Impact of Air/Cond Hose Length ?? 2

[jcrowell]

First post here. Seems like a smart group of guys around that can maybe help with this question.
We build off-road machines with the engines mounted at the rear, maybe 20' away from the cab, and use an engine driven a/c compressor. This approach has worked great for many years, just not so good in the mid-east with desert ambients of 120+F. So, some of our competitors claim they have improved performance by hydraulically driving the compressor and mounting it closer to the cab and minimizing the Freon hose length. My contention is that there should not be that much difference in performance assuming the hose ID's were properly chosen, and that the compressor speed remains the same, evaporators and condensers are the same. I would sincerely appreciate any insights all of you might have!

 

[MintJulep]

A longer liquid line will increase the pressure pressure drop between condenser and expansion device and will adsorb heat from the ambient. Both of these will decrease sub-cooling, or potentially even cause some boiling of the refrigerant in the liquid line. All of these reduce evaporator performance.

A longer suction line will increase pressure drop and again adsorb heat from the ambient. These will add super-heat. Decreasing compressor performance.

 

[IRstuff]

You might consider wrapping a couple inches of styrofoam insulation around the lines and see how that works...

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MiketheEngineer]

Using hydraulics will cost you more horsepower -

 

[jcrowell]

Thanks, Mint. I'm obviously not a refrigeration engineer or I wouldn't be asking these questions! Do you agree with IRstuff about insulating the hoses? And can you expand a little on "adsorb" and "sub-cooling" since I'm not sure what happened to those 1970's brain cells that sat thru Thermo 232.

 

[jcrowell]

Mike, absolutely agree. Also more noise from the gear motor and complexity for service. It just goes against my grain to cave in and copy the competition.

 

[MintJulep]

Oops. Absorb. Run a cold substance through a pipe or hose surrounded by a hot substance and the hot goes in.

Can you compensate with insulation? Sure, but not entirely. And you can't insulate away pressure drop.

I agree that the hydraulic option is unlikely to be more efficient.

 

[jcrowell]

Mike, I can see where larger hoses and reduced pressure drop might be a simple improvement. But assuming the condenser is doing a perfect job and brings the cooled gases down to ambient (as good as it gets), then I don't see how the hose can get any hotter than ambient on its way to the cab. Or, put another way, if the condenser is not perfect then exposure of the hose to ambient might actually cool the stuff more?? Comment?

To be fair with my system description, we actually bundle the a/c hoses with several hydraulic hoses that might be causing the refrigerant to heat back up. We'll have a look at that, too.

Many thanks for your input.

 

[MintJulep]

Are you running the hoses on the outside of the vehicle where they are exposed to the same air that goes through the condenser? Probably not. They're under hood, where things are a bunch hotter than outside, right?

If you already have a hydraulic power system that might change the efficiency trade-off.

 

[Compositepro]

A large liquid line will store liquid and take a long time to fill after the compressor starts. A long suction line carries vapor so pressure drop is more of a problem. The temperature of the evaporator depends on the pressure. A pressure drop in the suction line will raise evaporator pressure and temperature unless you have a thermostatic expansion valve. It will also reduce compressor suction pressure which can have a significant effect on compressor capacity. Using larger tubes to reduce pressure drop means less velocity in the tube and oil can start to segregate.

All these issues can be dealt with but the trade-offs are complex. So the answer is, it depends. But, I agree that hydraulic drive will not be energy efficient.

 

[jcrowell]

The hoses run along the top of the frame and are exposed to the elements. Hopefully the attached picture helps. What I think from all this is that the return hose is probably the most important and could probably see an improvement by going up one size and adding some insulation? Probably wouldn't hurt to go up one size on the supply, too?

 

[evelrod]

I'm not a refrigeration qualified engineer, my problems in this area are legend around my shop. However, relocating the compressor closer to the cab and using an electric motor for power is a fairly common practice. Several OEM hybrids are doing this.

Rod

 

[MintJulep]

One more thing. Rubber hoses are permeable to refrigerants to some degree. More hose surface means the refrigerant leaks out faster.

Is space in the cab at a premium?

Put a heat exchanger near the compressor in place of the evaporator coil and pump chilled water to the cab.

 

[rmw]

I am not a fan of the hydraulics, but one thing that the hydraulics will give you is constant compressor speed where the engine driven compressor is a slave to whatever the engine is doing at the moment. Not knowing how the engine cycles (or not) that might be the difference in the total mass flow of refrigerant along with minute differences accountable to the hoses.

Also, with hydraulics they may be getting the condenser away from engine or power train heat too, and that would give them a leg up.

Having had way more AC experience on Motor Coach size busses than I would have liked to have had in my life, and being an avid Eagle (brand of motor coach) fan, I give MCI some real grudging respect for their AC location.

The Eagle A/C compressor/condenser system is engine driven through a jack shaft to the compressor with a pulley drive to the belt driven condenser fan. The condenser is located athwartships on the passenger side, right behind the drive and tag axles. One day while driving in the rain, with the AC on and the AC fan in operation, I noticed that the spume from the road spray coming out of the wheel wells was curling right around and right back into the Condenser intake. I realized then that when it wasn't raining, that would be hot air from the engine compartment and air that had come through the A/C commpressor and engine radiator on the other side of the bus opposite the condenser.

MCI on the other hand has a hydraulic driven system that is constant speed, no matter what the engine speed, and it is located in what would on an Eagle be the rearmost cargo bay on the drivers side. No hot air recirculation from the engine compartment there, just pure first cut cool air from the front of the bus.

I tell that tale to try to help you get your thought process juices going to see if it might be more than just hydraulics that the competition is aceing you on.

rmw

 

[jcrowell]

rmw, both us and the competition run the engine at constant speed. But since the rest of their system is different than ours in terms of components involved, my hunch is that they just have a better, cooler running assembly of parts than we do and that it really has nothing to do with the fact that theirs is hydraulically driven unless they also built in a faster spinning compressor in the process.

It's just that a lot of our in-common customers think that because their hoses are shorter, that's why theirs runs cooler and consequently we get a lot of requests for a hydraulic system. I've just never accepted this conclusion while I can admit that we might just have to upsize and re-spec everything to get cooler.

I'm thinking now that we need to put gages on both sides of the evaporator and gages on both sides of the compressor and get a set of readings on the current hosing, then another set of readings with some larger hosing to see if anything is improved. All done with same-weighted amounts of Freon.

All this is about extreme extremes. Amarillo in August ain't squat compared to Abu Dhabi.

Merry Christmas you guys and thanks for the input!

JC

 

[potteryshard]

The usual caveat: I know nothing about this. Nonetheless...

It would seem that suction line pressure would most appreciably affect the performance of the expansion valve; in other words, it needs the lowest possible pressure to expand into to create the cooling effect. A slightly elevated pressure in the suction line would conversely seem to aid the compressor.

Both lines would seem to work better if separated from the hydraulic tubing; direct heat conduction from the hydraulic lines would seem to dominate over the environmental heat inputs. If the lines could be phsyically separated that might be best, but placing the refrigerant lines in a heat-blocking overjacket might be a alternative.

Still, the easiest means to improve performance might be to install (and or retrofit) a small electric booster pump at the outlet of the evaporator to increase the pressure differential across the expansion valve. I would expect that 12 V heremetrically-sealed units are available for the camper and truck markets... If paralleled with a bypass valve this might allow an AC performance boost when needed. If a turbine-type booster fan could be found, the bypass wouldn't be needed.