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Originally Posted by AC_Hacker
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Precisely one of those. Copeland reckon their scroll compressors are good from about 40Hz to 70Hz, so the 15KW compressor I have here (rated at 50Hz) shoudl be good for between 12 & 21Kw. This can also be varied by altering the evaporation temperature by changing the supply fan speeds.
I'm hoping that my generic Scroll compressor copes with the VSD. Some motors can suffer severe stator eddy current losses with VSD drives, and overheat. I don't really want to have to spring for a new compressor if I can avoid it (another $1k or so).
The VSD runs the compressor fine, but of course I've not been able to do any extended run tests yet.
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Originally Posted by AC_Hacker
Also, could you please go through your HX selection process step by step? That's a mystery for some folks. I tried to clarify it on a design, but there still seems to be remaining questions...
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This is a bit of a hairy one. I did it this way, but others do it differently. Any way you do it you need to break the HX up into zones, and perform the calcs for each zone, then add it all together.
- De-superheating
- 80% Vapor
- 50% Vapor
- 20% Vapor
- Pure liquid / Subcooling
It's quite a complex set of calculations. I did it once (about a year and a half ago). This time I just got on the phone to the HX vendor, gave him my numbers and he plugged them into his simulator software and came up with the same condensation temperature I did originally. All the HX design software I've seen has been very manufacturer specific, and the don't unlock features like refrigerant side calculations unless you pay a load, or sell lots of their product. Therefore I've not seen anything out there we as DIY'ers can get our hands on. Also, a lot of the software lacks R290. I ask for sizes based on R22 and it works out pretty close.
The other thing about condenser sizing, is everyone seems to do it differently. I've seen algorithms that have 30 or 40 iterations and sub-steps.
I actually don't recall now where I got the heat transfer coefficients for different vapor quality levels. I'll have a dig through my reference material and see if I can dredge it up.
I've got 8.5M2 of surface area in the condenser. The vendor told me I could halve that, but at the expense of having to double water flow. Now given doubling water flow causes the power to rise with the square, it meant a pump 4 times as powerful, so my overall COP would have dropped. I would have saved $600 on the HX, but needed a bigger pump and more power consumption.
HX design for evaporators and water to water is _much_ easier.
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Originally Posted by AC_Hacker
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Yeah, I've been keeping a loose eye on that, but my general feeling it it puts the cart firmly before the horse.
I have a strong background in electronics, embedded control and therefore control algorithms. My philosophy is that a control system can quickly be developed to solve any problem economically. Planning the control system before having a defined problem to solve is a bit like arguing over the colour you want to paint a bikeshed before having worked through the problem of designing and building it.
I'm happy to share the designs and algorithms I'm developing and working with, particularly with regard to the EEV's, but I tend to be very "hands on" when it comes to developing stuff like that. Unless the hardware is built and I can get measurements from it, I tend to shy away from theorising about it.
BTW, SSR's are great for compressors. I built a custom thermostat control for an old 1/2hp window banger years ago that used an off the shelf SSR to control the compressor. It seemed to cope with the inrush start current fine, and probably cycled the compressor every 20 minutes for about 3 years before I decommissioned it.
I used this :
SSR5A - SPST 3-15VDC 5A Solid State Relay - Technical Data
You guys in the US are unlucky with 120V. At 240V we can use a much smaller unit, and therefore sustain much smaller losses across the device.