some general advice on System design
Hello,
I am new here, and after being grilled by the Registration process, I am really glad to be here among you experts.
I have a design question, and I would really appreciate your opinion.
I have a situation where a small 12V R134a camping freezer design is not as efficient as I would like it:
1) There is sweating / condensation around the door sill
2) The return line to the compressor ices up, while the cooling inside is not very effective (even after playing around with gas pressure).
So I asked for a Yoder loop to be plumbed in between the condenser and the filter dryer to address the condensation issue.
I then also asked that the length of the evaporator tube (number of turns around the inner compartment - it is a chest freezer design) be increased - to address the ineffective cooling while the return line is icing up.
I realise gas pressure etc all play a role in the icing up issue. My theory is that the longer evaporator as well as Yoder loop will make the whole system a little more tolerant of variations in gas load, compressor speed, freezer temp, ambient temp, and so on. To my mind the compressor will be under the same or less strain, and it won't suck liquid/near liquid back in.
The design engineers however tell me that the current length of the evaporator tube is calculated correctly for the small compressor (Secop PBC 2.0 and 2.5 for the bigger models) that they are going to employ.
They reason that the longer pipe work will strain the compressor, while I feel the longer pipe work will make the life of the compressor easier, and make the whole system a bit more lenient.
What do you think?
I will really appreciate any ideas / suggestions / feedback you can give me.
Regards
Leon
Re: some general advice on System design
Leon,
First things that stand out, is door seal sealing 100%
Also do tubes have good contact with wall of freezer, as if there is not very good contact, poor heat transfer (assuming it has internal skin inside box with tubes attached).
Not sure what they do on domestic, but something like heat transfer paste.
Re: some general advice on System design
Quote:
Originally Posted by
RANGER1
Leon,
First things that stand out, is door seal sealing 100%
Also do tubes have good contact with wall of freezer, as if there is not very good contact, poor heat transfer (assuming it has internal skin inside box with tubes attached).
Not sure what they do on domestic, but something like heat transfer paste.
Thanks Ranger1,
Yes the door seals and other isolation aspects are fine.
It will be tubes that have to make contact with an aluminium skin - I agree poor contact will cause poor heat transfer. I will investigate how they do this to rule out (or in!) this possibility.
Regards
Leon
Re: some general advice on System design
More info needed Leon.
Capacity of fridge/ freezer ( Liters.)
Copper tubing length and pipe size used for the Evaporator....or the number of turns and pipe size.
Box insulation thickness.
Condenser? Static or fan forced.
Re: some general advice on System design
Quote:
Originally Posted by
mikeref
More info needed Leon.
Capacity of fridge/ freezer ( Liters.)
Copper tubing length and pipe size used for the Evaporator....or the number of turns and pipe size.
Box insulation thickness.
Condenser? Static or fan forced.
Hi Mike,
I don't have all these details, I will have to find out some of them.
One example:
60 liters capacity
Evap tube length = 11.3m, 6 loops around the inner aluminium bin (will have to get the pipe size).
Secop PBC 2.0 compressor, used at 2 speed settings (I think 2500 rpm and 3500rpm, but not sure now).
The condenser is fan forced - copper tubes with aluminium fins.
Box is 65mm insulated with a foam who's name I forget
But I am trying to get your opinion on my general theory, rather than specific help on this specific example:
My theory is that the longer evaporator as well as Yoder loop will make the whole system a little more tolerant of variations in gas load, compressor speed, freezer temp, ambient temp, and so on. To my mind the compressor will be under the same or less strain, and it won't suck liquid/near liquid back in (so much).
The design engineers however tell me that the current length of the evaporator tube is calculated correctly for the small compressor (Secop PBC 2.0 and 2.5 for the bigger models) that they are going to employ.
They reason that the longer pipe work will strain the compressor, while I feel the longer pipe work will make the life of the compressor easier, and make the whole system a bit more lenient.
