-125C all hydrocarbon autocascade cycle

[Kevin Hotton]

Hi XS'ers,

I have been studying autocascade cycles using Aspen Hysys software. I believe I have a good cycle using only three hydrocarbons (iso-pentane, ethane, and methane). The cycle uses two phase separators along with an auxilary condenser. Cycle efficiency is good with a COP of 0.26. I would use a 15,000 Btu/hr 410a window AC unit (compressor displacement 15 cc/rev) with a cooling duty of 270W. The main problem I see is the fact that the auxilary condenser has a very high cooling duty 1900W and its discharge temperature is only -75C. I guess a Temprite coalesing oil-separator could be used to avoid frozen oil before the first phase separator. The pressure-ratio is a good match for a 410a rotary compressor at PR=4 with suction pressure at 6.5-bar_abs and discharge pressure at 26 bar_abs. Cooling with 270W load to -125C blows away any 2-stage cascade (and likely 3-stage cascades as well). The cost of a 15,000 Btu/hr AC window unit is about $500 and probably an additional $1000 may be needed for all the modifications (HX's etc.).

Standard R410a AC cycle has a suction pressure of about 10-bar (45F) and discharge of 30-bar (120F) so the compressor loading should closely match the standard AC cycle and therefore yield good electrical efficiency.

I would post the cycle simulation results but it is still not possible to upload a jpeg to this blog (I hope this will be fixed soon).

Kevin

[mytekcontrols]

Kevin,

If you host the picture elsewhere I believe it still is possible to show images. You would just have to use the link to image URL option. A good free option would be to sign-up for a DropBox account, and then place your images in the Public folder that it provides.

Using an all hydrocarbon mixture will probably allow you to get away with no oil separator. Even at -75C the oil will usually not freeze when combined with hydrocarbon refrigerants, especially if you use something like Zerol 150 (ISO 32) which has excellent cold temperature performance. A good example of what is possible can be seen in my CryoBUG project.

Good luck with your latest AutoC.

[wdrzal]

Kevin,

If you host the picture elsewhere I believe it still is possible to show images. You would just have to use the link to image URL option. A good free option would be to sign-up for a DropBox account, and then place your images in the Public folder that it provides.

Pictures hosted on a third party server usually disapear after time. Leaving the thread worthless when the pictures/data are no longer viewable in the thread.
I've seen pics elsewhere on the forum. It's been awhile since I tried.

With all the computer pros here someone should chime in on the problem.

Walt

[n00b 0f l337]

My suggestion is to force link to Facebook. For better or worse Facebook isn't going away.

[mytekcontrols]

Hello everyone -- anybody know what happened that caused the image upload problem to occur in the first place?

Anyway I know this has been getting , so back to the -125C HC AutoC...

Kevin what kind of timeline are you envisioning for this new project? Any specific application you have in mind, or are you just doing this for the fun of it? And this brings me to my last question; is this going to be built for real, or just simulated? Inquiring minds wish to know

[n00b 0f l337]

Inquiring minds indeed. This place has been far too dead lately.

[Kevin Hotton]

Hi XS?ers,

I am an Engineer/Thermodynamisist by occupation and greatly enjoy designing and simulating cycles. I work for a company Pioneer Energy (www.pioneerenergy.com). I lead the gas-separation division of the company.

I was researching autocascade cycles for possible use with our gas-separation process. However, I only need to cool methanol to -60C and at that temperature, a standard two-stage cascade is the most efficient. Some of you may remember the pictures I posted of the "large cascade" (with its 54,000 Watt cooling duty). Well wait about six months and I will post some pictures of my next MONSTER. It will be 190,000 Watts using twin Vilter 458XL 8-cylinder compressors powered with BIG (14.6 Liter Caterpiller 3406 engines burning natural gas).





Kevin

[Kevin Hotton]

I have been finalizing my autocascade design and some negative effects have been discovered. When I model an autocascade cycle, the simulation only shows inlet and outlet temperatures at all the various heat-exchangers. I strive to get a temperature difference of about 10C between all inlet/outlet heat-exchanging flows. However even if this is met there is no guarantee the there will not be a temperature cross within the heat-exchanger. If in fact this is shown to occur, the simulation is invalid (the cycle would not work). Such was the case with my initial -125C, 270W duty design.

I have since refined the design to correct this problem. Of course performance is degraded. The performance estimate now is 200W at -125C (using a 15 cc/rev rotary compressor in a "morphed" 15,000 Btu/hr window AC unit). I lowered the pressures in the cycle slightly to accommodate the compressor power. Suction pressure 6-bar, discharge pressure 24-bar (pressure ratio PR = 4.00). The refrigerants needed were, n-butane, ethylene, and methane.

Kevin

[Kevin Hotton]

I have been continuing to refine my autocascade cycle. I have looked at cycles with one, two, and three phase separators. The two phase separator cycle seems best. The best so far is a duty of 222W at -125C using the same 15,000 Btu/hr R410a AC window unit I have previously. The pressure ratio is still 4 (suction pressure 6-bar and discharge 24-bar). The hydrocarbons used are n-butane, ethylene, and methane. I will continue to look for errors in this cycle and if I find none, begin heat-excharger and cap-tube sizing. I model the compressor with isentropic efficiency of 0.68, volumetric efficiency of 0.88 and electric motor efficiency of 0.85. The resulting COP is 222/1208 = 0.184

Kevin

edit: this cycle result was based on the belief a 15,000 Btu/hr compressor was 15cc/rev. The fact that the compressor is actually only 13cc/rev reduces cycle duty to 192W.

[subzero 1979]

thermodynamics

[Kevin Hotton]

I have been continuing in my quest to find a good cycle for hydrocarbon refrigerants in the -120C range that could be built as a "morph'd" AC unit. With the realization that the largest 410a units that run on 115VAC are only 13cc/rev rotarys this is a considerable limitation. However, I do believe I might have a usable cycle.

I have studied both 2-phase-separator as well as 3-phase-separator cycles. It was looking as if the 2-phase-separator design might be the best. However I finally believe I have a 3-phase-separator design which is better. The key was using i-pentane instead of n-butane. When using n-butane I kept getting a bad temperature approach "pinch" in the first cascade condenser.

My best cycle so far uses: i-pentane, propene, ethylene, and methane. Pressure-ratio is 4 (24-bar high / 6-bar low). Duty is 218W at an average evaporator temperature of -120C.

Kevin