experimental evaporator

[chuckstrain]

hi i'm using a BD35F condensing unit, my evaporator is experimental, meaning i made it out of 1/4" tubing and it chills a copper plate that chills water running over it. my evap frost up well but it fluctuates, it frost then the frost disappears then frost again, also the cap tube .031 id 9' long frostup about an inch or two before it enters the evap, anyone have an idea of what i can do to improve the performance?

[Gary]

Before you can determine if a system is doing what it's supposed to do, you need to know what it's supposed to do.

[chuckstrain]

Gary,
thank you for the response and you are right I did not provide enough information and my knowledge is also limited. this is what I'm trying to do. I'm sitting this BD35F condensing unit with a fan cooled condenser on top of a reservoir of water which sits on top of a copper plate which sits on top of a coil consisting of 4 passes of 1/4" tubing about 10" long. the water is circulated over the plate and through plastic tubing. i only want the water to chill to about 60 degrees F. leaving the reservoir. my cap tube is over 9' long which I did so that if I have to remove some, well it is easier than adding. now i did not have it under load conditions but my evaporator did frost up through all passes right up to the end of the evaporator. but it would fluctuate frosting then unfrosting. today I'm putting it under load conditions and testing again, i expect to have to remove some cap tube.any help you could give me would be appreciated Gary, Thank you.- PS if you need any specific info please ask. I'm not a engineer just a service man trying to please his boss-respects, Chuck

[Gary]

When the reservoir is down to 60F, then we need to know the reservoir water in and water out temps, the condenser air in and air out temps, the high and low side pressures, the liquid line temp near the receiver outlet (or condenser outlet if there is no receiver) and the suction line temp near the evaporator outlet.

Also we need to know what refrigerant you are using.

[chuckstrain]

Hi Gary! yesterday i cut the cap tube down to 7' my low side pressure was about 5-psig, my high was about 100-psig, my ambient was about 86-f, and we were getting about about 58-f out of the reservoir. they will be testing it this weekend and when they get back I'll get all of the information you asked for early next week. I don't believe this is correct yet as i was getting better results with the 9'+ cap. we are using R134a. Thank you Gary for your time and input, Respects, Chuck

[Gary]

At 100psi your saturated condensing temp (SCT) is about 87F. Since the ambient is 86F, the SCT is only 1F over ambient. This means the refrigerant is picking up almost zero heat load in the evaporator. A full heat load would put the SCT about 20-35F above ambient.

Similarly, at 5psi the saturated suction temp (SST) is about -4F. This is about 62F below the temp of the water, confirming the lack of heat flowing from water to refrigerant.

There could be a variety of causes and/or multiple causes for this, but we can't tell what is wrong without all of the numbers.

All we can tell at this point is that something is very wrong with this picture.

[chuckstrain]

Thanks Gary, again, before I cut 2ft.of cap tube off I was reading 150 psig and 0 psig on the low. at any rate look for me monday afternoon or tuesday morning I'll have the info, respects, Chuck

[Gary]

Those numbers are even worse. Less heat transfer in the evaporator and higher head = liquid backing up in the condenser/overcharge.

How are you charging the system?

[123-steve909]

any chance you could put a TEV on it and see what that gives you. Capillary is a bit of a nightmare to set up and fitting a TEV will rule out the incorrectly sized expansion device

regards
steve

[Gary]

any chance you could put a TEV on it and see what that gives you. Capillary is a bit of a nightmare to set up and fitting a TEV will rule out the incorrectly sized expansion device

regards
steve

That's a good point, Steve. In order to evaluate and optimize the performance of the experimental evaporator, we first need to optimize the cap tube... and that is going to take a lot of work.

It involves balancing the subcooling against the superheat, step by step, with presumably much cutting and recharging... a nightmare indeed.

Installing a TXV could make the task much easier as the TXV will automatically optimize itself.

[Gary]

If it is necessary to balance the cap tube, here is how it's done:

Charge the system until the liquid line subcooling is 15F/8.5K.

Wait until the water in the reservoir is down to the desired temp.

Check the superheat near the compressor inlet.

If the compressor inlet superheat is less than 20F/11K, then very slowly remove refrigerant until the superheat is 20F/11K.

If the compressor inlet superheat is more than 20F/11K, then the cap tube is too restrictive and needs to be shortened.

Repeat these steps until the correct compressor inlet superheat can be achieved without exceeding the 15F/8.5K subcooling limit.

[Gary]

I should point out that the above strategy balances the cap tube output against the current heat load. Once this is done we can evaluate the current performance of the experimental evaporator.

If we then take steps to improve its performance (increasing surface area and/or increasing water flow), this will increase the heat load. Then we need to balance the cap tube all over again.

[bowral fridgy]

[quote=chuckstrain;144415] my evap frost up well but it fluctuates, it frost then the frost disappears then frost again,

Sounds like moisture in system causing restriction in capillary hence low suction pressure

[Entropie]

Maybe this helps: http://www.refrigeration-engineer.com/forums/showthread.php?t=1224

[dan wong]

being this is experimental; rather that keep cutting the cap tube, wouldn't it be easier using a adjustable valve (many ***** hose come with a shut off valve), keep adjusting this valve until you get the proper superheat.

[chuckstrain]

this is a 1/20 hp condensing unit. is there a a expansion valve that meters for that small of a unit?