I recently had a suggestion for a post on low ambient cooling. Here is what was asked: running DX cooling in times like today, a building had all the heat of a 82* day but out-door ambient has fallen to 50*. And a common service call is a unit that locked out on a freeze stat over night! Any comments? He also said: great post on being a student for life!! I totally agree!!
Most HVACR companies usually only think of the need for low ambient controls for the really cold weather (in other words — winter). If you look at the Application Limitations that are published for the specific product you are working on, it will usually show something like:
As you can see, this manufacturer does not recommend running the unit below 50*F unless a low ambient kit is installed on the unit. So, this is what the basis of this posting will be about — low ambient cooling when you are below the manufacturers published Application Limitations. This can vary depenting on the type of unit or manufacturer so you need to check out what the manufacturer’s limitations are for the specific equipment you are working on.
On light commercial (non-process cooling) applications, one solution would be to install an Economizer on a packaged roof top unit. This would allow cool outside air to be used to cool the space below 50 degrees. The compressors or mechanical cooling could be “locked out” through a thermostat or “klixon” type switch so it cannot run below a pre-set temperature. Economizers on packaged equipment are a great and “economical” way to provide cooling below the applicaion limitiations of the equipment.
If mechanical cooling is needed and the economizer could not keep up or the application of an econoimizer could not be used due to the type of equipment or location, the next suggestion would be to add some sort of low ambient “fan cycling” control to the unit to maintain head pressure/suction pressure. Again, on light commercial (non-process cooling applications) or the occasional residential customer who wants cooling any time they feel like it, this is probably the best suggestion.
There are a couple of types of controllers — the most basic and simplest is cycling the fan on a pressure switch. These are designed to regulate condenser liquid pressure at low outdoor ambient temperatures by controlling the airflow over the condenser. The low ambient pressure kit will energize the outdoor motor when the liquid line pressure reaches 300 +/-10 PSIG for R-22 models and 360 +/-10 PSIG for R-410A models. It will stop the motor when the pressure falls below 150 +/- PSIG for R-22 models and 240 +/- PSIG for R-410A models thus keeping the pressures high enough so the coil does not “freeze”. This type is fine for the occasional need for low ambient cooling. If the space is going to regularly need low ambient cooling, then a true fan cycler type control is the best way to go.
This type of control monitors the liquid line temperature (which of course corresponds to pressure) and then modulates the speed of the condenser fan(s) to maintain head pressure. The main caution with these are making sure you set it up for the correct type of motor — ball bearing or sleeve bearing — so you don’t start having motor failures. As the unit is running, there is a sensor (resistor) strapped to the liquid line and insulated and then, based on the temperature it is reading, allows the fan to speed up or slow down (or if too low — turn off the fan eventually) to maintain a very consistent head pressure. Most of these controls will work with both the ball bearing or sleeve bearing motors and allows you to select which one your application has.
This is a very “simple” description of low ambient cooling but, hopefully, you can see how important this is to proper operation of cooling equipment. One other very important thing to mention is, if you are going to do low ambient cooling on any piece of equipment, be sure it has a functioning crank case heater on the compressor. Without a functioning crank case heater, the oil in the compressor will absorb liquid refrigerant (since one of the principles of refrigerants is that they always migrate to the coldest spot and the compressor in winter/early spring is that spot) and when the unit tries to start, the liquid will begin to boil off and carry a lot of the oil out of the compressor which could lead to early compressor failure.
So, hopefully this answers the original question/suggestion proposed by a dealer/follower of this blog. By the way, he also said: “great post on being a student for life!! I totally agree!!”
For information on specific type low ambient controllers you can go to your distributor or you can check one out at: http://www.hoffmancontrols.com/pages/816-10DH.htm