Why Low Ambient when you have an Economizer

[BronYrAur]

Just a general question here. It seems like every specification for rooftop units calls out for both low ambient controls and an economizer. If you are bringing in cold outside air through your economizer, your compressor is off. So why do you need low ambient controls?

 

[imok2]

Lets say you are on full OSA and and the temperature is 65 degrees but the enthalpy is still too hi for comfort in the building so you need the compressor on to dehumidify
Say a Tech made a general check on the health of an existing airconditioning system and discovered that the indoor section had frozen up, and the cause of this was running the airconditioning when the outdoor temperature is below 65 degrees. The solution is to add a 'low ambient kit".
The low ambient kit works by slowing the fan to maintain head pressure so that the TEV (thermostatic expansion valve). will meter refrigerant properly. The fan speed is controlled by a temperature sensor connected to the copper tubing on the condenser. In other words too low a head pressure = a starving coil = SST below 32 degrees = frozen coil and possibly compressor damage due to slugging, lack of oil return ETC,ETC.

 

[Yorkman]

I don't think I could explain it better than imok2. One thing to be aware of many two stage cooling units have two compressors. But if the enthalpy of the osa is lower than the return air enthalpy, the enthalpy controller switches the first stage cooling to osa/mixed air control, and second stage cooling becomes the number one compressor. Many times the first stage condenser fan is cycled off a pressure switch and the second condenser fan is enabled on an ambient temperature switch. This is a rather antiquated an inexpensive set up, today many manufacters are using VSD on their condenser fans. Just my two cents

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI