1 HVACR317 – Refrigeration Commercial Defrost Sequence

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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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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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• 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.

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1 HVACR317 – Refrigeration Commercial Defrost Sequence