Recip Pump Heat Rejection

[Funkytown]

Ok, so I have a 2250 HP triplex reciprocating pump. When testing it, I was inputing 1072 brake HP. I'm trying to size a cooler for this thing. Over 30 minutes of constant speed running, the power end lube temperature rose from 92 deg F to 106 deg F. I am trying to determine how much horsepower was dumped into the lube oil. The pump advertises a 90% mechanical efficiency, so it is feasible to think ~10% of the energy is rejected into the oil, but I do not believe this.

Here are the complete operating parameters and oil properties:
-Starting Oil temp: 92 F
-Ending Oil temp: 106 F
-Duration of test: 30 Minutes
-Heat capacity of oil at 100 F: .479 Btu/lb/F
-Density of oil at 100 F: 7.31 lb/gal
-Volume of oil resevoir: 350 gallons
-Total mass of oil in system: 2559 lbs
-Oil flowrate during test: 16 GPM

I have been told I could use the following formula:
Heat Rejected = (Heat Cap)*(Total mass of Oil)*(DTemp/DTime)

By plugging the numbers, you get 13.5 Horsepower, which is only 1% of the input power of the pump. Does this sound right? Any suggestions on what I have done wrong?

 

[rzrbk]

I think most of the heat due to inefficiency would be carried away by the pumped fluid; seen in some deltaT between suction and discharge. Some small amount of energy would also be radiated to the atmosphere thru the frame. The oil would only see heat due to bearing friction, etc.

 

[ccw]

Hi rzrbk

Your assumption that the 10% can be found in the oil is not accurate. This might be the case if the oil reservior, oil path boundaries and oil mass were adiabatic. Did the oil reservior get hot? Did the crankcase get hot? Then, there is thermal energy lost from the oil through these boundaries (conduction, convection, and radiation) during operation, as well as other thermal energy paths suggested by rzrbk.

You did not specify that you are running under maximum operating load during the 30 minute test. Do you size the cooler for 1072HP or 2250HP input?