restrictors, there are many similarities to capillary tubes. The size
of the system components and type of system must be considered when sizing this expansion
device. Sizing charts for the application of short-tube restrictors with R-22 can be found in
Ref. 23.

8 DEFROST METHODS
When refrigeration systems operate below 0 C for extended periods of time, frost can form
on the heat-transfer surfaces of the evaporator. As frost grows, it begins to block the airflow
passages and insulates the cold refrigerant from the warm/moist air that is being cooled by
the refrigeration system. With increasing blockage of the airflow passages, the evaporator
fan(s) are unable to maintain the design airflow through the evaporator. As airflow drops,
the capacity of the system decreases, and eventually reaches a point where the frost must
be removed. This is accomplished with a defrost cycle.
Several defrost methods are used with refrigeration systems: hot refrigerant gas, air, and
water. Each method can be used individually or in combination with the other.

8.1 Hot Refrigerant Gas Defrost

This method is the most common technique for defrosting commercial and industrial refrig-
eration systems. When the evaporator needs defrosting, hot gas from the discharge of the
compressor is diverted from the condenser to the evaporator by closing control valve 2 and
opening control valve 1 in Fig. 21. The hot gas increases the temperature of the evaporator
and melts the frost. Some of the hot vapor condenses to liquid during the process. A special
tank, such as an accumulator, can be used to protect the compressor from any liquid returning
to the compressor.
During defrost operation, the evaporator fans are turned off, which increases the evap-
orator coil temperature faster. With the fans off, the liquid condensate drains from the coil
faster and helps minimize the thermal load to the refrigerated space during defrost. In some
instances, electrical heaters are added to increase the speed of the defrost.
Defrost initiation is usually accomplished via demand defrost or time-initiated defrost.
Demand-defrost systems utilize a variable, such as pressure drop across the air side of the
evaporator or a set of temperature inputs, to determine if frost has built up enough on the
coil to require a defrost. Time-initiated defrost relies on a preset number of defrosts per day.
The number of defrosts and length of time of each defrost can be adjusted. Ideally, the
demand–defrost system provides the most efficient defrost controls on a system, because a
defrost is only initiated if the evaporator needs it. Termination of the defrost cycle is accom-
plished by either a fixed-time interval or a set temperature of the coil.

8.2 Air, Electric, and Water Defrost
If the refrigerated space operates above 0 C, then the air in the space can be used directly
to defrost the evaporator. Defrost is accomplished by continuing to operate the evaporator