Any benefit in insulating the Low side AC line?

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I have heard of people doing this but I have not seen it practiced quite oftem. What I am wondering is if I insulate the Low side AC line (the cold one, coming from the evaporator and going to the compressor) and prevent it from absorbing the heat from the engine, would it allow the AC system to:
1. operate more efficently?
2. lengthen the time between compressor 'off' to 'on' cycles along with shortening the on cycles (save gas)?
3. reduce the compressor discharge temp, ultimately reducing the deltaT across the condensor and allow cooler air to the radiator?
4. reduce the Hi side line temp, which might reduce the liquid temp going into the evaporator (cools the air in the car's cabin), resulting in cooler air?

Is it possible that if that line is insulated, you could cause hydraulic lock and ruin the compressor, assuming that you did not have complete evaporation of the liquid in evaporator?

Cars it would pertain to are an 06 Mitsu EVO 9 and a 2003 Toyota Matrix.

Sorry about all the questions but thanks in advance.
 
Insulation will make the system more efficient, but it won't be dramatic.

I don't know if the compressor on your car cycles. If your compressor cycles, or if it has a variable compressor, then there should be a small improvement in mileage. If your car uses the engine heater to modulate the cabin temperature when you have the air conditioning on, then no effect on mileage. Insulation would still speed up the cooling a tiny amount, plus it would help the maximum cooling on a really hot day.
 
My MB had insulation over this line from the factory. After 30 years it has gotten old/hard and flaked off (original AC still works great though).

I say yes, if the routing is such that it takes the hose by hot components (like in my MB) but be sure to use the right insulation and adhesive to prevent moisture and corrosion. if it runs with plenty of space from anything hot, I wouldnt worry much.
 
I think manufacturers leave that last section of the suction line uninsulated in case any of the refrigerant is left as liquid after leaving the evaporator. The compressor is made to compress gas, not liquid. By leaving the line uninsulated for that last bit before the compressor, any liquid refrigerant should pick up heat and turn into a gas before hitting the compressor. I think.

Anyway, don't you think that if insulating it had any benefit that the car makers would already be doing it?
 
There's a reason why it's uninsulated.

The whole point of the compressor is to add work to the refrigerant in the form of raising it's temperature. The compressor compresses the vapor entering it and discharges a high temperature/pressure liquid. The liquid loses heat in the condensor - the radiator-looking thing in front of your engine coolant radiator - so that the liquid refrigerant exiting the condensor is lower than the inlet. Next, the refrigerant will pass through an expansion valve and into the evaporator. Pressure and temperature are lowered across the expansion valve so that what enters the evaporator - the thing your fans blow across to give you cold A/C - is a mixed liquid-vapor at a lower teperature than that of the interior of your car. Heat is absorbed by the refrigerant in the evaporator until it's fully vapor. At this point the compressor does it's thing again to compress the vaporize refrigerant to start the cycle again.

Insulating the line between the evap and the compressor will NOT make things more efficient. The whole point of the compressor is to raise the refrigerant temperature high enough to lose a significant amount of heat in the condenser. If the suction temperature is lowered because of insulation, the discharge temperature will also be lowered. After the evap you WANT the refrigerant to lost heat to ambient so that what enters the compressor is fully vaporized.
 
Of course it will help. Many cars have some insulation there, BTW.
A step in the right direction.
Also, keeping your home air conditioner out of direct sun has a big impact on it's efficiency.
 
Originally Posted By: berniedd

I've never heard of hydraulic lock in AC in cars.


More commonly called "slugging". Occurs most often when newbs charge an AC system with liquid refrigerant instead of gas when the system is running. It is actually not a concern on a normally operating system as the "accumulator" between evap and compressor inlet serves as a liquid separator to keep liquid from reaching the compressor.

Originally Posted By: gathermewool
There's a reason why it's uninsulated.
Insulating the line between the evap and the compressor will NOT make things more efficient. The whole point of the compressor is to raise the refrigerant temperature high enough to lose a significant amount of heat in the condenser. If the suction temperature is lowered because of insulation, the discharge temperature will also be lowered. After the evap you WANT the refrigerant to lost heat to ambient so that what enters the compressor is fully vaporized.


I don't buy that. The only job of the compressor is to compress the gas, that's it. The temp of the gas is raised as a consequence of compression (a la Boyle). If the temp of the gas into the condenser inlet is even 1 degree cooler (assuming the air going through the condenser is cooler than the gas in the condenser, else it wouldn't be a condenser I guess) then the liquid out of the condenser will be cooler as well. Not quite 1 degree cooler, but cooler nonetheless. This will lead to a lower evap temp if you have complete vaporization in the evap core. Insulating the return line will lead to overall lower evap temps. If you still have any liquid refrigerant at the evap outlet, then it is a moot point as the refrigerant boils at a specified temp/pressure. Sorry to be unclear, I guess thats why I never became a teacher.

