Excessive Superheat

Excessive Superheat

I have an issue on a custom built system used to cool a scientific process tank. The system has a Danfoss – Maneuorp compressor with the following specs:

Compressor: MTZ50-1
Code Number: MTZ50HK1VE MTZ50HK(B)1VE

33355 Btu/H; R134a; 60 Hz; 208 – 230 Volts 1/0 (187 – 253 VAC)
2504 Watts; 12.6 Amps; and a Cut-out temp of 230°F.

The system is built with cap tubes because the evaporator (four cap tubes feeding four sections – one each section) is located inside the process tank. The tank contains a gas, cooled by the evaporator, and consequently varies in pressure, which would affect at TEV. It is supposedly sized for low temp operation – around -10F.

The condenser is a plate type, water cooled unit, and does a remarkable job.

During initial cool down, starting with the system at ambient temp (75F) the system operates at only 75 psig head pressure, and around 3 psig suction. It does a respectable job of removing heat, but the Superheat is so excessive, that it will cause the compressor to cut out on high temperature – not head pressure. With head pressure, only in the 70’s, the compressor is only pulling 10.5 amps.

Readings: Process Tank atmosphere 75F; 78# head pressure; 2# suction pressure; High side gas discharge; 211F; Suction line gas temp 64F. After 15 minutes: Process Tank atmosphere 27F; 75# head pressure; 2”(vac) suction pressure; High side gas discharge; 223F; Suction line gas temp 27F. At this point, the compressor cuts out, requiring approximately 45 minutes to reset. This occurred 3 times today, and only once did the system make it down to 0F.

The system has a liquid receiver, and a suction line accumulator, and a sight glass in the HP line. Hot gas enters the condenser at the above listed temperatures, and exits at a respectable 68F. Water enters at 55F, and exits at 65F. This provides steady state liquid feed in the sight glass to the cap tube distributor. I have the ability to alter the water flow, which I have, and will marginally affect the head pressure, but obviously makes no affect on the superheat, nor the cooling efficiency of the system. I am able to reduce water flow to allow head pressure to climb to approx 125, but does not seem to help anything except to draw full load current of 12.5 amps.

I believe that is all the pertinent information I have on the system. Can anyone suggest what is wrong? It is charged with uncontaminated R134a.

Since you are making your 1st post a 8PM on a Saturday Evening, I presume you would like a little help ASAP.
I am an Industrial type for the most part but will help you if I can.
Fortunately you have given us a lot of information to work from.
I am assuming this once operated in a satisfactory manner and no modifications have been made to it since then.
It sounds to me as if you have high superheat because of underfeeding. Are you sure you have liquid in the Sight Glass?
You are running almost 50 degrees of Superheat. Sometimes a sight glass that is completely empty is hard to tell from one that is full.
Is the sight glass before or after the Filter/Drier?
If it is after is there a substantial difference in temperature between the in and out? If so it could be clogged.
I assume there is an expansion valve before the distributer. has it's setting been disturbed?

Thanks for the ASAP reply NH3LVR! No, the system is new. I have been brought in to assist. The guy who designed the system is no longer involved, so the "balance" of the system is questionable at best.

Underfeeding was an initial problem, but that has been solved, the glass is full. Actually, there are two glasses, one before a liquid receiver, which feeds down line to a filter/drier, and the second line is immediately after the filter/drier.

The temp through the entire line exiting the condenser is quite uniform, including in and out of the receiver, and both sides of filter/drier

Will see an occasional few bubbles pass the glass, very occasional, but enough to see the glass is full.

There is no expansion valve. The distributor, which is inside the process tank with the evap, divides and feeds four cap tubes which each feed respective evaporator sections, all manifold back to single return line to suction line accumulator.

With the receiver and accumulator, I believe the charge would not be super critical, suffice it to say with full liquid feed. What blows my mind is that I am only popping about 75 psig head pressure with such high discharge temp.

Oops! I said the second "line" is after the filter/drier - I meant the second "sight glass" is after the f/d.

QUOTE AEGIS Cool

With the receiver and accumulator, I believe the charge would not be super critical, suffice it to say with full liquid feed. What blows my mind is that I am only popping about 75 psig head pressure with such high discharge temp

Hi AEGIS:)

I would say not enough head to push the liquid through the capillary lines leading to liquid starvation, low suction pressures, high suction superheats and very high discharge superheats.

Try rasing the head, if this doesn't work I would ditch the capillaries ane fit TEV's (that is a personal preferance others will disagree)

If suppose you could shorten the capillary lines or better still increase the ID of the capillaries until you acheive a resonable suction superheat:)


Kind Regards Andy:)

Andy, as usual is correct.
I believe you are looking at a design problem.
As Andy suggested you might try raising the head a bit more. Put an Ammeter on the Compressor Leads if you wish. Sometimes the increase in head will cause rather dramatic increases in the suction-which is what you are looking for here.
Do you have an automatic water valve to control the head?
If I could ask, is this a high purity application? Is the chamber made in such a way it can be opened and the evaporators observed?
You do seem to be drawing quite a bit of current @125lbs head pressure. But I do not work on small systems much so I will let others comment on that.
Do however check the Voltage at the Compressor and verify you do not have a voltage drop that could contribute to the problem of overheating.

