What happens when capacity of system is exceeded?

[cyfi66]

Thank you everyone for all the input, I think we have covered just about everything I can do on this car, I will think about whether or not I'm going to continue trying.

I am curious about some refrigeration theory though.

Lets say I am aiming for a 40 degree evaporator core - according to R134a PT chart the saturation pressure would be 35psig

In theory it doesn't matter what the head pressure is. If I have a clear sight glass, flooded evap and 35psig on the low side the evaporator should be 40 degrees across the entire core.

So why does head pressure matter if you've got pure liquid to the TXV, the TXV can open big enough to flood the evaporator and the compressor can push enough volume to keep the low side at 35 psi? I would think higher head pressure and higher high side temperatures would give better heat transfer because of higher deltaT at the condenser.

In this case it seems like bigger compressor would be the best benefit as it could continue to pull the low side down as the TXV opens more to completely flood the evap.

One of my concerns with going with a larger capacity TXV in this particular car is that it would open bigger to maintain superheat and low side pressure would increase making the temperature of the evaporator higher.

[tbirdtbird]

Theory, if correct, is one thing. Doing actual testing is another.
Write again once you do the water test

[JohnHere]

In theory it doesn't matter what the head pressure is. If I have a clear sight glass, flooded evap and 35psig on the low side the evaporator should be 40 degrees across the entire core.
So why does head pressure matter if you've got pure liquid to the TXV, the TXV can open big enough to flood the evaporator and the compressor can push enough volume to keep the low side at 35 psi? I would think higher head pressure and higher high side temperatures would give better heat transfer because of higher deltaT at the condenser.

Seems to me that it definitely does matter what the head pressure is because the higher the high side goes at, say, 85F ambient, the less condensing you'll get and the less the refrigerant changes state from a gas to a liquid.

So if that's the case, you wouldn't have the conditions you mentioned at all: A clear sight glass (which isn't a factor with R-134a and R-1234yf anyway), a flooded evaporator with only liquid refrigerant, and 35 PSI on the low side.

Just the opposite would occur: The low-side pressure would climb, the evaporator would be full of primarily gas, not liquid, and cooling would suffer proportionally.

You mentioned superheat elsewhere, but that isn't normally considered or discussed in MVAC because of the constantly changing operating conditions.

[swampy 6x6]

hi

low side freezing point temp is around the 28-30psi
On your gauges commonly seen 20--40psi but usually 25---35 psi . Yes there are exceptions

High side
R12 was common at 130--150psi in refrigeration
convert this to temperature
R12 at 130psi == R134a at 150psi

SUPPLYING THE EVAP WITH COLD COLD 100% liquid REFRIGERANT NOT gaseous mix

--Supplying ******consistent*** low temp refrigerant
high side ,must be consistent 150 psi idle 160psi 1500rpm 10psi rise . This supplies consistent temp liquid to evaporator
high side must be as low as possible down to 150psi . Lower psi = low temp liquid

****** Supplying evap with correct 100% full charge , to eliminate gaseous mixture supplied
Under charging is a threat to evap performance ,, many OEM but not all charge by weight to avoid excessive high psi in the high side gauge readings
Under charge limits evap performance

******* Tx valve operation
1/ By supplying correct psi 150 /low temp to tx when the high pressure/temp converts to low temp/pressure refrigerant . This consumes evaporator BTU cooling capacity to do this .
2 / Once thru tx valve ,refrigerant goes turbulent
The problem is the more turbulent = higher psi/ higher temp supplied refrigerant
150psi /temp once in evap performs far better by at least 10--20% compared to 250---290psi/temp .
The gas liquid ratio is far far better the lower the high side temp/psi occurs

Refer to a R134a enthalpy chart . Gives engineering data on refrigerant in a refrigerant system and shows how all parts interact

[DetroitAC]

Wow swampy, so many words typed and so little understanding of how things work. You even know there is a pressure vs enthalpy/entropy chart, but don't understand what it says.

[Tim]

Wow swampy, so many words typed and so little understanding of how things work. You even know there is a pressure vs enthalpy/entropy chart, but don't understand what it says.

If it takes 7 pages of posts to get to the bottom of the problem. You know the post has gone wonky.

[DetroitAC]

Thank you everyone for all the input, I think we have covered just about everything I can do on this car, I will think about whether or not I'm going to continue trying.

I am curious about some refrigeration theory though.

