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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8768
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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QF2; 18 oz R502; new12ft; .052 cap tube with copper drier;

This answer was rated:

QF2; 18 oz R502;
new12ft; .052 cap tube with copper drier;
new EK163 liquid line drier;
new Copelametic KAMB 007E CAA 800 compressor rated RLA 10.9;
actual RLA 8.6;
suction pressure psi/-16 degrees (gage and discharge 225 psi/103 degrees (gage);
condenser air inlet temp80 degrees; Evap air inlet 8.3 degrees.

Are these specs in range if not, what should I be looking for?

hvacdave84 :

By the readings you've gotten, it sounds like the system is chugging along as designed. is it having an issue that needs to be addressed? or were you just checking to be sure it's within parameters? as a general rule, you'll want a low-temp (freezer) evaporator to run about a -15 degree saturation (coil) temp. the -16 you have is right in line with that. the condensing pressure will usually correspond to a temperature 20-30 degrees above ambient (condenser inlet) air. you are on the money there. your evap inlet air should be closer to 0. that's the box temp we shoot for in low temp applications. are you checking it with the door open or closed? and, is the unit cycling as normal, or runing constantly? I won't get too far ahead of things here. let me know if there's a problem that needs addressed. if not, have a great day.


there is some question as to the original cap tube size. If the original was .049X10' each and was replaced with .059X12' each, what would be the difference in temperature range. the original cap tube was replaced with the.059. would it be necessary to change or modify the cap tube based on the above readings?


The cold control replacement is a -25 degree control in lieu of a -10 degree control but this should not be a problem except to adjust the control to prevent constant running. am I correct?

Hello.... HVACdave is on target with is remarks, its a bit of a hair split what difference the change in the capillary tube will make, there are many variables... if it works as you have it, things will be fine.

you want the suction line at the compressor to run cool or frosty at times to the compressor, but not frosting the compressor service valve at all.

If you have those conditions you have done a good job.

Let us know if you have any other concerns... and be sure to rate HVACdave positively on his analysis, it is right on target... an excellent rating means a lot to us on this end.

Customer: replied 4 years ago.

A recheck of temp and pressures this morning after 20 hours of running I have 5 buckets of ice inside.


current readings on box:

Box temp: 03 to 05 degrees

Condenser air in: 80 degrees

High side gage: 230 PSI/100 degrees

Low side gage: 14PSI/-22 degrees

Hello again


click that link to see the chart on R-502

You are 5 to 10 psig on the head pressure,
suction pressure should be 20 to 30 psig.

Anything within 10 or 15% of that is good if the suction line is cool or just frosty at the compressor.


Customer: replied 4 years ago.

If the original cap tube was .049 X 10' (2 pieces) and was replaced with .052 with a length of 12" (2 pieces), would error in length make the difference? By the chart I just found, it looks like the .059 cap is too long. Reading the Supco chart it looks like 79" would have right. Am I correct?

Will the unit function O.K. except that the box temp doesn't get to 0 degrees. can I shorten the tube at next service in about 6 months.

Hello again, You are roughly equivalent in capillary tube restriction in your selection of the .052. If your earlier statement of .059 is the case then you are a bit oversized... However it is the system *function* that is most relevant. And your system is working quite well. That is the deciding factor, not any chart. The chart does not know how much the compressor rings and valves have worn and that changes its net capacity.

What you have done works to provide ideal operating conditions... and *that is all that counts... it can easily be the 'right' cap tube will not perform ideally on a *worn* compressor system.

It was an intelligent choice as the reduced restriction going to .052 from .049 is about 20% less restriction than the .049... that in itself would have run the suction pressure up by about 3 psig. and the box temperature up by the same would still have been a functional box.

However you offset the loss of restriction by extending the tube length to 12'

Regardless any typographical error... you are within 2% to 5% of ideal.


Customer: replied 4 years ago.

final point. the compressor is new.

