Cap tube question...

[JSU]

Hey, I currently have a dd unit with 10ft of .031 cap tube. I have come to the conclusion that is too long (suction line wouldnt freeze up no matter how much refrigerant I put in). I have some .041in laying around. Would 10ft of it help out the temps???? (10ft of .031 is 240w load which is way toooo much)

[Ssilencer]

Why dont you use just .028 cap tube instead?
BTW, what gas are you using and for what heatload?
It's imposible to anyone give you some advice with the info oyu posted.

[JSU]

k thanks! btw the suction on my compressor is realy small like less than a 1/4. My suction is about 5ft, so i guess ill add some tubing there. Guess I need to shorten my cap tube abit.....

[JSU]

I know that.... that is the size on my compressor as in factory.....

[JSU]

ok I just adjusted the cap tube length to 5ft of .031 and it went down to about (it was still dropping) -8c then I put a load of 1ohm with 5v and 40a (i believe its 25w) and the temp sky rocketed to 50f~ . This is with r404a and some propane low temps pressure is 15psi~ and high side is 200psi~. The temp out of the condensor is 85f~ when ambient is 80f~.

Iam almost out of propane do you think it could have become contraminated???

[kayl]

low side pressure should be lower, too much gas
should be about around 1PSIG

[JSU]

The compressor is realy small, I dont think it can do any lower with a high side psi of 175~

[100]

"1ohm with 5v and 40a (i believe its 25w) "

that sounds to me like 200w.

P = V x I

R = V / I

That would be 8§Ù @ 200W. Or did you mean you had an extra 1§Ù resistor?

P = I^2 / R
P without extra 1§Ù res = 200W
P with extra 1§Ù res = 177W

I hope that was helpful. I'm led to believe that peltiers are between 60 and 99% efficient, depending on the proximity of each pole.

BTW what CC&HP is your compressor?

[Epsilon]

Originally posted by 100
"1ohm with 5v and 40a (i believe its 25w) "

that sounds to me like 200w.



P = U*I

I = U/R :)

40A PSU maybe, but with 1ohm @ 5v, he will only draw 5A.
5*5 = 25w, so he is correct ;)

[Gary Lloyd]

Originally posted by pc ice
that is true..6ft would be closer. and you would need to extend you suction line to around 7 ft of 3/8 indiameter for the whole sution line with no small diameter spot's between.

5ft .031 with r507 give's me around a 250 watt heatload.holding -40c+ erea with single stage diy...

A 1/4 suction line is sufficient for the flow in these systems, although 3/8 is probably a little better so long as the velocity is sufficient for oil return. And shorter suction line is better than longer.

5 ft of .031, using R507, is tuned for about 290 watts of heat load. 7 ft would give you a little better temps at 250 watts load. With a lower load, a little longer would be better. 10 ft of .031 is good for about 200 watts. It all depends on the load.

[100]

Thanks epsilon! I misred and thought the pelt was drawing 40a :p: wow, that's a nice pelt indeed... :D

Gary, do you have charts for cap-tube lengths or do you just work it out in your head!?

[JSU]

Gary I got that but why is it goign to 50f with 25w of heat if it has a 290w heat capacity???

[100]

What condensor cooling do you have?
What temp is your condensor-out pipe?
What sort of design is your evap?

[Redwolf]

Restriction?

100 is right... could simply be just the evap.

Maybe the exit of the Captube is now plugged. Because of that dirty oil you poored out the other day.

[JSU]

The load is set for 200w with .031cap tube, can I add 2ft of .028 to make it handle 170w???

[Gary Lloyd]

Originally posted by 100
Thanks epsilon! I misred and thought the pelt was drawing 40a :p: wow, that's a nice pelt indeed... :D

Gary, do you have charts for cap-tube lengths or do you just work it out in your head!?

The existing cap tube charts assume that you want fastest pulldown to a target temperature. If we make the cap tube more restrictive than this, the pulldown is slower, but a lower evaporating temperature is achieved. However, going too restrictive causes skyrocketing temps at heavy loads.

On these systems the goal is to reach lowest possible evaporating temperature without regard to pulldown time, thus the ideal is to allow just enough refrigerant flow to handle the highest expected heat load.

I am using existing charts as a starting point, but then shifting to more restictive sizing based on results reported in this and other forums. There are no charts designed specifically for our purposes.

[Gary Lloyd]

Originally posted by JSU
Gary I got that but why is it goign to 50f with 25w of heat if it has a 290w heat capacity???

Why is what going to 50F? Where are you measuring this?

What refrigerant are you using today?

What are the following temps?

Low side:

Evap temp
SST (saturated suction temp) or low side pressure
Suction line temp near compressor

High side:

Cond air in temp
Cond air out temp
SCT (saturated condensing temp) or high side pressure
Liquid line temp near condenser

[JSU]

That was with 5ft of .031in with r290 and some r404a. Now iam usin 5psi of propane and the rest r404a. Ill post temps in a minute

[JSU]

I have 4 resistors on a 1/2in piece of copper with a temp probe in the middle of that and a temp probe in the actualy evaporator. With 12v @ 12a The temperature sky rockets to about 50f then the resistors get to hot to keep running. This happenes no matter hpw much refirgerant I put in. I had the low side at 35psi once and it still culdnt hold the load. Ima currently vacuuming out the systemn and when I am ready to realy test it (the rest weerr just test runs) ill let you guys know the temps.