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HVACR317 – Refrigeration
Commercial Defrost Sequence
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Diagram 1
CFM
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Description of Diagram 1
• The first diagram represents a typical self-contained reach-in box.
• It is a low-temp system using hot gas defrost.
• The compressor is CSIR. • The LAC (low ambient control) would
be used if the condensing unit is installed outdoors.
• The run cap is optional.
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Diagram 2
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Description of Diagram 2
• Like any system, fan, fan, and compressor will sequence in parallel.
• The start cap will provide the required torque to the start winding.
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Diagram 3
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Description of Diagram 3
• The BEMF energizes the relay and drops out the start cap.
• The system will cycle normally on the (Honeywell/White Rogers/Johnson Controls) remote bulb thermostat.
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Diagram 4
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Description of Diagram 4
• When the box temperature reaches 0 - 10°F (typical setting), the thermostat opens. EFM and TM continue to run.
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Diagram 5
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Diagram 6
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Description of Diagram 5 & 6
• When the timer switches to defrost, the compressor stays on and HGS energizes.
• The EFM is de-energized in defrost.
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Diagram 7
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Diagram 8
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Diagram 9
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Description of Diagram 9
• This system is also low temperature, but using electric defrost.
• This type of system has an evaporator fan “delay” sequence.
• It uses two “DTT” type controls.• It also has the ability to disengage the
timer out of defrost.• Step by step sequence is coming up.
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Diagram 10
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Description of Diagram 10
• Just like the last system: fan, fan, and compressor are running.
• The DTC timer is line-to-line (like the last system).
• A different type (or model) DTC is used; notice the extra terminals on the timer.
• The device between 3 and X is a solenoid (more on that later).
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Diagram 11
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Description of Diagram 11
• The only path is through the heater and “limit”; all else is off.
• The limit “act as as” a DTT (like the domestic units). But it has a higher temperature (cut-out) rating than the domestic types. It is basically a high temperature safety. The “actual” DTT is the SPDT one just below the “limit.”
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Diagram 12
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Description of Diagram 12
• When the SPDT DTT senses that the evaporator coil is warm enough, it will switch up to the top circuit, completing a path to the “clutch solenoid” on the DTC.
• The solenoid will swap the contacts back into refrigeration mode (no waiting).
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Diagram 13
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Description of Diagram
• The evaporator fan has not come on yet due to the warm evaporator coil.
• When the DTT cools back down, the switch will drop back in and the evaporator fan will restart.
• The purpose of this control (and sequence) is to prevent a “hot pull-down” situation after each defrost cycle.
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Diagram 14Hot pull-down example: Presently, the system is running normal (R134A). The system pressures will generally run 5 to 20 psig.
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Diagram 15When the system goes into defrost, the evaporator coil heats up and thepressures are basically equalized.
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Diagram 16When defrost is terminated, the extreme heat load on the coil could cause an overload on the electrical circuit and trip a breaker.
Delaying the EFM is one way to keep the pressures down at initial startup.
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Diagram 17Evaporator gets cold, EFM goes on.
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Diagram 18
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Description of Diagram 18
• Like all (domestic OR commercial) automatic defrost systems, this defrost circuit can be terminated by time and / or temperature.
• The main advantage in using this type of timer is the refrigeration mode can be instantly brought back on if the DTT duration lasted just a few minutes.
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Diagram 19
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Description of Diagram 19
• If the SPDT DTT should fail, the limit will provide a lock-out condition if an overheating condition occurs.
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Diagram 20Electric defrost reminders: the element is either on or below the evaporator coil. DTT and limit are both located on evaporator coil.
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Diagram 21
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Description of Diagram 21
• This system is a medium temperature application (EFM is line-to-line).
• The defrost cycle simply shuts down the compressor circuit (sometimes done in the late evening hours). A simple timer is used.
• The LP switch is used as a temperature control (not a great way to go, but it is common).
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Diagram 22
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Description of Diagram 22
• Using an LP switch as a thermostat:− Set the LP switch to its recommended
settings (C.I. and C.O.); attach gauges.− Place a thermometer in the box and
close the door.− When the box reaches the desired
temperature (e.g., 35°F), set the C.O. of the LP switch to the low side pressure shown on the gauge.
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Diagram 23
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Description of Diagram 23
• High or medium temperature system • Uses off cycle defrost. • “Off cycle” defrost can be set up by
adjusting the LP switch differential higher than normal.
• The 30 psig differential allows sufficient time for defrost.