new member and have a non typical application

[danev2]

So I am a mechanical engineer and I am toying around with an obsolete auto style AC system in a helicopter.
I work at Md helicopter in Mesa AZ, and we have a 80s era designed system, that used to use R12, tube and fin condenser/evap, but got changed to R134a and a sanden compressor in the 90s.None of the other components were changed.
The system in this helo and other models is really completely obsolete but customer demand requires a workable system to be installed. This unit as it is, is very inefficient, a large horsepower drain on the engine , has compressor failures (oil starve).
The design direction I am looking to, is to use micro extruded tube condenser and evap units. Redesign the location of the condenser in the nose of the craft and repackage all the housings and rudimentary ductwork.
Most of the applications I find out in industry are geared toward either large commercial building scale, residential. Or full large volume automotive production. I need to find a company that makes small scale pieces (8x12x2 inch ) and also make only a few of them on a custom order basis. My design limitations on a helicopter, are primarily space, then weight. Cost is not a real big factor.
I also need to find a relatively simple analysis program geared toward small AC systems, and use it for system level macroscopic analysis and system sizing (balance the condenser with the 2 evaporators, and the sanden compressor, then make sure it has the capacity to cool the thermal load of the cockpit. (in high temp- phx115deg AND high humidity (Houston, Mississippi, 90% rel hum) conditions.

As an additional upgrade we are going away from R134a and I am proposing to use R1234yf refrigerant. Europe already has it mandated in vehicles there, aerospace applications are not far behind, and we sell a number of craft there and Africa.

Can someone point me toward a company that can make these small size cond. and evap pieces. ?
and software program to analyse the system?
thanks!
DV
Sr. Project Engineer
MD Helicopters

[ice-n-tropics]

Rotary wing AC ain't easy!
Challenge for copters is weight control of AC so fuel load can be optimized. Highest priority is cooling of electronics. Next priority is pilot and last is Passenger comfort as the lowest priority.
Parasitic energy draw of AC must be considered and will comprimise passanger comfort especially in Mesa in vicinity of the GM test track.
I'd start with a high isentropic efficiency compressor like the Sanden scroll TRSE 70.
Pressure controls must control extreme high pressure from affecting copter safety or drive belt failure.
Compressor fail safe controls are recommended.
Use 4% OCR min (oil circulation ratio) for scroll compressors at extreme operating conditions.
TXV with approximately 65 psi MOP. Test OCR per ASME standards.
Use all aluminum components for weight.
Forget HFO1234yf flamable crap.
Analysis program should include AC system component balance for heat dissipation greater than heat absorption plus compressor energy input.
Compressor charts are 10% or more overly optimistic because they do not consider up to 8% OCR at some conditions and they always LIE.
hotrodac

[danev2]

I will add your comments on my list of notes.
Electronics are already covered thu surface plate heat exchangers and receive outside bleed air.
So it's pilot, copilot cooling
the Mesa GM test track is longo gono Its all houses now, in a idyllic community where everybody conforms, call SyncroniCity.
No I'm not kidding it's called that
they never found any buried concept cars
Desicion to use HFO1234yf was made at levels above me.

[bohica2xo]

I would be inclined to add an oil separator after the compressor so that higher OCR's can be used while controlling the oil at the compressor. It also leaves a handy spot to store the pieces of a scroll compressor when it does what it always does. MacD should be able to fabricate such an item.

The last 3 fords I have had with a scroll compressor & 1234Yf - BBQ all cooled well as long as you drove the car like a 2 stroke dirt bike - always above 3500 rpm. A gas turbine fixes the scroll performance issue with a fixed rpm at least.

You mention micro extruded components. The MVAC world would call that a Serpentine condenser, and the industry has largely abandoned them for Parallel Flow units. The system volume has decreased with more refrigerant exposed to the interior surface, while offering more exterior area as well.

Lytron was always my go-to for aviation quality heat exchangers in small runs. Automotive stuff tends to be thin & getting thinner. If you want a 2" thick core for an evaporator Lytron does a nice plate & fin design. Vacuum brazed aluminum. They have helicopter experience.
.


Ice- Do you know what you get when you charge a 134a system with R500? Cold Air.

[ice-n-tropics]

I like oil separator also because it increases reliability and improved efficiency, but it comes with a weight penalty.
If MD uses a 28 Volt DC motor for compressor drive then my design (used for military vehicles) with a 85.7 cc swept volume, modified scroll clutch assembly, serpentine belt drive compressor could be used to optimize cooling with the limited power input to the compressor. With electric drive the scroll's downside of low cooling at idle rpm is avoided.
I would love to be the lawyer for the deceased when the evaporator leaks explosive HFO1234yf and alkyl benzene lube into the cabin. The passangers could compare experiences with Ricky Nelson. Similar result as a loose Jesus fastner.
hotrodac

[danev2]

Very interesting Info
I will look up Lytron and who is MacD?
Ice n tropic. Please email me I would like to discuss your 28V belt drive compressor for our 902 series craft.
dainius.valatka@mdhelicopters.com

thanks!

[bohica2xo]

Lytron http://www.lytron.com/heat-exchangers/custom/

MacD? Mcdonell Douglas? Any aircraft plant should be able to design & fabricate a suitable oil separator.
As far as a weight penalty, there should not be that much metal to hold a pint of oil.

Certainly not the weight penalty of the 135 amp 28v electric motor to run the compressor - holy serial loss batman. Every single one of those archaic belt drive 28v units in fixed wing GA have been turds.

Gas turbines still have constant speed pads right?

[ice-n-tropics]

bohica 2xo
Roots with separator and Wankel have disappeared along with vane rotary. Speed pads are new to me.
Lots of R500 in TX tractors. Smell during service is obvious.
Agree that separator could be made (of aluminum) with little weight penalty smaller than a aluminum accumulator. I have a compact Japan design separator with a capillary tube coiled inside so that it so that the oil return (to the scroll inlet) does not result in unacceptable recirculation of discharge gas.
Helicopters could use a 30cc scroll direct driven by a 28 V 8000 rpm motor for a less than 100 lb AC.
hotrodac

[bohica2xo]

Ice:

The "pads" are on the accessory gearbox with a gas turbine. They run at a fraction of the turbine speed.

With a turboshaft engine the N1 turbine spins the compressor & the accessory gearbox at a fairly constant speed. The N2 turbine spins the output shaft through a separate gearbox. The slip between N1 & N2 is sort of like a torque converter. The N2 turbine can actually be stopped completely while the Compressor & N1 still spin.

The starter, generator, fuel pump, hydraulic pump, etc all bolt down on pads to connect to the N1 power. Usually a spline shaft & 4 bolts.

The OP has not mentioned which helicopter he is working on, but a little bird like the MD500 has an Allison 250 turboshaft engine with an N1 speed of about 52,000 rpm. I don't think there is a choice for an 8k rpm pad output, but I think you can get 4k, and perhaps 6k. Been a long time since I saw one of those engines.

In a direct drive application the OP might benefit from using a newer constant run compressor with the computer controlled volume.

We are on the same page with the oil separator - they work.


Here is a quick animation of the Allison 250 assembly
https://www.youtube.com/watch?time_cont ... qRmKdfuF6Q

[ice-n-tropics]

Thanks, I learned a lot today
hotrodac