Calculating Refrigerant flow in a Sanden TRSE07

microHydro on Sat November 13, 2010 8:04 PM

I'm trying to design a system with a Sanden TRSE07 Scroll Compressor. When I check two refrigerant flow calculations against each other, they don't reconcile. Please help me find my error.

Sanden lists the TRSE07 volume at 77.1 cc per revolution. Converting this volume to cubic feet, I get 0.0027 ft^3. The Density of R134a at 43 PSIA is .232 lb/ft^3 (from R134a ideal gas constant 53.3). Thus at 2000 rpm I calculate 76 lbs /hr of refrigerant flow.

Looking at the TRSE07 Performance curve (Curve) I see 13,300 BTU/hr cooling at 2000 rpm. I attempted to calculate the refrigerant flow from this curve using the equation for heat flow:

Qc= mc[cp*deltaT+Effective Latent Heat]

Qc is heat flow, 13,300 BTU/hr

mc is refrigerant flow lb/hr

cp is specific heat for R134a, .204 btu/lb

deltaT is superheat, 9 F listed on the curve

Effective Latent Heat, 53 BTU, came from charting the operating point of curve on an enthalpy chart. I used Discharge pressure 256PSIA to assign a condenser operating temp of 143F. With 18F subcooling (listed on the curve), the constant enthalpy line descends at 55 btu/lb. Latent Heat ends at 108 BTU/lb.

Solving for mc, I get 242 lb/hr.

Help! Why are these flow rates so different (76 lb/hr versus 242 lb/hr). It would seem that the volume method shown first would produce a theoretical max flow GREATER than the actual flow calculated in the second method. How do you use the 77cc/rev compressor size to design refrigerant flows?

Enthalpy chart for R134a below:

Edited: Sat November 13, 2010 at 8:19 PM by microHydro

ice-n-tropics on Sun November 14, 2010 7:37 PM

The chart is close to correct but optimistic because the oil mass flow is forgotten (and other reasons).
Cordially,
hotrodac

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

Edited: Mon November 15, 2010 at 10:07 AM by ice-n-tropics

microHydro on Sun November 14, 2010 9:45 PM

Hotrodac,
Thanks for your reply. Your expertise is greatly appreciated.

Would a volumetric efficiency for this compressor help me account for the inactive inlet chamber volumes?

I was using 43 psia at the suction inlet to the compressor. This is the pressure listed on the Sanden performance curve. What do you mean saying, "It's much higher average pressure than 43 abs. " --Wait, I think I get it. As the pressure increases thru the scroll path, part of the scroll volume has more refrigerant mass. How do I calculate the higher average pressure?

How could I more accurately draw the operating point for the TRSE07 performance curve on the r134a enthalpy chart? I have much to learn, and I hope this chart could be a good teaching tool.

How would you go about calculating the refrigerant mass flow in this TRSE07?

Edited: Sun November 14, 2010 at 9:55 PM by microHydro

NickD on Mon November 15, 2010 10:40 AM

Years ago, like around 1965 got a refrigeration engineering handbook so I could understand how to repair my own AC system. Scanned it was more like it just to get a general idea without working out tons of equations. Understand they have CAD programs for that today.

Briefly, flow rates on a compressor are maximum and ideal, don't forget you have an orifice restricting those flow rates. And while 1 BTU is the amount of heat energy required to raised one pound of water 1*F, takes a lot more than that for change of state, refrigerants equally rely on change of state. See this missing from your specifications. Heat exchanger components are rated in BTU per hour using the now outdated English system of measurements, those metric guys like calories. The heat exchange rate is dependent on either temperature rise or fall.

Just to complicated of a subject that can be covered in an 800 page text book for this board, I feel. In real life engineering, really can't do what you want to, in a vehicle, given a space and a targeted cost for the components, plus that all important deadline date to get it done.

ice-n-tropics on Mon November 15, 2010 2:18 PM

Your on the right track except DuPont shows the density in the range of .9 lb/ft3
I'm limited on attaching a chart, but your superheat zone could be enhanced and the pressure changes as the refrigerant is condensed and vaporized
The volume of the cross sectional area of the crescent shaped intake pocket X the scroll wall height is the swept volume displacement . The gas spirals inward for a total travel distance of about 630 degrees as it is compressed. The 43 psia is measured externally to the intake port, therefore the pressure drops in the pocket due to gas flow restrictions as the crescent chamber closes and seals between the 2 involutes (fixed and orbiting)
hotrodac

