refrigerant pipe installation practice

[jedax81]

Hi guys,
I still don't get it why we don't insulate liquid lines in split a/c application. Is this has something to do with subcooling?

According to my analysis, since liquid is formed through heat rejection in the condenser, it will have a tendency of removing more heat while it is exposed to ambient till it reaches to the evaporator unit.

In that case what if the ambient is too hot? boiling temperature of a R407c is -42 degC.

I really don't have a clear explanation on this. So if someone out there can explain it well, I would appreciate.

[US Iceman]

This is an interesting subject...

If the liquid line is warmer than the ambient temperature it is rejecting heat to the air. That is a good idea. If the liquid line is insulated the insulation slows down the heat rejection, which would not be helpful.

If the liquid is colder than the ambient temperature and the line is not insulated the pipe will absorb heat from the ambient and reduce the subcooling. Again, this is not helpful.

IMO, this is a matter of heat transfer and accomplishing what you need to ensure the refrigeration system performs as designed.

[Segei]

I think that heat transfer won't be significant(one direction or another). Heat transfer coefficient is low(air and natural convection). Surface of the liquid pipe is small(no fins). Temperature difference between liquid refrigerant and ambient air isn't significant as well.

[jedax81]

I would agree with iceman and would agree with segei as well. It will have a minimal amount of heat rejection or absorption because of no fins and forced convection. Heat transfer coefficient on copper is good for conduction but on natural convection it is minimal.

As per our design, the condensing temperature is at 60 degC & ambient is 43 degC. So it is reasonable to have our liquid line un-insulated.

Thanks to you both Segei & Iceman.

[RANGER1]

I thought at the outlet of condensor ,the liquid line has a orifice so liquid line is always cool ( like a pre expansion ) . Don't know the reason as its just part of its dasign .
That is why it is insulated .

[jedax81]

Correction, that 60 degC condensing temp is at saturated vapour line. So the temperature may go down to certain degree wether below of that 43 degC ambient or above. The software Coolpack can tell its exact liquid saturated temp.

I never thought an orifice would also installed at the outlet side of the condenser. As far as i know it is only in evaporator. If it is used as a pre-expansion then that would dropped down the pressure without enough subcooling.

By the way guys, how can you decide whether that subcooling or superheating is enough for that design? Do you have reference or formula? I've been searching for its formula but find none.

[US Iceman]

If the saturated condensing temperature is 60°C and the condenser provides no subcooling, then the liquid leaving the condenser is 60°C. In an ambient of 43°C there is a 17° temperature difference. I agree the heat transfer is minimal by convection or radiation but it does exist nonetheless. Therefore in this example the liquid line is trying to subcool as much as the temperature difference and heat transfer mechanism will allow.

This whole argument though is based on the liquid line in a purely horizontal plane where you are only dealing with pressure loss due to friction in the pipe & fittings. If the liquid line flows in an upward direction you will need more subcooling to offset the static head penalty (and that is larger than the friction loss in a well designed piping system!).

[jedax81]

Iceman,
Theoretically it will come out the same pressure & temperature on the condenser but if you look on the p-h diagram of R407c (for example) the temperature is slightly offset & will have different saturated liquid & vapour temperature. This thing confuses me though I would still agree with same temperature & pressure.

Regarding the subcooling & superheating, I'm talking about deciding the value of it. Like if I have a certain design should I just decide to have 2 degC subcool & 5 degC superheat so I can get an analysis in p-h diagram?

[US Iceman]

I did not consider the glide so there can be slight differences of course. I was thinking of a pure refrigerant or azeotrope.



Regarding the subcooling & superheating, I'm talking about deciding the value of it. Like if I have a certain design should I just decide to have 2 degC subcool & 5 degC superheat so I can get an analysis in p-h diagram?


Do not use arbitrary values. The amount of subcooling required is determined by the friction loss and/or static head penalty of the liquid line size and installation. You need just enough subcooling to ensure the liquid quality is 100% liquid to obtain the rated capacity.

However, if you provide additional subcooling the TXV capacity can increase which may be helpful.

Superheat: you need just enough to keep liquid out of the compressor. More is bad...

[Oregonbythesea]

Iceman, your are so right, now, if I could just get customers to understand that. In my line of work, commercial fishing, the users always want to tweak the settings. The hardest thing to make them understand is ice forming on the suction valve of the compressor is not a bad thing.

[Bachuss]

Hi Im now in my 3rd year of refrigeration and wondering if you would clarify what I have taken this thread.

If you have sufficent sub cooling to Just fill the sight glass, you should work out pressure drop from the sight glass (including pipe length, bends and componants) to the TEV and add subcooling equivilant to the comparative temperature to compensate?

[sterl]

On a single compressor circuit: Sight glass should be right next to the Liquid Solenoid, which is itself the biggest player in causing vapor generation upstream of the TXV.

But the objective remains: The liquid should remain liquid state until the Expansion point....And thus a little subcooling is required. On an Air Cooled System: the condensing liquid is itself always going to be warmer than the air going over the condenser and you will not be able to "ambient subcool" to anything below that temperature, so the only thing that can happen in a liquid line run through that same air is that the liquid gets cooler. IF THE CONDENSATE LINE NEEDS TO RUN THROUGH A WARMER THAN AMBIENT SPACE it should be insulated...otherwise it should not be.