SC and SH changes as the ambient fluctuates

[Chef]

The system parameters are condensing temp is 35C and subcooling is 2K, evaporating temp is -10C and superheat is 4K at the outlet of the evaporator.
The air into the condenser is 30C.

Without any change in the evaporator temps the first question is what do you think the system parameters would look like if the air temp rose to 40C.

Its an R12 (old old unit) with cap tube inside the suction line and has a condenser fan and is sealed so it is critical charged and as far as I know its a rotary compressor.

Its possible the temp of the air in could drop to 20C but I doubt that and think 25C would be the lowest but thats the query I have - what will be the SC and SH at 20C air in?

Any Ideas?

Chef

[Tesla]

I haven't measured the SH or SC much on R12 but have worked on many R12 fridges when I was young. It would behave just like a domestic fridge or bottle cooler. When the ambient temp drops the condenser backs up with liquid reducing the surface area for condensing keeping the head pressure up (rule of thumb about 21k above ambient equivalent condensing pressure). And vise versa. There are bottle coolers that operate below 0C and 40C with out any probs. But when I worked in Brisbane a company would remove some refrigerant when it got above 40C to keep them running and it worked. On some very big systems they trim the charge for hot/cold seasons to improve efficiency. No doubt the SC will increase a little in cooler ambients. Back in the days of R22, R502 and R12 we generally charged the system to pressures not weight of refrigerant and it was common to control temp via pressure, even humidity for meat coolers. We would run into problems with over condensing at about 0C or below and partially cover the condenser for very cold ambients. We would measure the SH when down to temp at the end of the cycle and aim for 6k SH at comp for the safety margin.

[Chef]

Hi Tesla
The increase of SC as the air temp falls is as we expected as well and we both think the SH will increase as well.

The issue comes as the ambient goes to 40C, as I say the SH will fall till the SC reaches 0C (at about 35C ambient) and will then rise after that as ambient gets hotter. He on the other hand says the SH will continue to fall as ambients increase. This is the sticking point?

Chef

[mad fridgie]

The question is if the ambient rises, then does the load rise, and due to higher SCT a reduction in nett refrigeration capacity?
This being the case would your evap run out of liquid?
If so SH would increase or does the SST just follow the load and cabinet temperature?

[Chef]

The question is if the ambient rises, then does the load rise, and due to higher SCT a reduction in nett refrigeration capacity?
This being the case would your evap run out of liquid?
If so SH would increase or does the SST just follow the load and cabinet temperature?

Well this is the key point, if the ambient increases a little the SC falls and pushes more liquid into the evap which would look like a reduction in load so a drop int SH.

But after the SC has passed 0C then less and less liquid is passed to the evap and so it appears as an increase in load and so the SH will now increase, thats what I think but he insists it will still fall.

I know the SST will follow the load as the capacity is reduced but it was a question where too many variables changing would confuse the issue so we decided to agree the evap would remain constant - wrong but just for the discussion!!!

Chef

[mad fridgie]

So lets say that you SST is constant and the load is constant, the only thing that changes is the liquid pressure and temperature.
So at high ambients you would have high pressure and high temp with no liquid sub cooling. Your vapour to liquid fraction would increase (more vapour) so less liquid would mean less refrigeration effect, so the coil would be deemed to be starved, there fore the load and sst are constant then the extra load there for must be absorbed by the total vapour content, thus increasing the superheat.

[TRASH101]

Well this is the key point, if the ambient increases a little the SC falls and pushes more liquid into the evap which would look like a reduction in load so a drop int SH.

Is this right? If the SC falls then so does the liquid density and the only thing that I can see that would allow more volume through would be a reduced frictional loss due to the fall in density as the SCT remains 35C until the SC is less than 0k.
The way I see it is that the refrigerant enthalpy (with r12) from SC 2k - 0k is 231kj/kg to 234kj/kg.

[Chef]

Is this right? If the SC falls then so does the liquid density and the only thing that I can see that would allow more volume through would be a reduced frictional loss due to the fall in density as the SCT remains 35C until the SC is less than 0k.

If the SC falls due to a rise in ambient it is because the pressure has also risen pushing all of the liquid into the evap, when the SC is zero there is no hold up in the condenser and all the liquid is in the evap. I am not sure there would be a great difference in the liquid density.
It is more about where the charge is located and the pressures.

Its a good point about the frictional loss changing - it does change a lot with changes in amount of SC.

What allows more volume through is the rise in pressure in the condenser until the SC=0C.

Chef

[Chef]

so the coil would be deemed to be starved, there fore the load and sst are constant then the extra load there for must be absorbed by the total vapour content, thus increasing the superheat.

Thanks Mad
It is not easy to convince some of the way it works but I agree with you and given the constraints it is the only solution, we will see if I have a convertee?

Chef

[lana]

Hi there,

Everything depends on the initial design conditions and equipment selection.

Let say that the condenser is selected for TD=10°C with ambient 30°C discharge temperature would be 40°C with say 4°C SC. At design conditions the cap tube is selected to give Pc-Pe pressure drop.
When ambient temp rises then Pc rises (also Pe) and therefore the same cap tube passes more refrigerant into the evap. If evap load is constant then SH will fall. But if evap load in increased due to ambient temp rise then this statement is not true. Amount of SH change will depend on the load increase.
SC on the other hand will drop due to high ambient temp. BUT this also depends on the capacity of the condenser. If condenser has not enough capacity then Pc rises a lot and system parameters will change accordingly.

Cheers