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superheat
03-03-2003, 09:38 PM
Came across one that confused me completely. Store gets a new walk-in-freezer, so they use the old one as a cooler. The system is way too big for a cooler and the compressor dies. This guy likes extended warrenties with his stuff. I can only get those with new CU. My local dealer will not give an extended warrenty where the CU does not match the evap, but they will give an extended warrenty on a matching CU even though it is way oversized.
I asked about the right sized compressor for the load and a new TEV. They told that was the worst thing I could do. I do not understand why a short cycling compressor is better than a compressor with 2X sized coils. Someone explain this to me.
I put in the CU with extended warrenty.

750 Valve
10-03-2003, 10:50 AM
Install a fan in the old implosion/explosion port, suck some nice moist air in for more load!!!!!

Dan
12-03-2003, 12:24 AM
Came across one that confused me completely. Store gets a new walk-in-freezer, so they use the old one as a cooler. The system is way too big for a cooler and the compressor dies.

That confuses me. I don't see the system as being way too big. We warm up boxes all the time with great success. What you might have is mismatched compressor with too small a motor at the higher suction pressures it is exposed to for its displacement, but that should be easily handled with a CPR valve and different low pressure control settings.




I asked about the right sized compressor for the load and a new TEV. They told that was the worst thing I could do. I do not understand why a short cycling compressor is better than a compressor with 2X sized coils. Someone explain this to me.

This baffles me too. The load on a freezer and the load on the same box as a cooler is not significantly different. I am talking in the neighborhood of 10 to 20% in normal applications. The freezer may have larger coils than you would see in a cooler, but the size of the coils should not be important.

What displacement differences are we talking about here?

superheat
12-03-2003, 05:18 PM
I do not have the details you ask for. The job is out of town. I will be there again in a couple of months for a PMA. I do not recall the size of the CU, I think it was a 3 HP. Evap, CU, and TEV were sized right for the freezer. We changed the TEV for a higher back presure and I think we installed a 2 HP CU. I do not remember for sure. The system was about 10 years old and had been a cooler for 2 years.
I was just wondering about the comments made by the distributor. He was willing to sell me an extended warrenty on a low temp CU operating on a 35 degree box, but told me I am asking for trouble with a smaller compressor on the 3 hp condenser/evap combination as it existed.
We did the right thing: convince the store manager to spend more money than he wanted to spend. He was cussing his boss for requiring an extended warrenty, when he just wanted to change the compressor. I gave him labor for free on the new TEV (increased the mark-up a little though). Everybody is happy, but I am a little confused.
I posted this question to gain a little design knowledge and try to stump people who are better at this than I am.

Andy
12-03-2003, 10:06 PM
Hi,
as Dans says the duty at the high temp from the coil will not change much, especially if you keep the td the same.
Size the unit to match the evaporator capacity at the condintions you want. ie if the coil did 4kW at -33 deg C you want the new condensing unit to do 4 Kw at the new conditions say - 5 deg C.
If the unit is too small for the coil you should select the tev to match the unit not the coil.
Regards. Andy

Dan
15-03-2003, 02:44 AM
He was willing to sell me an extended warrenty on a low temp CU operating on a 35 degree box, but told me I am asking for trouble with a smaller compressor on the 3 hp condenser/evap combination as it existed.

Ahh. I didn't understand. So the choices were a 2 hp medium temperature compressor on the original 3 hp condensing unit versus an entire condenser unit changeout. I think I see the distributor's point of view, since he is being asked to warrant the performance of the equipment.

BUT! I am puzzled that you would go down in horsepower when changing from a low temperature unit to a medium temperature unit.


If the unit is too small for the coil you should select the tev to match the unit not the coil.

Thanks, Andy. To exaggerate a condition why this is correct, let's imagine a 10-ton coil with a 2-ton condensing unit. A 10-ton TEV would not work at all during normal steady state conditions, and during a pull-down condition would permit the 10-ton load the evaporator is capable of, outpace the poor 2-ton unit to the point of inevitable failure.

But a 2-ton valve would minimize this risk. One wouldn't want to put a 10-ton valve on a 2-ton coil either for the same reason, even though the coil cannot develop as high temporary loads as the 10-ton coil can develop.

I wish we knew what compressor model was replaced and what compressor model was installed. It would be a good study question.

My last thought on this is that the customer would have been better served (in a purely theoretical sense) if
Superheat installed a properly rated compressor for the duty on the existing condensing unit.

There is a key relationship between the size of the evaporator and the size of the condenser, regardless of what is in between. Whatever heat goes into the evaporator must be expelled from the condenser.

So you might say that larger evaporators require larger condensers....for potential transient loads. I see the evaporator as the promise and the condenser as the delivery of the promise.

