1 reply to this topic

[geoneo]

Hi Everyone,

 

To begin I am an engineer with only 2 years of practical experince which means I just have enough experience and education to make me dangerous but to my credit I know to check and double check and defer to the more experienced regarding sizing of equipment.  OK this is what I would like some help with for those whom are willing...

 

I have an appliction that requires a heat pump to reject ~800,000 BTU/hr to an evaporator (using the heat rejected by the condenser to heat a process evaporator).   The heat pump (screw compressor) quoted had a 600 BHP motor for the screw compressor using R142b as a refrigerant.  OK something is not correct!  I would expect (using hysterics) that the size is off by about 10 magnitudes!

 

Here are my quick calcs for everyones reference.  Am I off in my assertions?

 

Heat Source 110 F (Process Condenser - Tin)

Heat Delivered 180 F (Process Evaporator - Tout)

Co-efficient of Performance (COPhp) = Qout/Win;

Ideal Carnot-cycle heat pump related to heat delivery temperature and temperature lift:

COPhp = Tout / (Tout - Tin) = (180+460) / (180-110) = 9.13

Assuming actual COPhp is 70% of ideal = 9.13 * 0.70 = 6.3

 

Evaporator Duty (Qin) = 0.75 MM btu/hr

Win = Qin / (COP -1) = 0.142 MM btu/hr; 297 kW per MM btu/hr

Win = 42.2 kW (this seems more reasonable to me for the compressor)

 

Qout = Qin + Win = 0.892 MM btu/hr (amount of energy we would need to provide via an alternate source)

 

Any help with this from you that deal with heat pumps would be appreciated.

 

Also I am sure the community  could use some sizing tips and calcs from you because my searches on this sight left me with nothing.

 

Thank you

Edited by geoneo, 23 June 2014 - 10:09 AM.

[Art Montemayor]

 

Geoneo:

 

A heat pump is another name for a mechanical refrigeration system.  You can find many threads in our SEARCH database that refer to, explain, and show how to calculate the size of such a system.

 

Are you sure you are using refrigerant “124b”?  I have not heard of such a refrigerant nor is it listed in the NIST thermodynamic properties database.  Perhaps you mean:

• R-124 (Ethane, 1-chloro-1,2,2,2-tetrafluoro); or
• R-134A (Ethane, 1,1,1,2-tetrafluoro).

I believe you are going about the wrong way to size your system.  You just can’t assume a Coefficient of Expansion.  This is never done in sizing this type of thermodynamic equipment.  You should first draw your heat pump cycle and label all the equipment, identifying the related design temperatures and pressures.

 

A heat pump rejects (at the condenser) and absorbs heat (at the evaporator).  Knowing the heat rejected at the condenser (800,000 Btu/hr in your case) and the exact type of refrigerant, you can calculate the estimated compressor hp by employing the design suction pressure and temperature as well as the discharge pressure.  I assume the screw compressor is single stage.  You should submit a detailed sketch with the requested information (or as much as has been furnished to you by the vendor or compressor manufacturer.  If you do this in an Excel spreadsheet, you facilitate our members in reading and understanding your sketch and calculations.  That’s the way engineers work.  We all have to have peer checking and the more clear and detailed the calculations are, the easier and faster the checking.  It’s up to you.  This is not a hard task; it just has to be explained accurately and in detail so that we can be of help.