sight glass installation

[MarkD]

Hi.I am new to this website and in browsing it I find it to be very nicely done.
My question today is,I think I learned in taking classes that a sight glass only should be installed on a system with a txv valve.A co-worker asked why,and I could not give him a reason.Is this the case, and why or why not? Thank you for any input.

[Sandro Baptista]

Hi MarkD,

What kind of sight glass? Liquid and Moisture indicator, which is connected in the liquid line? Or is a sight glass to indicate the level of the liquid receiver, for instead?

[taz24]

Hi.I am new to this website and in browsing it I find it to be very nicely done.
My question today is,I think I learned in taking classes that a sight glass only should be installed on a system with a txv valve.A co-worker asked why,and I could not give him a reason.Is this the case, and why or why not? Thank you for any input.

Hi Mark.

The sight glass is just for the TEV (txv) and does not need fitting with a cap tube.

The sight glass shows if pure liquid is delivered to a TEV, the valve opens and closes so there will be a fluctuation in the level of liquid delivered to the valve.

With a cap tube there is no fluctuation because the system is criticaly charged. So when a cap tube system is down to temp the system will be running with full liquid delivered to the start of the cap tube, but if the system was under extreme load there would appear not to be enough liquid in the system and the liquid would not be full bore to the cap tube (there would be vapour bubbles aswell).

So with a cap tube, the sight glass would appear to bubble in all but perfect running conditions.

Hope this helps

taz

.

[MarkD]

Thank you for the fast response.Your explanation makes good sense.I do appreciate it,and will pass it along.Again,thank you very much.

[Chef]

The sight glass is just for the TEV (txv) and does not need fitting with a cap tube.

There is absolutely no reason why a sight glass should not be fitted and many reasons why it should. Any tool that can help diagnose a system is useful. For diagnosis purposes a sight glass in a cap tube system is more useful than in a TEV system.

In the TEV both subcooling and superheat can be vital indicators but thats not the case in cap systems as subcooling is non existent at operating temps but a view of the liquid/vapour would give an invaluable insight.

With a cap tube there is no fluctuation because the system is criticaly charged. So when a cap tube system is down to temp the system will be running with full liquid delivered to the start of the cap tube, but if the system was under extreme load there would appear not to be enough liquid in the system and the liquid would not be full bore to the cap tube (there would be vapour bubbles aswell).

So with a cap tube, the sight glass would appear to bubble in all but perfect running conditions.

When the cap tube system is down to temp it will have quite a lot of bubbles in the liquid.

If the system is under extreme load then the evap temp will rise and so will the pressure, this in course will increase the compressor mass flow rate and excess liquid will build up in the condenser - the result will be ALL liquid at the cap tube entrance and not bubbles as you suggested, also it will have an increased flow rate because the condenser pressure has increased.

Cap tube systems do indeed have fluctuations in flow dependant on load and ambients - just like a TEV system does.

MarkD I thank you for suggesting this and think the idea of a sight glass in a cap system is brilliant - I shall be fitting one to a system as soon as it arrives.

In answer to your original post - your co-worker is correct and it can be fitted to both systems. However all knowledge on glass appearance has been published for TEV systems only and so a new set guidelines will be needed to interpret glass appearance for a cap system.

Chef