sluggeing on start up Help

[wetzel12345]

True freezer- box temp at -10 installed a compressor 200 btus less than old one. not a big deal, runs a little longer,
Had to run more piping to fit new compressor about a foot longer . Charge called for 12 oz , unit finally got down to temp rechecked charge 404a system 7 psi 75 F amb ,174 head sh was 40 degree.. ok low on charge, added charge to compensate, now the head @ 215 suction @ 16 , sh @15 , amb @ 70 degrees sc @10 JUst about 30 Delta Tfor condenser

NOW ITS SLUGEING ON START UP (ARG) it has a chincy accumilator in the evap section: if i just add a crank case heater will this definly solve my problem
:rolleyes::rolleyes::rolleyes:

[NoNickName]

Presumably the slugging is caused by condensation in the crankcase, because liquid from evap is trapped by accumulator, therefore I think the heater would solve the problem.

[Gary]

I assume this is a cap tube system.

I'm thinking the added charge is too much for the accumulator. i would remove refrigerant until the superheat is about 30F and see if that takes care of it.

[wetzel12345]

but why 30 degrees super heat, for a low temp system is supposed to be 4-6 or is this wrong for my cap tube system

[Gary]

but why 30 degrees super heat, for a low temp system is supposed to be 4-6 or is this wrong for my cap tube system

Are we talking about coil outlet superheat or compressor inlet superheat?

For clarification, you should put an F or C after your numbers.

[wetzel12345]

[quote=Gary;147891]Are we talking about coil outlet superheat or compressor inlet superheat?

For clarification, you should put an F or C after your numbers.[/quote

compressor sh and it is Fahrenheit... evap super heat 4-6 F degrees compressor 10-15 F why should i do 30 degrees Fahrenheit superheat? wont that starve parts of the evap???:eek:

[Gary]

Compressor manufacturers call for a minimum of 15-20F sh at the compressor inlet. 20-30F sh works just fine.

[US Iceman]

Compressor manufacturers call for a minimum of 15-20F sh at the compressor inlet. 20-30F sh works just fine.

They call for this much suction superheat or that is what they show the compressor ratings based on? Big difference in my book.

[Gary]

Liquid droplets in the suction line disappear between 5-10F SH, but I would consider this too close for comfort.

Back when I was doing the research for my books I contacted several compressor manufacturers. Some called for 15F minimum and others wanted 20F minimum to protect against flooding.

Consequently, in my books I call for 20F SH minimum. 20-30F SH seems to work very well.

[US Iceman]

wont that starve parts of the evap???


Yes, it will. Most evaporator manufacturers base their DX coil performance data on a specific amount of superheat (from the coil). If you exceed their value (whatever it is) the coil capacity will decrease.

BTW, if that's your last name you and I are probably distant cousins. Welcome to the RE forums.;)

[Gary]

Yes, it will. Most evaporator manufacturers base their DX coil performance data on a specific amount of superheat (from the coil). If you exceed their value (whatever it is) the coil capacity will decrease.


It should be stressed that you are talking about evaporator outlet superheat here and not compressor inlet superheat... and they are not the same.

[US Iceman]

It should be stressed that you are talking about evaporator outlet superheat here and not compressor inlet superheat... and they are not the same.

That's true, however the superheat has to come from somewhere. If it comes from the evaporator, which is more most superheat comes from, then it would cost you some capacity.

I suspect this system being worked on is very close coupled so the superheat would not in all likelihood come from the suction line.

Adding to this (but off topic somewhat) is that the additional superheat could cost you some compressor capacity too. Therefore you take a double hit on the negative impact this would generate.

[Gary]

Well... we can't see the system from here.

We know there is an accumulator in the evap section, so we want to be sure we are not confusing the coil outlet superheat with the accumulator outlet superheat.

Also, it is not unusual to have suction/liquid heat exchange on these systems.

This could be the cap tube soldered to the suction line or perhaps wrapped around the accumulator.

Any of this would increase the difference between coil outlet superheat and compressor inlet superheat.

[Gary]

Generally, it can be assumed that the designer would take all of this into account, so that a safe compressor inlet superheat would also result in an efficient coil outlet superheat.