Prof Sporlan
15-03-2003, 04:35 PM
Thanks, Andy. To exaggerate a condition why this is correct, let's imagine a 10-ton coil with a 2-ton condensing unit. A 10-ton TEV would not work at all during normal steady state conditions, and during a pull-down condition would permit the 10-ton load the evaporator is capable of, outpace the poor 2-ton unit to the point of inevitable failure.
If in this case, the 10 ton evaporator is rated at a 10F TD, then operating this coil on a 2 ton condensing unit, you are effectively trying to operate the coil at a 2F TD.

This, of course, cannot happen, as it would require the TEV to control within a 0 to 2F superheat range.

What will happen is the 2 ton TEV will effectively starve the 10 ton evaporator under all load conditions... the superheat the TEV controls will match the operating TD of the coil. As the box reaches design temperature, superheat and TD should work their way to some reasonable value, say 10F. But the evaporator, in this case, will only have about 20 percent of its surface active, which is basically what we want.

With some care, the TEV could be adjusted to a lower superheat/coil TD. But it would not be reasonable to expect we can adjust the TEV to make our entire 10 ton evaporator active. This would require less than a 2F superheat control.

If TEV superheat control isn't below the TD of the coil, then one can figure the coil is likely being starved.

Gary
15-03-2003, 07:51 PM
I would envision a liquid filled coil, with 20% activity, so I'm not sure the term "starved" applies. I could be wrong.

Prof Sporlan
16-03-2003, 02:42 AM
I would envision a liquid filled coil, with 20% activity, so I'm not sure the term "starved" applies. I could be wrong.
Interesting viewpoint, and it is probably more accurate than thinking the first 20 percent of the coil is doing all the work. We could certainly have a situation where we have "sewer flow" in the evaporator due to insufficient refrigerant velocity, which would be due to the oversized evaporator coil. And the inside surfaces of the evaporator tubes are not getting wetted, resulting in a loss of 80 percent of the coil capacity in this case. In either scenario, the evaporator surface is being underutilized to match the condensing unit capacity.

The Prof will define "starved" in the case to "underutilized". :)

Andy
16-03-2003, 07:58 AM
Hi,
mix match equipment is never a great success, but it can be made to work provided the person doing so understands the basics.

With some care, the TEV could be adjusted to a lower superheat/coil TD. But it would not be reasonable to expect we can adjust the TEV to make our entire 10 ton evaporator active. This would require less than a 2F superheat control.
I think the Professor sums up the coil TD to condensing unit duty quite well. What we are doing withe the smaller 2 ton TEV is extending the suction line into the coil, with the extra evaporator superheating the suction gases, but adding little in the way of cooling. A draw back with this is the suction superheat will be higher and also the discharge temperature on the compressor, high ambients should be avoided with this set up.
Regards. Andy

frank
16-03-2003, 06:25 PM
I think it was a 3 HP. Evap, CU, and TEV were sized right for the freezer. We changed the TEV for a higher back presure and I think we installed a 2 HP CU. I do not remember for sure.

How do you size a TEV for a higher back pressure but fail to remember what you did? - sounds like sloppy workmanship to me.

Surely, once you have done the maths you know the job?, I can recall jobs we did 10 years age.

frank

Prof Sporlan
17-03-2003, 02:07 AM
Nice explanation, Prof. Coincidence is, it's almost identical to the expaination I've been drawing up for the description of a fault experienced years ago
The Prof won't mind if you plagiarize some of his musings to get your explanation more precise. :D :D :D


This is about a water chiller accidentally piped backwards and the system subsequently experiencing frequent LP trips and high space humidities :)
Running the media your are cooling counterflow to the refrigerant is key to getting a evaporator employing a TEV to perform properly.

Gary
17-03-2003, 11:15 AM
I believe the Prof's original assumption is most correct. At the higher established TD dictated by the TEV's superheat setting we'll find that-first-percentage of the coil provides all the heat necessary for complete evaporation.


Hmmmmm... With high superheat at the coil outlet, the TXV will be running wide open, feeding more than enough refrigerant to match the compressor capacity.

It seems to me that how much of the coil is flooded with (inactive) liquid would depend upon system charge and heat load. If there is sufficient charge with normal to low load, the coil would fill with liquid in an attempt to satisfy the TXV, making the last of the coil the more active portion.

Andy
17-03-2003, 12:01 PM
Hi Gary,

the coil would fill with liquid in an attempt to satisfy the TXV,
never a truer word have you said. To acheive better refrigerant control the expansion valve would require a higher than normal superheat setting, this setting would be impossible to acheive due to the very low TD.
I would sugest that the TEV sensing bulb would need to be moved from the coil outlet to an end link, (if possible) somewhere about 50% of the way thru the coil. Better control would be then possible as the rest of the coil would just be a suction line.
Regards. Andy:)

Gary
17-03-2003, 12:47 PM
Interesting things could happen with the humidity here. If we fill the coil with (saturation temperature) liquid, maximum humidity would be removed. If the bottom of the coil is superheated (to entering air temp), any moisture dripping down from the active top would re-humidify the air, minimizing humidity removal.

