-125C all hydrocarbon autocascade cycle
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). :yepp: 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
Watch for update of something BIG!
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).
http://i1345.photobucket.com/albums/...ps35ceaf61.jpg
http://i1345.photobucket.com/albums/...ps013d2f1f.jpg
Kevin
Autocascade design refinement
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
Improvement in -125C Autocascade cycle
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.