RISING LIQUID LINES
If liquid refrigerant has to rise from the condenser or receiver to an expansion valve at a higher level, there will be a loss of static head, and the refrigerant may reach its boiling point and start to flash off. Under such circumstances, bubbles will show in the sight glass and will not be dispersed by adding more refrigerant to the system.
R404A condenses in a circuit at 34°C and is subcooled to 30°C before it leaves the condenser. How much liquid lift can be tolerated before bubbles appear in the liquid line?
Saturation pressure at 34°C = 15.69 bar Saturation pressure at 30°C = 14.18 bar Permissible pressure drop = 1.51 bar (151 000 Pa)
Specific mass of liquid = 1022 kg/m3
Possible loss in static head
9.81 x 1022 (where g = 9.81 m/s2 15.1 m approximately
Where a high lift cannot be avoided, the liquid must be subcooled enough to keep it liquid at the lower pressure. Subcooling can be accomplished by fitting a subcooling coil to the condenser, a watercooled subcooling coil, a suction-to — liquid heat exchanger before the lift, or a refrigerated subcooler.
To reduce the risk of these troubles, the condenser should always be higher than the evaporator, if this can be arranged.
The same effect will occur where the liquid line picks up heat on a horizontal run, where it may be in the same duct as hot pipes, or pass through a boilerhouse. If the liquid in the sight glass flashes even with the addition of refrigerant, the possibility of such extra heating should be investigated. To cure this, insulate the pipe.
Posted in Refrigeration and Air Conditioning