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Chinese universal evaporator dissected (lots of pictures)

digidocs on Tue September 02, 2014 12:56 PM User is offline

Hello AAC forum!

I'm in the process of designing/building a mobile air conditioning system for a homebuilt aircraft and am currently in the learning and experimenting phase. Thus far it's been very difficult to get solid technical details (BTU capacity and *measurement conditions*, air side pressure drop, etc.) on evaporator cores---so I decided to take one apart and measure myself. If you have any tech/catalog data on evaporator cores, I would love to see it.

This is a Chinese BEU 404 universal evaporator that comes in quite a few configurations. This particular unit is a 24V version set up for use with external ducts and vents. I procured it for about $70 on eBay. For scale, it's about 16" wide, 11" long, and 5.5" tall.

First the front comes off, and we can see the core for the first time. I was surprised to see that the core itself was only about 12" wide and that there is substantial space on either end. Also, the manufacturer attached a thin plastic mesh (like window screen) to the front of the core. Perhaps to reduce water carry over? They also smushed the fins at each place where the mesh was attached with wire.

Next the lid comes off. The evaporator housing and blower housings are combined---lots of screws. A large amount more fin damage is visible. The blower motor mount is interesting. The motor has a ribbed rubber sleeve that is clamped between the two case halves. Seems to work well and also damp a bit of vibration.

Here's a couple little tighter shots.

The TXV is marked XIN JING 10J and QKF-1020 1.5T.

Surprisingly, the core was still under pressure (or vacuum) when I pulled this rubber plug. I guess that means it's tight?

Here's the side of the evaporator core. It has 32 passes of aluminum tubing. It also looks like there's a fair amount of corrosion/scale left over from the brazing process. It makes me wonder if the corrosion will continue over time.

The dimensions of the flow area of the core are approximately 11.75" wide x 4.5" tall x 4" deep. Weight is about 2.4 lbs.

Also, weighed the blower motor/fans: 2.1 lbs and and the two halves of the case: 2.5 lbs for both halves.

Next, I took some pressure and flow measurements. Didn't think to take pictures though. Basic setup was a digital manometer, with a pitot probe to measure total pressure and aquarium air stones to measure static pressure across the core. The air stones were an experiment but seemed to work pretty well. The static pressure measurement from the stones weren't affected by local velocity which is exactly what I was hoping for.

Without the front hole plate, I measured an average flow rate of 260 CFM on high. Electrical power was 4.8A at 27.6V. The flow profile through the evaporator was pretty non-uniform, so I took 24 measurements on a grid just in front of the opening (table below). It's difficult to quantify the accuracy of this measurement, but I think it's in the ballpark. Using the measured flow area of the core, I get a core velocity of 891 fpm, which sounds fast based on my limited experience. dP across the core was .53 in H2O (dry core).

Core dynamic pressure distribution:

BTU measurements will have to wait for now, as I don't have a compressor and condenser setup yet. Hope you enjoyed the post and if you have any manufacturer data on evaporator or condenser core performance---I'd really appreciate a look!

Take care,

ice-n-tropics on Tue September 02, 2014 9:15 PM User is offline

1) Your future longevity could be linked the fail safe design of the compressor drive when the compressor fails /locks up. Consider a shear pin drive link. Nippondenso technology compares drive rpm to compressor rpm to recognize clutch slip. A heat sensitive fuse in special clutches will disengage the clutch electrical circuit due to slippage during abnormal compressor torque.
2) Water will spit out the air discharge when the air velocity is above 500/600 ft/min . The low air resistance of that screen is little help.
3) Smashed fins are less that 3% importance but you could straighten them.
4) Evaporator design looks like about 8000 BTU and 170 cfm after air ducts and louvers installed
5) Cu tube corrosion status looks OK
6 4 inlet tubes and 3 outlets are a puzzlement
7) Bottom coil tubes will be colder that upper because TXV spews liquid and vapor but liquid seeks lower tubes due to gravity. Best practice is a machined brass distributer downstream of TXV to proportion the same amount of liquid refrigerant into all 4 circuits.
8) Condenser capacity should be 30% + greater than Evap to dissipate compressor input energy.