[US Iceman]

One issue I worry about is if the suction superheat is too high (for any reason).

On low temperature systems the discharge temperature has a tendency to be elevated anyway (more or less depending on refrigerant used). If the total superheat (suction line + evaporator) starts to get too high the discharge temperature reaches increased levels and can cook a compressor.

Supermarket systems were/are terrible with this problem.

[Gary]

One issue I worry about is if the suction superheat is too high (for any reason).

On low temperature systems the discharge temperature has a tendency to be elevated anyway (more or less depending on refrigerant used). If the total superheat (suction line + evaporator) starts to get too high the discharge temperature reaches increased levels and can cook a compressor.

Supermarket systems were/are terrible with this problem.

I hear ya. A couple miles of suction lines can pick up a lot of superheat.

I can assure you that 30F inlet superheat does not result in high discharge temp.

[NoNickName]

I think that 30F (16K) is excessive too. The discharge temperature depends on isoentropic coefficient, pressure ratio AND superheating.
The higher the difference between evap and cond pressure, the lower superheating should be to counteract tendence of increasing discharge temp, especially on double stage recips.

[US Iceman]

All of this talk about superheat is getting off topic a little. (I'm guilty too)

The questions I have on the system are:


What happened to the compressor that was replaced?
Does it look like the accumulator was added to the system?
Was someone else working on this before you changed the compressor?

Ordinarily, adding a small section of pipe to refit a compressor would not be a big deal. One thing I don't know though is: What is the total capacity of the unit itself? 200 BTU's is not a lot of lost capacity, however, if the original unit was let's say 1000 BTU's then it's 20% short. If you were to say the unit capacity was 7,500 BTU's and the replacement compressor is 7,300 BTU capacity. then it should not be a big deal. Sometimes they don't match exactly.

[Gary]

I'm thinking this could all be settled by recording coil outlet SH, compressor inlet superheat and discharge temp while removing small amounts of refrigerant.

[wetzel12345]

Are we talking about coil outlet superheat or compressor inlet superheat?

For clarification, you should put an F or C after your numbers.

I was taught that the suction line should not exceed 60-65F because this will cause the discharge line to get above 225f and if this happenens copeland says it will start breaking down the oil 250f is the o crap temp and 275F is you are going to be replacing that compressor level

The original compressor was an aspera 2650btu/hr
that was mechanically bound new compressor is tecumsah 2450btu/hr

And the accumulator was in from factory

The suction line is soldered to the cap tube and last time i checked the discharge temp it was 171 F

Tonight i am going to installl a crank case heater and increase the SH up to 20 either be removeing some charge or unrapping some suction line so the liquid that possaby migrates down will boil off with the amb air fingers crossed.



Btw all the wetzels in my family orignatted from nurenburg germany and moved to weatherly PAback in 1763.

[US Iceman]

I think I would pull a little refrigerant out of the system gradually until the slugging disappears. The pull a little more until the superheat stabilizes at around 15° at the compressor suction.



Btw all the wetzels in my family orignatted from nurenburg germany and moved to weatherly PAback in 1763.


I believe that's right. Some of them moved to West Virginia, which has a county named after one of our great uncles, Lewis (more than one great, to be sure).

http://en.wikipedia.org/wiki/Lewis_Wetzel

My side of the family changed the spelling a little during the Civil War. Seems someone made some enemies.;)

[wetzel12345]

like i said it only does it on start up and for 10-12 secondes so like i said im going add the ccheater and take off some insulation to the suction of compreeor so it adds comp SH and hopefully the amb air will boil off the liquid if the cc heater doesnt a lil over kill i know but it should work

[US Iceman]

like i said it only does it on start up and for 10-12 secondes so like i said im going add the ccheater and take off some insulation to the suction of compreeor so it adds comp SH and hopefully the amb air will boil off the liquid if the cc heater doesnt a lil over kill i know but it should work

The crankcase heater should not hurt anything. but, I would wire it so that it is on when the compressor is off. You should not need the additional heat when the compressor is running.

What happens to the superheat at the compressor after it has run for a few minutes?

