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Sight Glass Use in Refrigeration Piping

Sight Glass Use in Refrigeration Piping

Sight Glass Use in Refrigeration Piping

(OP)
Can someone tell me what the sight glass is actually used for in HVAC DX cooling refrigeration piping?  Thanks.

RE: Sight Glass Use in Refrigeration Piping

A sight glass on the liquid line before the TXV would  let the technician know if any liquid flashing is occurring before the valve. This flashing could be from loss of subcooling  or too much static and/or friction pressure drop in the liquid line before it reaches the TXV.  There is a big difference between a bubbling sight glass and a low flow rate sight glass. If bubbles are entrained in the liquid, this is sign of a pressure drop causing liquid flashing, or an undercharge of refrigerant causing vapor and liquid to exit the receiver because of no subcooling  

RE: Sight Glass Use in Refrigeration Piping

Many sightglasses have a magic dot in the middle that changes color to indicate moisture in the system too.

Quote:

causing vapor and liquid to exit the receiver because of no subcooling

There's never any sub-cooling in a receiver.  You've got gas on top and liquid on the bottom.  By definition you have a saturated condition in a receiver.

 

RE: Sight Glass Use in Refrigeration Piping

MintJulep
Never say never but for the most part you are correct if the receiver is in a hot ambient, sub-cooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. Receiver bypasses are nothing more than a liquid line solenoid valve controlled by a thermostat, which will bypass liquid around the receiver to the liquid line.

RE: Sight Glass Use in Refrigeration Piping

A little off topic but...
When a system is operating, subcooled liquid enters and exits the receiver quicker than the pressure and temperature can achieve an equilibrium. The liquid/vapour interface is the only location where saturated conditions exist.  This is why when you measure the outlet temperature of a receiver it is almost always subcooled; in reality, most of the receiver is full of subcooled liquid if the unit is operating properly.  Some systems do bypass liquid to gain some efficiency, particularly if they have mechanical subcooling.

As imok2 said, a sightglass is used to determine whether a solid column of liquid is present in the liquid line.  This helps in troubleshooting many problems such as refrigerant charge, lack of subcooling, plugged filter/driers... etc
 

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