Regards
Leon
Re: some general advice on System design
Copper tubing exposed inside your 60L is fine, so long as it is fed from the bottom and has a suction accumulator installed before suction line exits refrigerated space. 5/16th tubing is recommended as 1/4 Inch is too restrictive.
General Theory? Your compressor speed should be set by the control circuit resistor. No resister = 2000 RPM. Resister value ( Ohms and at 1/4 to 1/2 watt) determines compressor speed...up to a maximum of 3500 RPM.
Extra tubing length in the Evaporator is an advantage since you have sub zero temps at the compressor. Oh, running a heat circuit for condensation issues should be tapped directly from the compressor, not on your exit from the Condenser :)
Re: some general advice on System design
Quote:
Originally Posted by
mikeref
Copper tubing exposed inside your 60L is fine, so long as it is fed from the bottom and has a suction accumulator installed before suction line exits refrigerated space. 5/16th tubing is recommended as 1/4 Inch is too restrictive.
Thanks Mike, I will read up on what a suction accumulator is.
But I take it you agree that extra length evaporator (inside the cooling space) will not strain the compressor by adding resistance in the pipe (if wide enough), but could actually help the heat transfer inside the freezer a bit.
Also when you say fed from the bottom, is that also a standard design consideration? Must it flow upwards so that the fluid will remain near the entry point, and the boiled off gas would rather be at the return line side? Makes sense.
Quote:
Originally Posted by
mikeref
General Theory? Your compressor speed should be set by the control circuit resistor. No resister = 2000 RPM. Resister value ( Ohms and at 1/4 to 1/2 watt) determines compressor speed...up to a maximum of 3500 RPM.
In this design the freezer controller can present two different resistors to the compressor controller, giving the machine a "LOW" and "HIGH" speed. I forget which speeds those are at the moment, but my point was that I think the system dynamics will change when the speed changes. E.g. maybe in LOW it won't ice up the return line, but in HIGH it might because it is forcing more coolant through the pipe work faster, while the ability of the evaporator to take up heat is the same (same length).
Quote:
Originally Posted by
mikeref
Oh, running a heat circuit for condensation issues should be tapped directly from the compressor, not on your exit from the Condenser :)
Are you sure about this? Everything I've read seems to show the Yoder loop comes after the condenser, and carries only very little residual heat. Putting it just after the compressor would make it very hot, the door sill will be very hot. There are even some forum queries about a hot door sill, and the reason was that the condenser was not effectively dissipating heat (clogged with dust or broken fan, etc), letting more of the residual heat flow into the yoder loop.
Thanks & Regards
Leon
Re: some general advice on System design
Quote:
Originally Posted by
mikeref
Copper tubing exposed inside your 60L is fine, so long as it is fed from the bottom and has a suction accumulator installed before suction line exits refrigerated space. 5/16th tubing is recommended as 1/4 Inch is too restrictive.
OK it seems they use aluminium with ID of 0.75cm. That is somewhere in between the 1/4 inch (0.635cm) and the 5/16 inch (0.793cm) you mention, closer to the bigger size.
Is aluminium OK? I understand the following potential problems:
1) welding aluminium & copper can cause problems long term as dissimilar metals could corrode each other etc.
2) aluminium has less heat conductivity than copper. Is this true?
Regards
Leon
Re: some general advice on System design
Wow. Post 7 Question 1. Standard design would have a ripple plate Evaporator that is a match for low and high speed. Accumulator is built in.
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Q.2. Low and High speed is simply two resistors, giving you two specific compressor revolutions.
To change compressor REVS, one has to change the value of the Resistors.
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Q.3. We are talking about a small 12 Volt Freezer. ( Yoder Loop?) Anti sweat, using discharge heat in small appliances doesn't generate high temperatures.
If the condenser is clogged...clean it. If the Condenser fan kicks the Bucket....replace it.
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Regards, Mike.