I have more than one problem with insulating lines on cars though. How flammable is the insulation you would use? Is there enough metal in the return line exposed to ambient temps to make a signifigant difference? For the condensate that gets between the insulation and the line, what corrosion effect will it have on the line? I have pulled apart corroded aluminum, and rusted AC parts due to trapped condensate problems.
 
I'm not the very best at explaining things, either. The purpose of the compressor is to impart work on the system in the form of compression, to change the state of the fluid in the system. The consequence, as you say, is the purpose. A phase change takes place across the compressor and then again across the evap. More energy is required to change the phase of a liquid than to simply raise/lower it's temperature, so more energy is absorbed from your car's interior evaporating the mixed-phase refrigerant than running cool liquid through evap. What enters and leaves the condenser is a liquid.

The heat lost across the condenser is relative to ambient temperature – air flowing across and cooling the condenser – and the temperature of the refrigerant entering the condenser. A higher differential between the refrigerant entering the condenser and ambient temperature, the higher the efficiency.

I do concede that the main purpose of not insulating the compressor inlet line is to prevent a mixed fluid – vapor/liquid – from entering the compressor.
 
I sort of agree with gathermewool but not really. The whole job of the A/C is to pump heat out of the cabin air but for the pumping to be most efficient I think it helps for the low side to remain cool (low pressure) up until the point the refrigerant is compressed. The compressor is going to work to get the low side down to the low side switch setting.

So I think the compressor will work harder pumping the extra lowside pressure down. But if the A/C isn't maxed out it shouldn't make the cabin air any warmer, if that makes sense.

The expansion valve assure the evaporator liquifies all the refrigerant, and on orifice tube systems (GM and Ford) the accumulator in the low side catches any liquid.
 
Yeah but doesn't it stand to reason that the entire lowside from the compressor intake to the expansion valve or orifice tube are equalized in pressure. That is why they can put the low side switch after the evaporator. Although I do agree insulating the low-side is not going to accomplish much for reasons that gathermewool mentioned.
 
There's a difference between efficiency and what provides the coolest operation. In short, insulating the low side in the engine compartment may lower the overall temp, but that's not to say it's more efficient.

Also, think about lifecycle costs. Is the manufacturer worried about lowering the evap outlet temp a couple degrees for a system that adequately cools and conditions the air or are they worried about their A/C system lasting for a long, long time? The compressor is temperature activated, and will begin pumping the refrigerant out of the evap and into the condenser when the temperature rises enough to ensure only vapor is being pump. Adding an additional amount of super heat with uninsulated line that will absorb engine compartment heat seems like a good idea to me.

IMO, this is also why the A/C compressor won't run while the engine is cold. With out the hot defroster air vaporizing the refrigerant, the compressor would probably be damaged, pumping liquid refrigerant.

I'm not an A/C engineer; I'm just thinking logically about this. This is my educated opinion, not fact. I hope I don't come across as trying to correct any one or tell you how it is.

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While Chis is correct, the lack of insulation can only add more btu's to the condenser load. While elevating it will produce more differential to the ambient air flow across it, I don't see how increasing it doesn't affect the system's overall performance from a cycling aspect.

It would be like sending your cold air return ducting on your house outside to expose it to sub-freezing temp JUST to assure that the air:air heat exchanger had a higher differential to reheat the air in the plenum. Granted we're dealing with a medium that experiences a phase change ..but ..
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That said, the stuff boils to a given -F temp indexed for a given low side pressure.

I think cycling would be reduced. I don't think performance, in terms of temperature, would change.
 
Originally Posted By: Gary Allan
While Chis is correct, the lack of insulation can only add more btu's to the condenser load. While elevating it will produce more differential to the ambient air flow across it, I don't see how increasing it doesn't affect the system's overall performance from a cycling aspect.

It would be like sending your cold air return ducting on your house outside to expose it to sub-freezing temp JUST to assure that the air:air heat exchanger had a higher differential to reheat the air in the plenum. Granted we're dealing with a medium that experiences a phase change ..but ..
21.gif


That said, the stuff boils to a given -F temp indexed for a given low side pressure.

I think cycling would be reduced. I don't think performance, in terms of temperature, would change.


I guess to answer the question of how much superheat you have at the pump suction at all times you would need to monitor pressure and temperature constantly. If the worst you see is only a few degrees of superheat at the pump suction, I'd guess the engineers were right in leaving the compressor suction line exposed to the heat inside the engine compartment. If the amount of superheat was excessive - I'm not really sure what would be considered excessive - I guess I'd have to change my mind about whether insulation is really a bad idea in all situations. I guess we'll have to trust that that designers of the system were smart enough to add in enough conservatism with out being overly conservative.

Edit: Regardless, I still believe that erring on the side of conservatism is best.
 
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one point that needs to be considered is how the a/c works in the defrost mode, it may very well need the heat in the engine compartment to vaporize the ref. to protect the compressor in the winter.
 
Originally Posted By: duaneb9729
one point that needs to be considered is how the a/c works in the defrost mode, it may very well need the heat in the engine compartment to vaporize the ref. to protect the compressor in the winter.


Good point, but wouldn't the hot air be great at vaporizing the refrigerant?
 
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