1. Originally Posted by Lennyb
Changed both fan motors and returned TXV back to center, de-iced both side of evap coil. Box got down to 16 Deg. and SH was still 50 Deg.
My question is that I'm going by simple math equations. when my gauge temp says a neg temp it gets added to the suction line temp not subtracted correct?
No.

SST temp of -5°F and a vapor line temp of 20°F. Equals a SH of 25°F.

2. And in your post, you said you have a 16* box. since your suction line temperature at the coil isn't going to be above your box temperature. And you said you have 50* superheat, at best you have a Suction line temp at 16* and a SST of -34* for 50* superheat.

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If the valve is frosting over its restricted. Clean the strainer if it has one, or replace the valve. Some valves can be torn apart and cleaned, I prefer to replace them. You want to see 6-10 deg superheat at the bulb as someone already posted.

4. I disagree about just looking at the valve being iced over indicating a problem. If the line temperature is below freezing, what's going to happen? Even with a properly operating valve??

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Originally Posted by ryan1088
I disagree about just looking at the valve being iced over indicating a problem. If the line temperature is below freezing, what's going to happen? Even with a properly operating valve??
If there is a distributor, typically that is where the final pressure drop occurs, meaning that is where the frost line will start.

That said, it isn't a diagnosis, but a clue to look in one direction rather than another.

6. Originally Posted by jpsmith1cm
If there is a distributor, typically that is where the final pressure drop occurs, meaning that is where the frost line will start.

That said, it isn't a diagnosis, but a clue to look in one direction rather than another.

especially on medium temp.

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The poster stated the evap was iced. After he thawed the ice, the suction pressure & box temp are dropping as they should but his discharge is still hanging tight at 175. He needs to properly charge the unit, and now that he has adj the txv, he may need to re-adj the SH once box temps are near spec. In addition, he needs to test and confirm his defrost cycle is set and working correctly or possibly an abundance of humid air leaking into the box due to either improper use, no door strips, etc. And I am assuming he knows how to take an accurate temp reading at the outlet of the evap under the conditions.

8. Just curious this is kinda on topic. when you are doing calculations on walk-in coolers/freezers i usually get my Sh by the inlet of the compressor would that be good place to grab it or is it always "better" to grab it at a few inches downstream from the evap a little bit past the bulb.
The reason i do it at the compressor is because its a lot easier and i dont have to take off the cover for it its right at the valves. Just wanting your opinion? thx

(total superheat compared to superheat of the evaporator only)
Last edited by akelesis; 08-20-2012 at 06:55 AM. Reason: re-worded some things (-:

9. Evap outlet as close to the bulb as possible is the best place. You do want to check at the compressor as well generally you need a minimum of 20deg sh at the compressor.

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Originally Posted by ar_hvac_man
Evap outlet as close to the bulb as possible is the best place. You do want to check at the compressor as well generally you need a minimum of 20deg sh at the compressor.
Yep.

11. Originally Posted by ar_hvac_man
Evap outlet as close to the bulb as possible is the best place. You do want to check at the compressor as well generally you need a minimum of 20deg sh at the compressor.
Minimum or maximum?
What would be the acceptable range for superheat at the compressor?
I have thoughts on this but am interested in everyones' opinion.

12. Originally Posted by koolkahuna
Minimum or maximum?
What would be the acceptable range for superheat at the compressor?
I have thoughts on this but am interested in everyones' opinion.
For a freezer, you'd want to see 4-6* evaporator superheat...The Idea is to fill the coil with as much liquid refrigerant as possible to maximize heat removal without flooding.

Most Compressor manufactures say you can go down to 15* compressor superheat, but recommend a 20* minimum compressor superheat to insure no liquid is returning in the suction vapor.

If you adjusted your evaporator superheat to 4-6* but are running below 15* compressor superheat, you will need to increase your evaporator superheat, for compressor protection sakes.

I've never seen a listed maximum compressor superheat value. But I think a good maximum compressor superheat value would be one that keeps the compressor discharge line temperature below 225*. If your discharge line temperature gets much above 225*, oil on the cylinder walls will start to be damaged.

If your evaporator superheat is set at 4-6*, but your compressor superheat is so high, it is unable to maintain discharge line temperatures below 225*. You need to have a serious look at your suction line insulation.

If your insulation checks good, you will need to find a way to lower your compressor superheat in order to lower your discharge line temperatures.

A lot of guys will crank the TEV open in order to lower compressor superheat...This is not the correct thing to do. Your evaporator superheat was already at 4-6*. Now you will most likely be flooding the evaporator & continuing the refrigeration effect throughout your suction line. You don't want to refrigerate the suction line.

A better choice would be to install a de-superheating valve at the compressor.

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I have systems where the compressor is a block of ice and they run great. As long as you maintain evap. SH 4-6* low temp 6-10 med. temp 10-12 high temp (everyone has a diff opinion on these) know that you compressor is safe from flooding unless like said before you have a long a\$\$ lineset that isn't insulated causing high discharge temps due to lack of compressor cooling.
To do a proper service always check the Stinky5
1-suction/discharge pressures
2-superheat/subcool
3-T.D across coil
4-Amps/volts
5-Pipe temps(all 3)suction, discharge, liquid

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