Two smaller condensers instead of one larger?

[recover]

Hi. I'm a physicist who has a bunch of refrigeration equipment and an HVAC license, but little experience except for a few custom sytems I've built, servicing the AC units to rental property that I own, and my own antique vehicles, and a few friends and family who badger me.

Would you experienced experts consider a plan I have, before I complete the intall. I haven't plumbed it yet, but am otherwise ready to.

So I'm renovating a 1973 Rv and installed 2x Dometic 110 volt 14,000 btu split mo_bile units, condensers below deck. They use tecumseh compressors, capillary metering systems, and have a two speed condenser fan, run on Nu-22 (R-22 zeotrophic replacement). One unit is a year older than the other and has a differently engineered metering capillary system (looks different).

They both work great. The problem is that the older unit easily ends up kicking on the high speed of the fan, and it is way too loud for my easily annoyed self to tolerate. The fan speed kicks on when the refrigerant temperature entering the dryer, between the condenser and the evaporator hits, I think 130 degrees F., the thermocouple sensor is not faulty. The newer unit never requires high speed. Unmounting the two condensers and putting them in the open with tons of airflow doesnt change this, it isn't a mounting location problem. The condensers are identical. I think it is related to the design of the units, but can't get answers from Dometic because it all acutally cools fine and the numbers are fine, just a bit different. The refrigerant temp of the "better' unit never gets much above 120, the older "problem" unit gets to 130, the high speed fan kicks on, and it drops right down and cycles.

So, in my inexperienced and easily annoyed genius, I have installed a second but smaller condenser. I plan to plumb it between the main condenser and the dryer, and hope that it will cool down the liquid refrigerant so that the fan can be on low speed, and be very quiet.

I plan on filling the system using superheat readings, but expect that the lines to/and the smaller condenser between the main condenser and the evaporator will be filled with liquid refrigerant most all of the time.

do any of you experts see any obvious problems with this plan, before I recover my refrigerant and plumb this smaller condenser into the sytem.

The seemingly obvious other way is to get a bigger main condenser, but I don't have room underneath for anything bigger, but two smaller ones fit.

[monkey spanners]

If you increase the systems highside volume without increasing the systems low side volume, there is a risk of flooding the compressor with liquid when it restarts. A suitably sized accumilator may be required.

A cheaper option would be to fit a variable speed drive to the condneser fan motor on its highest speed and do away with the two stage speed control, this will give you just enough air (and nosie) to keep the system working as it should without the fan running flat out unless it really needs to.

[recover]

if one has an oversized accumulator for a given system volume, will that cause any obvious problems? I would think that it would just spend most of its time empty, but I wondered if that would cause it to sequester too much of the systems oil. The current total system refrigerant is 3 lbs, and the new condenser/lines I guestimate would hold another 1 lb. most accumulators that Ive seen are for larger units and hold at least a 3 lbs. smaller automotive accumulators might work, some auto ACs are in the 14000 btu range but rarely do they list their capacity. I wouldn't think that the fact that they are made for r134 would matter if you used good O rings and such.

I have played with a variable speed on the fan motor, initially one that I managed manually as I watched the temperature out of the condenser, and it for some reason still needs to be in the noisy range, it was possible to improve on things with such a setup. Why there is such a difference between the two units, I don't know. The indoor part of the split is too expensive to replace with one like the "quiet" version.

[recover]

It will not help you anyway.

thanks for the articles, makes senes. I suspect you are correct, unfortunately.

but I don't fully understand how, if it increased the subcool, meaning decreased the liquid refrigerant temperature but didn't affect the pressure (it shouldn't affect the pressure since it will be cooling mostly or all liquid), how that might not keep the sensor from triggering and keep the fan on low. the high fan is trying successfully to remove more heat (increase subcool), and wouldn't a second condenser also remove more heat? how does the system know if the heat was removed from the liquid (air cooled subcooler) or if the removed heat resulted in a phase transition (condenser)?

of course sometimes things that seem obvious aren't.

[al]

So back to basics, condenser is clean, fins still attached and not corroded, condenser fan motor not worn out and the condenser fan blade is spotless and on the correct way?

if so, what about upsizing the condenser,also comparing the two units, is there a difference in pipe size on the condensers by any chance? Also are the fan motors and blades identical?

al

[Brian_UK]

Also back to basics but...

Are you sure that you don't have any non-condensibles in that system raising the temperatures?

[Chef]

So I'm renovating a 1973 Rv and installed 2x Dometic 110 volt 14,000 btu split mo_bile units, condensers below deck. They use tecumseh compressors, capillary metering systems, and have a two speed condenser fan, run on Nu-22 (R-22 zeotrophic replacement). One unit is a year older than the other and has a differently engineered metering capillary system (looks different).

As you say both systems are identical except for the metering device I would look at that first.

If the older unit has a slightly smaller or longer capillary it will need a little more pressure to push the refrigerant through. Generally the only way this can happen is to backup the refrigerant into the condenser and reduce the condensing volume thus raising the pressure (and temperature).

When the temp rises the fan goes high and so effectively increases the size of the condenser and more refrigerant is stored there, the evaporator pressure falls a little and so the flow falls followed by the temperature and you go into a cycle.

As a physicist I am sure you appreciate feedforward and feedback.

Adding the extra condenser would help to releive the fan cycling a little but it would also mean you are running the system 'out of spec' and reducing its efficiency. It is not really the best way to go as it is not fixing the core issue.

Your best bet would be to change the capillary.

[mad fridgie]

Before you consider the condenser, you do need to check the load on the evaps, you may find that the older unit may be acting as a master (larger load), thus high HOR, higher discharge pressure. Are you seeing a difference in suction pressure and superheat.

[recover]

thanks guys. if I thought it was simple, I would have just done it by now. I appreciate you considering this seeminly minor, but to me interesting, situation. I know its trivial in the greater scheme of keeping our world chilled.

and I posted incorrectly in the first description, the sensor that triggers the high speed fan is directly on the compressor outflow tube. and the temp is 140. Im pretty sure that, as I read you guy's thoughts, the best approach is to either just accept the noise cycling when it is very hot, or suck up the expense of replacing the evaporator/metering/compressor module ($1200 or so) and keep the old one for spare parts down the road.

but putting the second condenser in the mix would only take me a few hours, so I might just try it first, maybe even do it series, then if that doesnt work out well, parallel.

putting the second condenser in parallel had crossed my mind, but I worried that since they have different capacities, the smaller is proably 2/3 the capacity of the larger, that there may be some significant difference in the pressure changes that occured with condensing, and that the ration of refrigerant that went through one vs the other might be unpredictable or that there may even be some "back gurgling" if the pressure change from one was notably different than the other.

I have pumped out the system, and refilled, so I did check for non condensables. the fans on the condensers are identical. Ive spent so much time on getting this thing right, Ill probably just buy a new inside unit, and hope if functions like the quiet one. Still considering things.

The RV in the old days, kids young and happy. Several years ago, moving the fiberglass over to a new chassis. Its the most crazy, modern, sooped up, overdone, little thing around.
n_b.jpgn_a.jpg

[frank]

If the 2 systems are identical, you would expect temperature/pressure readings to be identical.
Have you measured temperature and presures on each system? Are they identical?