Sight Glass And Moisture Indicator
The heart of any air conditioning system is the refrigeration system. In an R-12 system, the sight glass should be clear and colorless.
A combination sight glass and moisture indicator is essential for easy field maintenance on any system, and is required on any field installed system unless some other means of checking the refrigerant charge is provided.
A sight glass in a convenient means of determining the refrigerant charge, showing bubbles when there is insufficient charge, and a solid clear glass when there is sufficient charge. however, the operator should bear in mind that under some circumstances even when the receiver outlet has a liquid seal, bubbles or flash gas may show in the sight glass. This may be due to a restriction or excessive presure drop in the receiver outlet valve, a partially plugged drier or strainer, or other restriction in the liquid line ahead of the sight glass. If the expansion valve feed is erratic or surging, the increased flow when the expansion valve is wide open can create sufficient pressure drop to cause flashing at the receiver outlet.
Another source of flashing in the sight glass may be rapid fluctuations in compressor discharge pressure. For example, in a temperature controlled room, the sudden opening of shutters or the cyclinng of a fan can easuly cause a reduction in the condensing temperature of 10 º F. to 15 º F. Any liquid in the receiver may then be at a temperature higher than the saturated temperature equivalent to the lower condensing pressure, and flashing will continue until the system has stabilized at the new condensing temperature.
While the sight glass can be a valuable aid in servicing a refrigeration or air conditioning system, a more positive liquid indicator is desirable, and the system performance must be carefully analyzed before placing full reliance on the sight glass as a positive indicator of the system charge.
Refrigerant Sight Glass?
Flashing And Clearing?
Over And Over And....?
Is the refrigerant sight glass flashing and clearing?
What do I mean?
Does the glass become clear, only to resume bubbling later?
Then after a few minutes, clear again?
Repeating this pattern over and over?
Here's a few suggestions.
The pattern could be the result of the expansion valve metering the refrigerant into the evaporator.
As the valve opens it increases the flow, out of the liquid line, causing the glass to flash.
When the valve closes, the flow is restricted and the refrigerant 'backs-up' in the liquid line. The glass becomes clear.
Normally this should not happen, if the refrigerant lines are properly sized and installed.
But if it does, the installation of a receiver is recommended.
The liquid receiver allows additional refrigerant to be added to the system. It stores any excess and releases it when the expansion valve opens.
Problem solved!
But what if the refrigeration equipment has a receiver, but still repeatedly flashes and clears?
Are you using fan cycle controls to maintain high side pressure?
Many many times I have watched a clear glass begin flashing after the condenser fan motors starts, only to clear again after the motor stops.
Why?
You have to visualize what's going on inside the condenser coil and/or receiver.
After the motor starts, the head pressure drops. This dropping pressure causes the refrigerant in the coil and/or receiver, to fall out of liquid form and evaporate.
The glass flashes.
When the motor stops, the pressure increases. This additional pressure, forces the gas back into liquid form.
The refrigerant sight glass clears.
What can be done?
Sometimes, adding more refrigerant helps.
You can also try adjusting the fan cycle control to cycle the fans more quickly, with less pressure difference between cut-in and cut-out settings.
But, when all else fails, I just live with it!
It's really not a big deal and doesn't affect the system performance.
Why sweat the little things?
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The materials provided on this website are provided for general information
purposes only and do not constitute legal or other professional advise
on any subject matter. Real-world-refrigeration.com does not accept any
responsibility for any loss which may arise from reliance on information
contained on this site.