How to find proper compressor?

[jayryu]

I have a walk in freezer it is not cooling enough at all. The condition is as follows.
Designed box temp.: -5F
Actual box temp.: 14F
Box size: 6 x 6 x 6.4ft
condensing unit : unknown.
compressor : Tecumseh AJ2425A
TXV : Sporlan GFE-1/2-ZP
Unit cooler: Trenton Low profile 1 fan unit, unknown model #.
Gas : R414b

I think the compressor is not enough capacity to cool down the box design temp. So I am going to change the compressor to more proper one of R404a compressor but I don't know what size of compressor it needs. Of course I have to change the TXV too. I imagine original compressor was more larger capacity, but someone has changed the compressor only before w/ some reason. The receiver outlet tube size of condensing unit is 3/8" and suction tube is app. 5/8".
I think the compressor has to be of at least 3500 btu/hr (-20F evap temp) capa. for the size of the freezer box. The owner want to change the compressor only due to the cost.

The question is if I increase the capacity of compressor only to 3500 btu/hr (-20F evap temp) one . it will be ok with the condensing unit?

[chillerman2006]

Hi jayryu

sorry to say your method will not work

to design a system you have to calculate your heat removal requirement first

that will then enable you to specify an evaporator

then next you match up condensor/compressor/txv to that evap

increasing the size of a compressor in a already designed/balanced system

will just cause problems that can only be corrected by going back to designed spec compressor

R's chillerman

[jayryu]

Thank you chillerman2006,
I have calculated the heat load of the wif box about 3800 btu/hr.
AT present, I think the system is obviously unbalanced. The compressor looks not original. Evaporator capacity looks 3800 btu(at -20F), and compressor is only 2500 btu/hr(at -10F evap temp.). Therefore I think the original compressor has been changed to this present compressor w/ some reason. I am doing my best to reduce the cost to minimum. With this condensing unit, is it possible to change the compressor only to R404, low temp abt. 3600 btu/hr compressor?

[chillerman2006]

Hi Jayryu

from my minimal understanding of design

your compressor needs to be a match for the evaporator or very slightly oversized

maybe someone can correct me but I would be looking at a compressor of 3800btu

hopefully your txv is sized correctly for mass flow and only the power assembly needs changing for R404a

R's chillerman

[chemi-cool]

By using a bigger compressor you will not solve anything. All the parts have to fit each other.

after you have calculated the heat load, you need the right capacity evaporator,the refrigerant you want to use, the right size TXV, the right size condensing units.

No cutting corners in refrigeration, only in carpentery, choose your side.

[mikeref]

Jayryu, How do you know compressor is not origional?

[jayryu]

The compressor model is AJ2425A. Condensing unit with that size of compressor has to have 1/4" liquid line, but the liquid line size is 3/8". That means the condensing units is more than 3500 btu/hr capacity minimum with R-12 system. One more thing, TXV valve size at present is 1/2 nominal ton. But with that size of compressor, TXV has to be 1/4 nominal tonnage if the compressor is original. Therefore I imagine the original compressor was at least 3500 btu capa. and it was replaced with this small size compressor by someone with some reason.
This condensing unit works properly as SH -- 23 F( at compressor suction), SC -- abt. 14 F.
But the best box cool down temp. is 14F.

[jayryu]

Hi Chemi-Cool,
I appreciate your post, but I myself knows very basic. I can choose easy way, but I try my very best for my customer always. Replacing evaporator, c/u is the most easiest thing and profitable. But I do my best to save the money for my customers anytime possible. At this matter if I had enough data of Evaporator, condensing unit, etc. of course I have already changed the compressor only, it saves a lot of money for the customer.
But this time, there were no data at all and only my experience and guesses. Therefore I wanted every recommendation of you gentlmen. Thanks again.

[nike123]

You could take measurements of evaporator dimensions, number of pipe rows, number of pipes and pipe diameters, fins spacing. Also, size of fan and number of blades, blade angles, fan rpm. Also, picture taken by digital camera will help to recognise what model of evaporator you have.
That will give us full picture of evaporator and we could then tell if evaporator is of right capacity.

Same goes for condenser.

By my quick calculation you need at least 5000 btu of capacity if your insulation thickness is 4" (floor, roof, walls) and light loading conditions with 18 hours of run time and ambient of 90°F.
Therefore changing compressor to 3500 btu will not help even if evaporator and condenser are matching to that compressor. Compressor will be running non-stop and maybe never reach -5°F at ambient near 90°F.

[mikeref]

Jayryu, from me quick search, the AJ2425A, or Z, has a capacity at -23.3 C of 2450 BTU. Only have current lists on compressor capacity handy at the moment and some of those are rated for only -15C. I always try and get as much history of machinery i work on from the customer so as to figure out the kinks, so to speak. Do you know if this freezer was getting to temperature, with current compressor on board any time before you started on this job?..Mike.

[jayryu]

The customer bought the restaurant only recently and is renovating the lot. He does not know any history. Even if he can contact the former owner, he would not get honest answer as a matter of fact.

[jayryu]

I have replaced condensing unit & TXV. Condensing unit is 3800 btu unit at -20F evap temp. Everything is working fine. Thanks everybody.

[chemi-cool]

Glad to hear.