Pressure Ratio (PR) as displayed on various screens in the chiller control is an expression of the ratio between the discharge pressure and suction pressure of the compressor.
It is calculated based on psia, not psig. That is, it based on the absolute pressures, not the gauge pressure. The pressures displayed on the chiller control/ and or compressor page are gauge pressures. In order to convert these to absolute pressures, it is necessary to add 14.7 psi to each of the psig readings. Then, divide the discharge reading by the suction reading.
In general, air cooled units typically run higher discharge pressures than water cooled units. Suction pressures will run the same between the two types of system. As a result of this the P.R. (pressure ratio) of water cooled units will run somewhat lower than the P.R. Of an air cooled unit.
The P.R. will vary as the outside ambient and the evaporator load varies. Higher evaporator load results in more load to the condenser and higher discharge pressures. Higher ambient temperature will also result in higher discharge pressures. Higher discharge pressures will result in higher P.R.´s.
Higher P.R.´s equal lower chiller efficiency. Lower P.R.´s equal higher chiller efficiency.
Typically, P.R. On air cooled units will run between 2.0 and 4.0. On water cooled units, the P.R. Will typically run between 1.8 and 3.2.
When routinely servicing a unit, it is a good idea to know, in general what range the P.R. Is for that unit. Higher than normal P.R.´s could be experienced in conjunction with higher condenser approach, and thus a dirty condenser.