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Compressor Suction Pressure Regulator

ice-n-tropics on Wed March 26, 2008 3:53 PM User is offline

Year: 2002
Make: Military
Engine Size: Hydrauli
Refrigerant Type: R-134a
Ambient Temp: 130 F

Brad or any of you guys,
Say the scroll (or piston) compressor displacement and drive ratio is sized to just use all the available hydraulic power available at 95 F with Ps = 25 psig. Same applies to a electric driven compressor.
Now, the Baghdad driver gets into the vehicle at 160 F Hot Soak interior and turns on the A/C.
The Ps starts at the T/P relationship of P = 279 psig and the Pd immediately spikes to 450 psig +.
The hydraulic supply maxs out at say 2500 psi and then the hydraulic motor that drives the compressor probably slows to maybe 20 % of normal speed. Very very gradually the speed increases as the driver suffers.
Opportunities for improvement:
1) Evaporator pressure regulator EPR set at 25/30 psig- What manufacturer?
2) TXV with liquid charge and MOP (maximum operating pressure). What is minimum MOP available for 1,5/2.0 ton TXV? Where to get?
3) Variable compressor with external control for displacement, e. g., VW's. How to make controller. The Europeans have a hand held controller/service analysis tool that I saw in Germany for servicing Sanden, Denso and Tamma/Seltec/Valeo compressors. Who makes it?
Thanks,
Old IV guy

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

mk378 on Wed March 26, 2008 6:33 PM User is offline

If you're needing to back off because the high side is not allowed to go over 450, forget the compressor for a minute and realize that the system is being limited by the condenser. If there was an infinite condenser you could pump as much gas as you wanted into it and it would all condense without the pressure rising. Really the only way to increase the capacity of this system without changing the condenser is to allow a higher discharge pressure so there will be a larger temperature differential between the condenser and ambient and thus it can condense a larger mass flow. But going much over 450 with R134a I would think has very little benefit.

Fixed gearing a positive-displacement hydraulic motor with a constant pressure supply is going to turn the compressor with a constant torque. I would expect compressor torque required is driven primarily by discharge pressure so again we look back to the condenser as the limiting factor. When the compressor starts to stall just as the design maximum high side is reached, the gearing is optimal. Or look at it another way, if you could increase the flow of the compressor under these conditions, something on the high side would just burst. If you restrict the input of the compressor it will require more horsepower to move the same mass flow at a given same discharge pressure. OK with the hydraulic drive it speeds up and seems to be having an easier time but really you haven't gained anything you're just wasting horsepower.

Given a particular condenser arrangement, outdoor temperature and maximum allowable discharge pressure you will only get a certain mass flow out of that condenser at maximum pressure. Assuming the evaporator can always completely evaporate it, the mass flow of liquid out of the condenser is what takes the BTU's out of the cabin.

ice-n-tropics on Thu March 27, 2008 9:55 AM User is offline

MK,
1) From conventional engine driven compressor experience, we agree that the main factor is Pd and condenser capacity and extra hp is available for start up peaks.
2) From low temp hermetic compressor experience it is common to use a evaporator pressure regulator valve (EPR or STV) just to get the compressor started without overloading the fixed max hp comp.
3) Hydraulic driven comp is very much like the hermetic, in that zero extra hp is available.
4) Comp hp and Pd (PSIA) is a direct factor of refrigerant mass flow and is also influenced strongly by Pd.
HP is a direct function to PSIA.
5) As mass flow decreases, Pd decreases.
6) Condenser face area and 24 V electrical power for condenser fans are at the max limit of real estate and alternator output available.
7) SH is only 2 to 3 F to minimize Pd. TXVs are externally equalized to minimize SH of evaporator and compressor out.
8) To get the A/C started and reduce overloading all the dual condenser capacity, it is necessary to reduce the refrigerant mass flow somehow.
Decided today to try a Sporlan direct acting adjustable EPR at the fixed displacement scroll compressor inlet.
Cordially,
Old IV guy


-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

NickD on Thu March 27, 2008 10:20 AM User is offline

ice-n-tropics, are you trying to say that this system never worked since day one?

mk378 on Thu March 27, 2008 10:54 AM User is offline

I'm saying just let the compressor slow down, this is not a problem with a hydraulic drive right? An electric induction motor under similar conditions would of course stall and burn out so something needs to be done to prevent that. If you're at maximum allowable Pd and you're not going to damage the compressor drive there is nothing else can be done at the compressor.

At startup you don't need full horsepower input to the compressor because the system as a whole is pumping heat across a small or even negative delta-T between inside and outside. The thermodynamics are favorable, a little power can pump a lot of heat. Because the condenser can still only reject so much heat, that is what sets the BTU capability.

The variable compressor would be cool though both if you wanted to reduce effective compressor size during startup and to regulate suction pressure and/or evaporator temperature to prevent evaporator freeze-up. I suspect the VW ones are just a 0 to 12 volt pulse width modulated drive to the solenoid valve. If you could borrow a car and put a scope on the compressor control signal that would probably tell you all you need to know about how to control it.

bohica2xo on Thu March 27, 2008 3:05 PM User is offline

Tex:

My hydraulic drive was not so limited on horsepower, but I was not running an effing scroll compressor either... Actually a constant speed drive on a scroll is the only way I would have one of them. I still have a bad taste in my mouth from that effing explorer with no cooling below 2500 rpm. Ok, back to the problem at hand.

