Variable fan speed controllers for condensers

Condenser fan speed controllers......


I have installed many, and if you have too you will likely be very happy with the results. The really sort out a lot of problems.
If installing , always make sure they are installed measuring latent pressures on the receiver or the liquid line as they work far more accurately rather than a simple head connection.
The advantage of using this type of head control , is that the fan will slow down and speed up to maintain preferred condenser saturation temperatures. This also eliminated condenser shock by low ambient temperatures and "cycling" fans that can cause a sudden low pressure at the condenser which will draw back or reverse flow in the liquid line pumping down the unit by starving the evaporator. (a liquid balance line usually fixes that problem)
Using a VS fan head control makes a system perform better by a constant pressure drop across the TX valve. This ensures maximum performance when setting superheat. It also keeps subcooling in the condenser constant.
And guess what else remains constant... The compression ratio. 
The only unfortunate downside to using one of these is that you lock the compression ratio all year by setting your saturation temps in the condenser to a permanent design temperature, This means that in low ambient weather , you are artificially re-designing the condenser TD to a higher value which creates a new tangent for a low ambient design which never existed (see the chart curves).
So your power consumption, port head temperature on the discharge valves remain the same, where as normally it would have lowered in lower ambients. Now this might cause some superheat deviation in the evaporator but low ambient... low problems really with load. It would have also offset more power consumption and lower discharge heat.
However folks. If you decide to use one, try not to oversize the condenser thinking this will solve your low ambient problems with a high ambient condenser design. These days of heat are far and few between. My chart has been sized for a 10K design with a 40.c ambient and a maximum condenser saturation of 50.c to show how far out it can get on an 18.c day..
These VS controls for the fan motor are best used where indoor loads are high in low ambients or for very critical humidity design.
Images of VS controllers from the products @ www.danfoss.com