A Look at Hot Gas Defrost
Hot gas enters (and heats) the drain pan and exits through
the pan check valve (PCK), then enters the coil via the suction
line. The thermostat cycles the HGSV so that only condensed
ne of the most common applications of ammonia re- liquid exits the coil (via the liquid line). The liquid check valve
frigeration systems is to produce and maintain space (LCK) prevents reverse flow, so the liquid is forced to go through
temperatures by circulating air through a refrigerated the defrost relief regulator (DRR) and into the suction line downcoil. If the temperature of the ammonia in the coil is stream of the SSV.
below 32°F (0°C), water in the air freezes and accumuThe DRR is set to relieve at 75 psig (618 kPa), keeping the coil
lates on the coil. The ice blocks airflow and acts as an insulator, temperature at approximately 50°F (10°C). This prevents excespenalizing coil performance. For efficient performance, the coil sive fogging of the defrost melt. The hot gas condenses at
must be defrosted periodically. The defrost cycle is a necessary approximately three times the rate that it evaporates during the
and important part of the design of the refrigeration system.
cooling mode. Each pound produces approximately 550 Btu
Over the years, various defrost methods have been used. One (580 kJ) of heat, or enough to melt 3.8 lbs (1.7 kg) of ice. Since
of the first methods was to arrange the coil in such a manner that the coil is heated from the inside, the ice melts rapidly. A 30-ton
it could be isolated from the cold room. Warm air was circulated (105 kW) coil with an ice coating of 1,200 lbs (544 kg) is cleaned
over it until the ice melted. Another method is to run water over in a little more than 10 minutes.
the coil. Careful design of the water lines
3. Equalizing period (two to five minBSV
into and out of the cold room prevents
utes).
Suction
freezing of the defrost water. Electric
• HGSV and SHGSV are closed.
SSV
TSTAT
heater rods inserted into formed holes
• Bleed solenoid valve (BSV) opens.
DRR
PCK
through aluminum fins work effectively,
This period is important for large
and this type is common for halocarunits.
The coil is allowed to equalize
Liquid
HEV
LCK LSV
bon systems. All of these have been
slowly
to suction pressure. This allows
Hot Gas
HGSV
used for ammonia coils, but the most
the SSV to close with no pressure difSHGSV
common method is hot gas from the
ferential and prevents “slamming.”
compressor discharge.
Figure 1: An ammonia air unit designed for Also, it prevents liquid “slugs” in the
Hot gas defrost is simple and effec- liquid recirculation with hot gas defrost.
suction line from being accelerated by
tive, it removes ice rapidly, and is relahigh-velocity gas. This also is a “quiet
tively inexpensive to install. However, the control valves selec- time” for the last bit of water to drain from the coil.
tion and the sequence of operation must be correct for reliable 4. Fan delay period (one to three minutes).
and efficient defrosts. Let’s look at a typical hot gas defrost
• SSV opens and BSV is closed.
cycle design.
Any water remaining on the coil is re-frozen. This prevents
Figure 1 is a diagram of an ammonia air unit designed for water blow-over when the fan comes on.
liquid recirculation with hot gas defrost. When the liquid sole- 5. Return to refrigeration mode.
noid valve (LSV) opens, cold ammonia liquid passes through the
• Fans are energized (and may be controlled by the space
expansion (or balancing) valve (HEV) and enters the coil. A com- temperature sensor).
bination of gas and excess liquid exit the coil through the suction
• LSV opens and is controlled by space temperature sensor.
stop valve (SSV) and return to the recirculation vessel. When
The defrost cycle can be initiated by time of day (TOD), accuthe defrost cycle is initiated, the following sequence occurs:
mulated run time (RT), or frost detection (FD). For systems with
1. Pump-out period (10 to 30 minutes).
many evaporators, a common means of initiation is TOD or a com• LSV closes.
bination of TOD and RT. It may appear that RT would be the most
• Unit fans remain on.
efficient method. However, if all units operate on RT, they may tend
The liquid supply is stopped and the fans circulate room air to ice up at the same time, and have to get “in line” for defrost,
over the coil to evaporate the liquid charge. This is done for which negates the advantages. The optimum time for each of the
safety reasons and to provide quick defrost.
four periods—pump-out, hot gas, equalizing, and fan delay— is a
2. Hot gas period (five to 30 minutes).
function of physical size and coil loading, which changes with the
• Fans stop.
seasons and traffic in the space. The period times and time of
• SSV closes.
defrost initiation should be reviewed periodically and adjusted
• Soft hot gas solenoid valve (SHGSV) opens for a timed inter- when conditions change.
val. This slowly pressurizes the coil—again, for safety reasons.
• After the SHGSV pressurizing interval the hot gas solenoid Ellis Norton is senior vice president of The Facility Group in
valve (HGSV) opens.
Smyrna, Ga.
By Ellis Norton
Member ASHRAE
O
88
ASHRAE Journal
w w w. a s h r a e j o u r n a l . o r g
October 2000