Sight Glass,

[Abe]

We were "told" that a full sight glass devoid of bubbles is indicative of a full or charged system. How true is this? Recently I have had an evaporator which iced over very nicely, but the sight glass still showed up half full and bubbling. This was a new installation, mind you, a 1.5 hp hermetic compressor with an externally equalised valve, Danfoss with orifice number 2. Room temperature +1 Celcius and running on R404. Is the orifice perhaps too big , Number 2, or should it have been a number 1. I note in a previous thread that the drier position sgould always point downwards from the receiver.

[Prof Sporlan]

We were "told" that a full sight glass devoid of bubbles is indicative of a full or charged system.

Of course, we are considering a system using a TEV. Systems using a cap tube or restrictor as a metering device shouldn't have a sight glass installed...

A #2 orifice on a 1-1/2 hp R-404A compressor seems appropriate, though additional information would be necessary, i.e., evaporator temperature, condensing temperature, subcooling, and compressor capacity, to determine if it is the optimal orifice size.

Also, one can have a situation where you have a properly sized TEV, excessive flashing in the liquid line, the TEV adjustment turned all the way out, and the system appears to be operating normally. The problem here, of course, is the TEV isn't doing any modulating, other than simply acting like an orifice.

A few bubbles in the liquid line now and then aren't going to cause problems. But excessive flash gas in the liquid line is asking for trouble. So, what do we mean by "excessive"? Enough to displace a significant amount of liquid flow thru the TEV.

Answers like this is what makes servicing refrigeration systems interesting...

[Gary]

I tend to ignore the sightglass. It's misleading. To me it's just a moisture indicator. Instead I monitor subcooling and superheat. I find overcharged systems everywhere I go.

[Gary]

I monitor subcooling at the start of the liquid line (receiver outlet). My purpose in doing so is to avoid overcharging the system, which I define as exceeding 15F subcooling. Beyond this limit, liquid is beginning to back up into the condenser, driving up the head pressure.

On all systems, whether TEV or fixed orifice:

My primary goal is to flood the evaporator.

My secondary goal is to cool (but not flood) the compressor.

My tertiary goal (TEV system) is to provide surplus refrigerant.

In no case is the subcooling allowed to exceed 15F.

There are instances where extra subcooling is needed, but there are better ways to acheive this than driving up the head pressure.

[Gary]

I don't disagree with any of this, Marc. The balance (or imbalance) between subcooling and superheat, resulting in inability to reach these goals within my limits, indicates mismatched components and/or the exceptions you mentioned.

Would I raise head pressure to get the system working? Yes. But I would know that I was doing so, and why. And I would recommend alternatives to the customer.