Andy
17-03-2003, 01:12 PM
Hi Gary,
looks like we have just stumbled across a low latent heat capacity coil, is that not how most things are invented?
Would be idea for storing vegetables no doubt, or even computer suites come to think of it:D
Regards. Andy

Gary
17-03-2003, 01:22 PM
Hi Andy


looks like we have just stumbled across a low latent heat capacity coil,

Or a high latent heat capacity coil for A/C in humid areas. An oversized coil can be manipulated to achieve either goal, and can be similarly manipulated by increasing or decreasing air flow.

Andy
17-03-2003, 06:28 PM
Hi Marc,

So the minimum available TD is equal to the intended TEV superheat setting.
have said that if the over sized Evaporator was equiped with a Electronic Expansion Valve (the one with a pressure transducer referance) we could expect a better utilized evaporator.

I personally would go the other way and close down a standard TEV to give a slightly higher than normal superheat, just to remove the fear of hunting and liquid over feed.
Thats provided the expansion valve is sized for the Cond Unit Not the evaporator.
Regards Andy:)

superheat
17-03-2003, 08:17 PM
Originally posted by frank
How do you size a TEV for a higher back pressure but fail to remember what you did? - sounds like sloppy workmanship to me.

Surely, once you have done the maths you know the job?, I can recall jobs we did 10 years age.

frank

Sorry for my loosy memory when it comes to numbers. I would be lost without the address book in my cellphone. I can't even remember my home phone number 1/2 the time. Hey, I can't be so good-looking and still be perfect in everything else too. My wife calls me at 3:00, if we have a date at 6:00. I might miss it otherwise. I can't recall the details on a job last fall. Maybe I should not have started this thread without more details, but I was bored and curious. It has created some interesting discussion. I was more curious about why both an oversized evap and condenser is worse than an over sized CU. The TEV was not even mentioned by the counter man. As the system is right now, I believe the evap is oversized according to CU and valve.
A low temp valve is set to close at about 0 degrees. It protects the compressor during pull-down and after defrost. A medium temp valve is set to close at 35 degrees. A low temp valve on a medium temp box would not be good on the medium temp compressor. It would leave a very dry box too. I would not do that ever.

The 2 HP compressor is the next size down from a 3 hp compressor at the store I buy from. I run a load size on every box when I change the CU. Maybe the 3 hp was oversized. I would definitely know if the 2 HP was too small by now. The load may only increase a little when you drop the box temp, but the compressor needs more power to pump the same amount of ***** for higher CR. The biggest load on a box is the infiltration. Maybe they added the door curtains after the box was commissioned.

I believe a 10 ton valve on a 2 ton evap would hunt like crazy. I don't beleive in an evap rating anyway. The capacity of an evap is rated in BTU provided there is a 10 degree TD. The potential for capacity is provided for by the compressor. The valve controls the amount of ***** through flow. So the valve delivers the potential capacity that compresser sets up. ei An undersized compressor will not have enough ***** for the valve to provide capacity. An undersized valve will not deliver the ***** the compressor has set-up. Correct compressor/valve combination goes most of the way to set the system capacity. You guys are going way out with some of these examples, but that is the direction of my curiousity. Eff. increases with the size of the coils, but up to what point? Is there such a thing as a condenser that is so big, it is dangerous to the compressor? Would a 3 ton condenser on a 2 ton compresser be over that line?

Dan
18-03-2003, 02:54 AM
So no matter what the compressor/evap match is, and no matter if the TEV is big enough, if the TEV superheat setting is higher than the matching TD then the TD will rise to equal the TEV superheat setting because the valve will close to force the establishment of it's superheat.



With some care, the TEV could be adjusted to a lower superheat/coil TD. But it would not be reasonable to expect we can adjust the TEV to make our entire 10 ton evaporator active. This would require less than a 2F superheat control.


To acheive better refrigerant control the expansion valve would require a higher than normal superheat setting, this setting would be impossible to acheive due to the very low TD.


It seems to me that how much of the coil is flooded with (inactive) liquid would depend upon system charge and heat load. If there is sufficient charge with normal to low load, the coil would fill with liquid in an attempt to satisfy the TXV, making the last of the coil the more active portion.

The relationship of Temperature difference and superheat has been a worthy discussion unto itself.:)

Gary
18-03-2003, 12:32 PM
The relationship of Temperature difference and superheat has been a worthy discussion unto itself.


I agree, Dan. I am still trying to wrap my mind around the concept, and adjusting my perceptions accordingly.

Reducing airflow would utilize more of the coil, and the dual heat exchanger (with TXV bulb in between) strategy could effectively reduce the TD without de-stabilizing the TXV.

Interesting stuff. :)