Condenser Control

Condenser Control: In a chilling circuit, the high-pressure gas from the compressor needs to be turned into a high-pressure liquid so that it can be expanded back to a gas, absorbing heat as it does it. This is chilling.

The device that turns the gas back to a liquid is appropriately named a condenser. A condenser is a heat exchanger with a large surface area and usually uses ambient air to cool it via electrically powered fans. The air takes away the heat produced (latent heat) when the gas is condensed. The variable that is controlled in the condenser is head pressure. For a given refrigerant, the head pressure and temperature are inextricably linked i.e. a certain head pressure is at a certain temperature when at the condensing point. Condenser fans control the head pressure. The harder they operate, the lower the head pressure.

This is where inverters can help in the control of the fans. Fan power consumption follow the ‘cube law’ i.e. the power consumed by a fan is proportional to the cube of the shaft speed. So, if you slow a fan down by just 20% its power consumption will drop to (80%)³ of its original or 51.2%, whilst still giving 80% of the airflow. From this, it is clear that if you can run fans slower for long then there are significant energy savings.

Let us take a worked example: a fan is being controlled on-off and is on for 50% of the time and this fan consumed 10kW at full speed, the cost per kWh is 12p.

This fan costs 10kW x 50% x 12p x 8760h/yr = £5256 per annum

If we use an inverter to control this fan and run at 50% speed all the time, giving the same airflow,

this fan would cost 10kW x (50%)³ x 12p x 8760h/yr = £1314 per annum.

This is a saving of 75% power or in this case £3942 per year. For reference, an 11kW inverter costs around £800.

There are other benefits of using an inverter for condenser control. Where appropriate, during cooler weather, the system head pressure can be lowered and this significantly reduces the work done by the compressor. The compressor is the large power consumer (often 100s of kW). So, by measuring ambient temperature, we can automatically work the condenser fans a little harder and take a significant load off the compressor. This can save a significant amount of energy.

On – off control of condenser fans naturally lead to ‘cycling’ of the head pressure since the fan is either flat out or off. Using an inverter, it will always be running at the appropriate speed keeping the head pressure constant allowing the chilling system smooth operation.

In summary, the benefits of using an inverter for condenser control are:

Reduced condenser fan power consumption.
Reduced chiller compressor power consumption in cooler weather (where appropriate).
Smooth control of head pressure.
Reduced fan noise.
Longer fan life (less starts and slower running).

Condenser fans should always be inverter controlled!

For specific advice on using inverters for condenser control call our engineers on 0115 944 1036 or email [email protected]