Hello again,

Thats good you have a new compressor. These systems as you know however operate across a broad range of day it is hot and the condensing temperature is high, say for instance that its 90 degree air across the condenser coil.. the condensing temperature will be about 25 degrees above that (depending on how dirty the condenser coil is at the time)... The head pressure will be high as well.

These are the key points.

As winter arrives, the air temperature across the condenser coil will be a lot cooler, perhaps 50 degrees F for instance...and the head pressure will be about *half* what it was in the summer.

With the pressure across the capillary tubes cut in *half* the refrigerant flow rate through the capillary tube(s) is cut in half..
. so that sizing a capillary tube is about hitting the most viable average condition.. And that varies with every job. It is not an exact science for that reason..

You can look on the spec sheet of the condensing unit and see that the capacity of a refrigeration system varies in the 30 to 50% range in these extremes.
From say 4,000 btu's when running at 105 degrees condensing temperature, to 6,000 btu's per hour when condensing at 70 degrees F.

The same is true for how cold you run the evaporator.
For instance if you run the evaporator on any given refrigerant low enough, the net refrigeration capacity at that temperature will drop to near zero...

and the horse power required will be 3 or more times higher than if you run the evaporator at a much higher temperature or pressure. These conditions are listed in the spec sheets of all condensing units.

Yet...with all of these variables, the manufacturer must select only ONE capillary tube.. that capillary tube is ideal for ONLY one set of conditions, and is not ideal for all other conditions.

Your sizing selection puts you as close to the sweet spot as a person can get... any variation from that just changes the ideal operating condition for any given system a few degrees... .say from condensing at 80F to condensing at 85 degreesF... ...and in the larger scheme of things makes no difference when you are as close as you are.

If you look on the box that the zupco capillary tube came in, you will see that it calls out a set condensing temperature for each tube size.... yet..... you will only be running that unit at that set condensing temperature about 1% of the rest of the time.

So much for 'exact sizing' ..can you see that? Its a critical insight that when understood will help you in big ways througout your career.


(My background is in custom built ultra low temperature machines, from -50F to near liquid nitrogen temperatures around 250 degrees F below zero... 6 compression stages.... a total of 20 horsepower to get the same amount of refrigeration effect (btu's per hour) that you can get with ONE horse power at just zero degrees F... I have done major projects nationally many into the hundreds of millions of dollar is not common for even professional engineers to have missunderstandings on these issues.)

I can stay with you as long as you need in order to understand the thermodynamics of these systems... If you are sure to rate my assistance so far *positively, I will hold the question open for you.

Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8768
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
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Customer: replied 4 years ago.

Thank you Phil, based on the data provided I think I will leave the system alone for now since it is running close to zero.

Just got back in to check on you, I apologize sincerely for my apparent disappearance. I've been working through some technical difficulties here in my office. I see you've been taken care of by a very experienced and well-versed associate. Thank you Phil.

Customer: replied 4 years ago.

based on the info provided by you and Phil, I am thinking that I should leave the system as is for know. do I have any problem with doing this? when I service it in the future I may shorten each cap tube to 79" from the current 120". if I am correct the correct length should be 79". do you agree?

The proof of the pudding is in the eating. and this thing is making great pudding right now. I know how nerves can be shaky when you stray from the literature, but in this business, the literature never covers all bases. the literature provides you a reference, this reference assumes exact design parameters are in effect.... which is almost NEVER the case. the extra length of the tube has apparently either not had any effect, or, has offset some other design hiccup that was stealing capacity from the system. either way you're getting results. If you had liquid flooding back to the compressor, or the head pressure running high, you would run the risk of compressor valve &/or seal damage, and my advice would be different. but as is, there's no indication of adverse operating conditions, so if it were me, I'd zip the unit up and walk away. Try not to stew on it too much. like I said the literature assumes perfectly accurate design parameters. If you take the literature as law in all situations, you will be pulling out copious amounts of hair on a semi-regular basis. Thanks again for affording us the chance to help out.