[wetzel12345]

just got back got a few numbers 141F discharge 262 psi head 80F amb 98F liquid line 16 psi suction -20F sat evap 20 F suction line thats with out 6 inch of insulation told her to call back if it does it more in a couple days

[Gary]

So... 141F discharge temp with 40F superheat.

Does anyone still think that 30F inlet SH will result in high discharge temp?

[wetzel12345]

well as long as the suction side is not abbove 60-65 F there theoretically should not be any issues

[Gary]

well as long as the suction side is not abbove 60-65 F there theoretically should not be any issues


Perhaps not a compressor issue (although a little cooler would be better for the comp), but a comp inlet SH that high 60 - -20 = 80F SH) may indicate coil starvation... unless that extra SH is being picked up in the HX.

I like to see 20-30F SH at the comp inlet (when the refrigerated space is down to design temp). If it is higher, I want to check the coil outlet SH to see if the coil is starving.

[NoNickName]

So... 141F discharge temp with 40F superheat.

Does anyone still think that 30F inlet SH will result in high discharge temp?

Yes, there's clearly something wrong there.
-38°F evap (16psi), 116°F cond (262psi), -20°F suction (which is 18°F superheating, not 40°F, if my math is correct) will result in a discharge temperature of 448°F with R22.

Check again.

[NoNickName]

And translating the units into metric:

-38.9°C evap (1.1 bara), 46.7°C (18.0 bara), -28.9°C suction (10°C superheating), discharge temperature 230°C

[Gary]

It's R404a.

And the suction line is 20F, not -20F.

And I'm wondering what formula you are using to predict discharge temp. It seems way off to me even if this were R22.

[NoNickName]

It's R404a.

And the suction line is 20F, not -20F.

And I'm wondering what formula you are using to predict discharge temp. It seems way off to me even if this were R22.

If the evap is -38F and the suction is 20F, then the superheating is 58F or 32K (wow).
The calculation for the discharge temp with R404a and these new data yields 289F which is 142°C, causing the protection device trip.
Double check with coolpack returns 115°C considering a real and adiabatic compression for a 15.8:1 compression ratio.



Does anyone still think that 30F inlet SH will result in high discharge temp?

Yes, I do.

[Gary]

If the evap is -38F and the suction is 20F, then the superheating is 58F or 32K (wow).
The calculation for the discharge temp with R404a and these new data yields 289F which is 142°C, causing the protection device trip.
Double check with coolpack returns 115°C considering a real and adiabatic compression for a 15.8:1 compression ratio.


The SST is -20F, not -38F.

[Gary]

Double check with coolpack returns 115°C considering a real and adiabatic compression for a 15.8:1 compression ratio.

Does coolpack mention that on average the temp drops about 50F/28K from the discharge valves to the discharge line?

[NoNickName]

The SST is -20F, not -38F.


16psi for R404a should be -38°F.
I'm not familiar with imperial units. Can you check for me, please.


Does coolpack mention that on average the temp drops about 50F/28K from the discharge valves to the discharge line?

That is your experience and mileage may vary. Besides, that is entirely irrelevant, because the discharge temperature is originated by the compression, and everybody knows that temperature reduces as refrigerant travels down the pipe.
Also off topic, as I still can't understand how this guy has got slugging with that amount of SH.

[Gary]

16psi for R404a should be -38°F.
I'm not familiar with imperial units. Can you check for me, please.

On my pressure/temperature chart R404a @16psi = -20F/-29C saturation temp.

[Gary]

That is your experience and mileage may vary. Besides, that is entirely irrelevant, because the discharge temperature is originated by the compression, and everybody knows that temperature reduces as refrigerant travels down the pipe.

It is relevant because we judge the health of the compressor environment based upon the optimum discharge line temp, not the optimum discharge valve temp.

For example, I consider 140-160F to be optimum discharge line temp. The actual discharge valve temp would be 190-210F.

[Gary]

Also off topic, as I still can't understand how this guy has got slugging with that amount of SH.

He has added more refrigerant than the nameplate calls for. During the off cycle the extra refrigerant is condensing in the evaporator, filling the accumulator (which is sized for the nameplate charge) and then overflowing down to the compressor. The compressor is slugging on startup.

[US Iceman]

Come on guys, don't start in again.