I would also suggest a TEV, possibly without MOP.

Thanks Andy, and NH3LVR for the feedback.

I'll try cutting back the water to the condenser, which should raise the head. I've slept since then, but I believe I was not getting sufficient liquid condensation with lower water flow, but perhaps I can play with balance. That, Andy, will allow me to sleep tonight! Thanks.

This process tank is semi-hermetically sealed and the distributor and cap tubes are inside. Pressure of gaseous atmosphere in process tank varies, and it would seem to me that such atmospheric pressure variations would wreak havoc on a TEV. Is that correct? Does a TEV not pressure balance off of atmosphere (surrounding it)? I've not dealt with them much. The guy who built the system apparently thought that cap tubes would be the way to go, as they would be immune to what happened around them, which kind of makes sense.

I don't know everything that is inside the process tank - and that's OK, good bunch of guys, working on the next greatest mousetrap. I know that opening this system would be an extravagant issue that these guys would rather avoid. If they are willing oneday, and want to open the system, perhaps I could relocate the control system to the outside of the tank, replacing the cap tubes with TEV as Andy suggests, and locating the sensing bulb on the suction line where it exits the tank and insulating it well. Just have to deal with hermetic pass through connections. I hope they will consider that, as the evaporator seems to be trapping some oil. The compressor has an oil level sight glass, which was 1/2 full when started. It is now only 1/4 full at rest, and was almost empty when running.

Would you suppose more head would also provide more velocity and help bring the oil around? They tell me the riser is only about 8 inches from the lowermost point of the evaporator, and is 7/8" tubing, same size as the suction line (all the way back to the compressor, which is only about 6 feet of tubing plus the suction accumulator) all located downhill from said riser, which only serves to go back up to an existing port through which the suction line passes, hermetically sealed to the tank.

The way the system is constructed, it would be a major feat to rebuild.

Couple of points. Compressor designed to operate at 12.5 amps, and is only pulling 10.5 (at the 75 psig head stated.) Also, voltage is rock steady at 240VAC. Compressor is rated 208-240.

Also, having poked around a bit on RE.com, I see Andy, "site moderator - and generally nice guy" <<need a smiley here - don't know how to do that, so here you go :-) >> that you graciously submit a lot of comments / answers. Most of my experience has been with dairy equipment, milk tanks, etc. and a few heat pumps (as few as I could get by with), and consequently most experience is with R12 and R22, medium temperature systems. R134a is new to me. Am I correct in assuming I should be seeing similar pressures with R134a as those of typical R12 systems, i.e., Head about 125 and suction around 5-10? This system is supposedly low temp -10 to -20F
(-23 to -29C). System did get down to 0F (-18C) last time!

(Standard Disclaimer here about being an Industrial Guy)
A TEV does not not pressure balance off of atmosphere . Even the internally equalized ones. I am prejudiced against Internally Equalized Valves. Although they work well in some applications, I have never seen a situation where the Internal Equal was better than the External.
Your Oil Trapping is almost certainly because of your Underfeed Problem.
I too have had the experience working with the Chambers the Engineers do not want to shut down. We just have to live with it.
I certainly agree with Andy about the need for TX valves here. The only problem may be selecting the Valve Size. In cases like this a Valve with a replaceable Orifice might be the way to go. If the first choice is too large or small simply change the orifice.

Quote:

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Originally Posted by AEGIS Cool

Am I correct in assuming I should be seeing similar pressures with R134a as those of typical R12 systems, i.e., Head about 125 and suction around 5-10? This system is supposedly low temp -10 to -20F
(-23 to -29C). System did get down to 0F (-18C) last time!

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Hi AEGIS:)

yes R134a is similar, probabaly a few psig higher head and suction.

If you are getting the back pressure ie -18 deg c the liquid supply is probably quite good (maybe a little shy)

Seems like a design problem, the high suction superheat is probably comming from a vary warm product in the vessel.
You could use a TEV injecting into the suction to cool the compressor, by reducing the suction superheat (probable size would be 10% of the compressor duty at the head and suction you have stated). This will stop the compressor tripping on overheat, but it will also reduce the system capactity, but at least it will run:)

Kind Regards Andy:)

Hello
If you can't fix it leave it that no one else will

I'm going to jump to the band waggon and say TEV for the evap and a cap tube with a solenoid to inject refrigerant to the suction line only after a certain trip point, this way you don't lose capacity till you need to.

Just a point guys you should not fit and do not need a receiver if you are using capillary lines, this will give the appearance of the system being fully charged when in fact it is not and why do you need it if you have a critically charged system. My first suggestion would be to remove the receiver and let the refrigerant back up into the condenser which will raise your discharge pressure and give the correct delta T across the capillaries by then adding enough refrigerant to fill the sight glass your superheat will drop and you can then play with the charge until you get it correct.

Worth a try.

Ian

Thanks Gents,

After reading Andy's reply on increasing head, I just about banged mine on the wall - duh - made all the difference when I cut the water back, as it did not have an automatic control.

I appreciate all the feedback, and hope that I may be of assistance to a fellow in need!