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reggie
11-05-2002, 01:05 AM
Prof since you put the fundamentals of superheat to sleep in a blink how about your assessment of sub cooling. Do you rely on the gauge pressure/temperature of the condenser coil?
What about if the liquid line has a receiver in it and is there a rule of thumb pressure drop over it or the pipe run?

Prof Sporlan
11-05-2002, 06:06 PM
Do you rely on the gauge pressure/temperature of the condenser coil?
Absolutely. :)


What about if the liquid line has a receiver in it and is there a rule of thumb pressure drop over it or the pipe run?
You have a similar situation as with reading evaporator pressure at the compressor, and measuring temperature at the evaporator outlet.

If you are measuring high side pressure at the compressor outlet, you have discharge line and condenser coil pressure drop to consider, in addition to the liquid line if you are measuring subcooling ahead of the TEV. A ballpark number you can use for the sum of discharge line and condenser coil pressure drop is 5 psi, though this number can go significantly higher.

For the liquid line, a 2 psi pressure drop should suffice, receiver included, though you need to consider the vertical rise or drop of the liquid line in your calculation. For every 1 foot of liquid line vertical rise, you lose approx 0.5 psi in liquid line pressure (0.25 psi for R-717). So if you have a 20 foot rise, figure you are going to have a 10 psi pressure drop in the rise alone. Conversely, with vertical liquid line drops, you gain pressure by the same amount.

Of course, you're better off if you can measure liquid line pressure directly. :)

From a practical standpoint, if the system has a sight glass, you can use it to determine if there is a problem with insufficient subcooling, though it won't tell you if you have excessive subcooling.

reggie
11-05-2002, 11:37 PM
By your explanation it seems that we are almost duplicating our calculation of superheat in reverse. The only difference being sub cooling is a reduction of saturation press/temp whereas superheat is an increase in sat press/temp.

You mentioned sight glass but can you recall Garys disdain outlook on it?
He claims it is virtually useless and swears by the level of subcooling? Whats your view and is there a td rule of thumb that will obtain us a perfect charge?

Dan
12-05-2002, 02:26 AM
By your explanation it seems that we are almost duplicating our
calculation of superheat in reverse.

That is one of the most seemingly understandable yet incomprehensible statements I have seen in a while. I had hoped that just copying and pasting it would help me to understand it better, but thus far, I have made no progress.:)

reggie
12-05-2002, 06:06 AM
lol.
Bad English im sure. What i meant was we are duplicating the method of using gauge pressure and pipe temperature to record our readings. The reverse i meant was that with subcool we are looking for a decrease in temperature not increase as in superheat. Although in both cases its only a temperature difference that we seek.

Gary
12-05-2002, 07:49 PM
As the Prof points out, a sight glass will not tell you if the subcooling is excessive. This, in itself makes subcooling far superior to sight glass. On the other hand, I am strongly in favor of the moisture indicators which are incorporated into most sight glasses.

Keeping in mind that many things should be checked before and after charging a system, here is a charging method which works well on virtually everything:

Monitor both (receiver/condenser outlet) subcooling and (compressor inlet) superheat while charging the system.

Charge refrigerant until the suction line is cool. Wait for the refrigerated space temp to drop close to design, then continue.

Charge the system slowly, stopping often to check subcooling and superheat, and giving time for the operation to stabilize.

The high limit is 15F subcooling. The low limit is 20F superheat. When either of these limits is reached, stop charging.

If the upper limit is reached first on a fixed orifice system, something is wrong. If the lower limit is reached first on a TXV system, something is wrong. In any case, stop charging when either limit is reached.