'378 is right, you could just let the hydraulic motor stall for a bit. Performance would suck, but the GI's are used to expensive crap that does not work & enriches the usual suspects.

Part of your issue is volume. You have dual evaporators that hold a lot of low density (hot) refrigerant on startup, along with all of the associated low side plumbing. The high side on the other hand has smaller plumbing, shorter runs, etc.

The compressor is capable of high mass flow, as long as it is not at a high pressure. A simplistic cure would be a 400 psi relief valve to shunt the compressor. Probably get a bit warm, but if you ran the relief discharge about 4 feet from the suction port you would probably get away with it.

Of course there are other more elegant fixes. Sequestering the refrigerant on shutdown. Feedback loop in the compressor speed control. And other expensive complicated stuff...

Are you running a hydraulic fan drive yet? If not, then your condensor is a big part of the roadblock.


Personally, I would increase the discharge line size to a -12 and add about 100 to 200 cubic inches of volume (preferably an oil seperator) between the compressor discharge & condensor, Like I did on that motorhome. That system was very similar to what you have. The suction side volume was huge compared to the discharge plumbing & condensor. Before I added that huge oil seperator it would spike on startup. After adding that discharge volume it was much more stable.

Let me know where to send the invoice.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

ice-n-tropics on Mon March 31, 2008 6:38 PM User is offline

Sorry for the delay,
True the CAPACITY IS LIMITED BY THE COND HEAT DISSIPATION BUT i'M WORKING ON THIS. oIL SEPERATOR HELPS.
VW comp has 400 Hz PWM signal about 4 volts as I remember.

Explorer pukes at idle.
Td is already too close to TPS trip.
R-1434a shunt makes extreme heat.
Explorer shunt was mid pressure and temp at 1/2 way into compression cycle.
Drive away scroll spike is severe. Considered orifice restrictor in suction but counter productive at mid range. Interesting idea on increased discharge volume.
Since you are prone to draw a line in the sand (PAG lab test etc.), just send invoice to San Antonio, attn: decedents of William B. Travis.
Remember the Alamo and Goliad.

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

bohica2xo on Mon March 31, 2008 8:34 PM User is offline

Tex:

Like I said, the shunt would get hot. Not a good plan at all, but you have to look at all of the options & discard them one by one.

The oil seperator would help several ways, we both know that. One that will hold a bit of oil in reserve would be a good thing on a combat vehicle.

I really think increasing the discharge line size & adding some volume would help quite a bit. The scroll has that nasty spike once per revolution, and volume will help damp that.

Pumps don't make pressure, they move volume. Adding restriction raises pressure. At any given spot on the P/T curve there is a corresponding Cv for the condensor. With a finite quantity of refrigerant you should be able to store some high pressure gas while the condensor catches up.

Are you running hydraulic fan drives as well? I hope so, because with a constant speed on the compressor you need full fan speed at idle...


As for the invoice, all I can say is get plenty from uncle sugar. They always wanted stuff from me for free, so I quit playing. Crane NSWC can kiss my...


B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

ice-n-tropics on Tue April 01, 2008 11:00 AM User is offline

Brad,
They wouldn't let me use hyd. fans on exterior of vehicle. Snipers are pretty clever evil doers. They could disable all the other hyd functions like loading shells and turret actuation.
Instead use 28.5 volt electric fans, but have to rob comp power to turn alternator. Darn alternators are only 50 to 60 % efficient at operating temp.
Nick,
Naw, never been A/C'd before. So many combat platforms were rushed to Iraq and Afghanistan w/o A/C & without any forethought to the consequences. Need some strategic thinkers in the Pentagon, case in point, USA supplier, Boeing getting fired from air borne tanker refuler business in favor of the French who hate us.
Cordially,
Old IV guy

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

bohica2xo on Tue April 01, 2008 12:09 PM User is offline

Tex:

Ahh, the picture clears. I thought you were working on the hummer. Yeah, the armor problem is a bit different, and they are fussy about hydraulics...

You should have a look at the systems in a Rooikat, they button up for NBC protection. The Rooikat & Ratel were designed to operate in that climate, instead of Ft Benning. Seeing 'kat in action is a real eye opener. 28 tons, 150 kph top speed, and a 1000 km range stand-alone. 105mm main gun. They consider it a "scout" vehicle. The Ratel is the predecessor, and is usually used as a command vehicle.

Of course BAE now owns LSSA, the maker of the Rooikat.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Tue April 01, 2008 1:32 PM User is offline

Not sure how far "outside the box" you're allowed to go here but switching to a refrigerant with a higher critical temperature would help in the desert with a small condenser. The obvious one would be R12, this property is a big difference between 12 and 134a. There seems to be only one suitable choice among the HFCs: the obscure HFC-236fa. It would be working with evaporator pressures near 0 psig (15 psia) though, a large displacement compressor and larger suction pipes would be necessary.

NickD on Tue April 01, 2008 7:13 PM User is offline

Seems like a nice vehicle to have for cruising around north Milwaukee or south Chicago.



Was an old German Tiger tank driver on the Military channel stating those Tigers really got hot on the inside, but was better than having your head blown off.

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