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

Edited: Mon November 15, 2010 at 2:39 PM by ice-n-tropics

microHydro on Mon November 15, 2010 10:15 PM

Thanks for the reply, gentlemen.

nickD: Funny you should mention an old 1965 Handbook. My textbook is a 1975 edition of Modern Refrigeration and Air Conditioning. No references to either R134a or scroll compressors! The change of state energy you are referring to is the "Effective Latent Heat", expressed as the length of the bottom operating point line (red) on the enthalpy chart, about 53 BTU/lb. This assumes that I correctly inferred the operating point on the enthalpy chart from the suction and discharge pressures.

hotRodAC: Regarding your statment, "The volume of the cross sectional area of the crescent shaped intake pocket X the scroll wall height is the swept volume displacement." Is Swept Volume Displacment different than Sanden's Max. Displacement (77.1 cc for the TRSE07)? How do I use it to calculate a refrigerant mass flow?

Edited: Mon November 15, 2010 at 10:16 PM by microHydro

ice-n-tropics on Tue November 16, 2010 11:31 AM

Don't know why it's called MAX. displacement. The displacement never varies.
Swept volume displacement = 77.1 (fixed).
Mass flow is basically calculated using same factors as BTU
e.g., working from compressor rpm and a Micro Motion Coriolis mass flow meter reading together with the liquid temp and pressure into the TXV and the temp and pressure of the vapor out of the evaporator we calculate BTU absorbed in the evap. from the refrigerant properties table. That's how the compressor BTU curves are drawn from calorimeter data.
Vehicle A/C designers calculate how many BTU they need at a engine rpm and wind up w/ how many compressor cc they need per mile to cool the vehicle. Then they can choose the size of compressor and it's drive ratio while considering volumetric efficiency.
If you start with the cc and comp speed to find BTU you need , in addition to refrigerant tables the volumetric efficiency of the compressor. The manufacture's graphs are more accurate. Firstly its just a rough ball park figure unless you know all the pressure drops and temp changes and oil circulation. It's too lengthily to calculate here and now.
hotrodac

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

NickD on Wed November 17, 2010 9:59 AM

Can screw the equations and play with it by breading boarding a system. Key is the evaporator outputting the desired BTU/hr. Orifice sized can be played with, but only a few small choices, as is the refrigerant choice, EPA dictates that. In my old manual, hundreds if not thousands of different refrigerants were listed, was shocked to see so many. But today, and at least for today, its R-134a, who knows what tomorrow will bring. Compressor can also be played with, if too small, won't get your pressures, oil is generally a percentage of how much refrigerant is used, about a 4:1 ratio.

Can tell you since I have many patents, US patent office does not want to see calculations, but a working model, as does your stupid ignorant boss when working as an engineer. They also want to see a working model and the price tag, namely the price tag. Amazing the degree of art that goes into a design, even use the term, the state of the art.

microHydro on Mon November 22, 2010 2:24 PM

Thanks again for the replies. I've spent some time reading a thermodynamics text and gathering better online resources, and have found a few answers.

For the original mass-flow calculation I used .232 lb/ft^3. As hotRodAc pointed out, this is wrong. From a r134a chart using 9F superheat and 43 psia, I find .813 lb/ft^3. I wonder why I could not calculate this? Is the ideal gas R value for r134a not 53.33? Sandon does not give a volumetric efficiency for the TRSE07, but they do for the TRSA12 (Curve). Using 91% efficiency at 2000 rpm, and the revised R134a density, I recalculate a refrigerant flow of 241 lb/hr. This matches the other original refrigerant flow calculation.

The higher average pressure during the compression cycle which is illustraited in the earlier "Scroll Compression Process" diagram I posted is apparently not the pressure to use in this calculation. As an interesting side note, I learned that the higher average pressure can be characterized as the Mean Effective Pressure (MEP). MEP x Total Displacment x RPM = power. However this is the Total Displacment of the entire compression cycle, not just the inlet volume of the crescent shaped inlet pocket (Sandon calls this the Max. Displacment, 77.1 cc).

Edited: Mon November 22, 2010 at 2:58 PM by microHydro

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