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Home Ice Breaker: Fan Cycling Controls

REFRIGERATION / CONDENSING UNITS

Ice Breaker: Fan Cycling ControlsByJ oe Marchese

Many refrigeration systems are installed using an outdoor air-cooledcondenser. When these systems are installed in a climate where the

outdoor temperature drops below 60F, some means of preventing the

condensing pressure from dropping too low must be incorporated into its

design.

Too low of a condensing

pressure results in too low

of a pressure drop across

the systems TXV, which

leads to reduced refrigerant

flow through the TXV and a

starved evaporator. This is not good for overall system operation.

Today there are several methods used to prevent a systems condensing

pressure from falling too low. On systems with multiple condenser fans, one

popular method is to cycle them on and off as needed.

Note: This method should not be used on single condenser fan

applications. It leads to rapid cycling of the fan motor, wide fluctuations in

condensing pressures, and unstable operation.

Cycled Operation

The fans are typically cycled on and off using a close on rise pressure

control. As the condensing pressure falls, fans are cycled off. As the

condensing pressure increases, fans are cycled back on.

Their operation needs to be staged so that not all of them are cycled off or

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RECENT ARTICLES BY JOE MARCHESE

on at the same time. Fans should be cycled individually; however, if the

condenser has two rows of fans, each pair can be cycled together. Also,

the fan(s) closest to the header of the condenser should not be cycled off

while the compressor is running. This will prevent wide condenser

temperature fluctuations that cause excessive expansion, contraction, and

metal fatigue. For example, in the sketch, fans 1 and 2 are not cycled; 3

and 4, 5 and 6, and 7 and 8 can be cycled together or individually.

Instead of using a pressure switch to cycle the fans, some systems use a

temperature switch. As the outdoor ambient temperature drops, the fans

are cycled off. As the outdoor ambient temperature increases, the fans are

cycled back on.

Using a pressure switch is more appropriate where the load on the

condenser will vary due to multiple compressor operation, stages of

unloading, or systems incorporating heat reclaim or hot gas defrost.

Control Adjustments

Proper adjustment of pressure or temperature controls is important to the

overall system operation. The controls should be adjusted so the

condensing pressure doesnt fall too low or rise too high; ideally there

should be no wide fluctuations in the systems condensing pressure as the

fans are cycled on and off.

It is best to follow the manufacturers recommended control settings. As a

rule of thumb, fans can be cycled off as the condensing temperature drops

below 90 and cycled back on as the condensing temperature rises to 110.

The controls differential setting needs to be wide enough to prevent rapid

cycling of the fans which will lead to its premature failure.

Fan cycling in excess of three minutes is considered excessive. For

pressure controls it is not recommended to set the differential less than 35

psig and for temperature controls not less than 5.

Lets look at a refrigeration system designed with an outdoor air-cooled

condenser that has eight fans (two rows of four), a 20 TD, and is using

HFC-134a as its refrigerant. In this example we will use three pressure

switches to cycle the fans one switch for each pair of fans.

So, fans 1-2 are not cycled off; 3-4 will have a cut-in pressure of 147 psig, 5-6 a cut-in pressure of155 psig, and fans 7-8 a cut-in pressure of 160 psig. Each of these controls will have a differential

setting not less than 35 psig, and may need to be increased to prevent rapid cycling (as long as it

does not cause the condensing pressure to drop too low).

This method of head pressure control works well in mild climates. In colder climates it may not be

adequate to maintain the systems condensing pressure. In such a case, this method can be used in

conjunction with the flooded condensing to prevent the systems condensing pressure from falling too

low.

Publication date: 09/05/2011

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J oe Marchese is an author and instructor and former HVACR contractor. He can be reached [email protected].

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Ice Breaker: Hydrocarbon Refrigerants

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icebreaker_ fan cycling controls _ 2011-09-05 _ achrnews.pdf

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