Lets say I am aiming for a 40 degree evaporator core - according to R134a PT chart the saturation pressure would be 35psig

In theory it doesn't matter what the head pressure is. If I have a clear sight glass, flooded evap and 35psig on the low side the evaporator should be 40 degrees across the entire core.

Correct, but only for a theoretical evaporator that has no pressure drop.

So why does head pressure matter if you've got pure liquid to the TXV, the TXV can open big enough to flood the evaporator and the compressor can push enough volume to keep the low side at 35 psi? I would think higher head pressure and higher high side temperatures would give better heat transfer because of higher deltaT at the condenser.

You are correct, higher condensing pressure means more heat transfer. In reality though, the condenser rejects exactly the amount of thermal power it is being fed. The condenser pressure floats up or down so that it can reject that supplied thermal power.

It matters because higher pressure ratios (discharge pressure/suction pressure) reduce compressor massflow. The lower you keep the condensing pressure, the more volume of suction gas the compressor will pump, and therefore the more refrigerating power at the evaporator.

In this case it seems like bigger compressor would be the best benefit as it could continue to pull the low side down as the TXV opens more to completely flood the evap.

One of my concerns with going with a larger capacity TXV in this particular car is that it would open bigger to maintain superheat and low side pressure would increase making the temperature of the evaporator higher.

If the existing TXV was within it's ability to flow liquid refrigerant, a larger TXV won't do anything significantly different. If the existing TXV is able to control the superheat then it's not fully open and it's within it's ability to control superheat to the value it's set for. If you put in a larger TXV and adjust it to control to the same superheat at the same conditions, system performance will be exactly the same. For example a smaller TXV might be 80% open at very high loads, still in control. A larger TXV might be 60% open and in control. TXVs and their behavior are about as complicated as AC systems so many other things will happen at different conditions. Bottom line is the TXV is a passive device, it's not an easy drop in performance boost.

[cyfi66]

You are correct, higher condensing pressure means more heat transfer. In reality though, the condenser rejects exactly the amount of thermal power it is being fed. The condenser pressure floats up or down so that it can reject that supplied thermal power.

Can you expand on this one? What determines the supplied thermal power at any given load? Is there a point at which the condenser cannot reject the supplied thermal power? Or is that when the pressure relief valve pops off?

It matters because higher pressure ratios (discharge pressure/suction pressure) reduce compressor massflow. The lower you keep the condensing pressure, the more volume of suction gas the compressor will pump, and therefore the more refrigerating power at the evaporator.

This makes sense. Why does increasing the compressor volume increase the refrigerating power at the evaporator though?

If the existing TXV was within it's ability to flow liquid refrigerant, a larger TXV won't do anything significantly different. If the existing TXV is able to control the superheat then it's not fully open and it's within it's ability to control superheat to the value it's set for. If you put in a larger TXV and adjust it to control to the same superheat at the same conditions, system performance will be exactly the same. For example a smaller TXV might be 80% open at very high loads, still in control. A larger TXV might be 60% open and in control. TXVs and their behavior are about as complicated as AC systems so many other things will happen at different conditions. Bottom line is the TXV is a passive device, it's not an easy drop in performance boost.

That is kind of my point - my theory is that upon inital startup under maximum load the TXV is wide open and still underfeeding the evaporator. Because upon initial startup max load the low side line from the firewall is cool to warm even with low side pressure around 40psi. After 15+ minutes the line becomes cooler to cold. It seems superheat is too high when under maximum load(the time I really want the most AC).

The concern is overwhelming the compressor by increasing the TXV tonnage - the bigger TXV will flow more resulting in increased low side pressure since the compressor volume cant pump enough to drive the low side down anymore. The result will be the same undesirable warm air from the vent. I would anticipate superheat would be correct at this point but the low side pressure would be higher than before.


Regarding the 7 pages of posts - this thread isn't and never was about solving a particular defect - it is about modifying a system beyond what it was designed to do. Something difficult to do without a lot of system design engineering knowledge.

[tbirdtbird]

You are repeating yourself. There are engineers, and there are practical hands on experts

[cyfi66]

You are repeating yourself. There are engineers, and there are practical hands on experts

There is no MVAC engineering forum in existence. Nor are there any textbooks that have the kind of info I am looking for that I can find. Technical hands on experts can really help understand the theory too. If the discussion is that much of an annoyance I'll let the thread die, but its just a discussion.