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

pippo on Wed September 03, 2014 7:35 PM User is offline

nice photos, and good info. Cool engineering project!! goodluck.

beware of the arrival

digidocs on Thu September 04, 2014 2:46 PM User is offline


Thanks for responding to my post. Your concerns about the compressor disrupting safe operation of the aircraft are 100% on point in my opinion. I looked at several configuration options for the compressor and was never able to find a solution that met my safety, reliability, and packaging constraints. Certainly it is possible to use an engine driven compressor, but the development and testing effort required to do so exceeds my budget. For that reason, I have elected to utilize an electrically driven compressor and to invest in a 2KW 28V alternator to power it. The compressor itself is manufactured by Masterflux/Tecumseh. This approach does limit system capacity to about 12,000 btu/h, but offers high efficiency, easier packaging/integration, and potentially reduced weight.

Based on what I've learned so far, this evaporator is probably not well suited to the job. My flow rate target is 300 CFM and it seems that I'm going to need 80+ sq in of frontal area to keep air side pressure drop and water carry over under control. I'm still struggling a bit to find numbers to quantify the required core volume, but the chart below and some educated guesses lead me to believe I want to be in the 220-250 cu in range. It seems that apart from weight, a bigger core is better because it reduces the required evaporator approach temperature, increasing the equilibrium evaporating pressure for a given thermal load which increases my system capacity. Finding performance data on evaporator cores has been extremely difficult so far.

The current project is going through the UAC and 4S catalogs to locate suitable cores and examining different packaging and location options for the evaporator. I'm trying to keep the evaporator inside the cabin where it's cooler and reduces the number of big holes I have to cut (and then reinforce) in the structure.

FYI, the missing outlet tube is hidden by the larger diameter line in the photograph. Good catch.

Hopefully these posts are interesting to some folks---I'll keep them up if they are.


ice-n-tropics on Thu September 04, 2014 4:58 PM User is offline

BTU ratings are not all from a comparible rating systems. GM/Harrison/ (HRD)/Delphi and commercial stationary manufacturers are ASHRAE ratings. 4 Sezins etc. use IMACA rating settings. Generally ASHRAE results in 25% less than IMACA. Pay attention to rating system when 30% oversizing condenser heat dissipation to evaporation heat absorbed + compressor input energy. Scroll compressors have higher isentropic efficiency and less input energy/BTU.
Problem with electric drive is overload when cabin soak temp is 150 F and suction pressure hits 350 psi+. A suction pressure regulator set at 30 psi max is recommended.
You'll carry more fuel w/ light weight AC. Best practice is a light weight TRSA05 scroll compressor driven by the engine. I've replaced 12 volt clutch on TRS105 w/ 24 volt clutch from SD7H15. Pay close attention to compressor rotation direction.
Your really fighting a weight battle w/ monster alternator and 28 volt motor. Preferred also are aluminum evaporator and condenser drier, fittings TXV etc.
I've had 100% success using 24 volt AC clutches, motors etc. w/ 28 volt supply.
I use vent a hood expanded aluminum filter screen to reduce H2O spitting. The installation orientation should be conducive to H2O drainage. This material will not rot as foam does.
Wet coil cfm of 225/250 cfm would approx. match 1 ton compressor capacity at 100 F ambient

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

Edited: Fri September 05, 2014 at 12:58 PM by ice-n-tropics

digidocs on Thu September 04, 2014 5:51 PM User is offline


I like your idea of using a small scroll compressor. If the mounting and drive arrangements under the cowling weren't so tight on space, I'd definitely consider that route---neat to know that some of the clutches are interchangeable. As it stands though, the bigger alternator only weighs about 2lbs more than the stock one and the compressor itself isn't too bad at about 14.5lbs. Eliminating the long hose runs from the cowling to aft fuselage should net 4-5lbs, but at least some of that savings will get spent on wire and controller.

Your comment about starting with very high temperatures is thought provoking. I seem to remember a post here that described a similar problem on a hydraulically driven application. The compressor I'm planning to use is a small brushless hermetic unit. I'll see if I can ask the manufacturer about what to expect in this scenario. Perhaps there's enough torque margin to get the cycle started, or as you suggest, I'll need a suction pressure regulator. Here's what the compressor looks like. It's a 16cc compressor and is variable speed up to 6500rpm.


Edited: Thu September 04, 2014 at 6:07 PM by digidocs

ice-n-tropics on Fri September 05, 2014 1:12 PM User is offline

The compressor appears to be rated 6500 btu at 95 F ambient and 45 F evap
OK for spot cooling w/ optimum louvers
2 KW alternator output probably 80% efficient or less at engine compartment temp
Ref FYI: Hydraulic driven compressor overload at 130 F ambient ( Iraq spec) was resolved by switching to higher HP drive and better viscosity profile oil spec

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

Edited: Tue September 09, 2014 at 12:51 AM by ice-n-tropics

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