Complete Heat Gain Analysis of Motor / Pump
Complete Heat Gain Analysis of Motor / Pump
(OP)
I have reviewed previous posts and couldn't find anything quite the same as this, but feel free to direct me to previous post or let me know if it is better suited in different forum.
Background: Was hired to design life test lab HVAC and chilled water systems for motor manufacturer. Owner has swimming pools in test facility with 30 pumps/motors per pool around perimeter pulling water straight out and back into pool (in closed room/chamber). Pool water is to be pumped through heat exchanger with chilled water to maintain 70 deg F pool water. Pumps are assumed to all be nominal 5hp. Space is to be maintained at 75deg F and 50-60% rh. Chiller will provide chilled water to pool water heat exchanger and air handler to condition the space (w/ chilled water coil). Owner's opinion and past experiences with their tests and equipment stated that approx 1/3 of rated hp will to to water as heat and only motor inefficiency will be rejected to space. They do not feel all hp of motor needs to be accounted for.
Question(s): Assuming 30 motors/pumps at 5hp each (pump and motor in conditioned space), what is heat rejected to pool water and what is heat rejected to space? Following ASHRAE calcs with driven equipment in space, total power (motor + inefficiency of motor) must be accounted for and all rejected to pool or space. Does the friction of the water in the pool from the pumps really all convert to heat? (conservation of energy) I'm assuming ineffiency of motor goes to space, and inefficiency of pump mostly goes to water (say 90/10 or 80/20 split), but does the remaining energy of the pumped water transfer to heat within the pool? Any tips on how to convey this to an owner so they understand and buy off on it?
Final thought: Other half of lab is similar but instead of driving pumps the motors drive fans all contained in test chamber/room. Does this behave the same way with the fans? (energy of moving air within room all converts to heat)
Sorry for the long post. Long time follower, first time 'poster'.
Thanks in advance
Background: Was hired to design life test lab HVAC and chilled water systems for motor manufacturer. Owner has swimming pools in test facility with 30 pumps/motors per pool around perimeter pulling water straight out and back into pool (in closed room/chamber). Pool water is to be pumped through heat exchanger with chilled water to maintain 70 deg F pool water. Pumps are assumed to all be nominal 5hp. Space is to be maintained at 75deg F and 50-60% rh. Chiller will provide chilled water to pool water heat exchanger and air handler to condition the space (w/ chilled water coil). Owner's opinion and past experiences with their tests and equipment stated that approx 1/3 of rated hp will to to water as heat and only motor inefficiency will be rejected to space. They do not feel all hp of motor needs to be accounted for.
Question(s): Assuming 30 motors/pumps at 5hp each (pump and motor in conditioned space), what is heat rejected to pool water and what is heat rejected to space? Following ASHRAE calcs with driven equipment in space, total power (motor + inefficiency of motor) must be accounted for and all rejected to pool or space. Does the friction of the water in the pool from the pumps really all convert to heat? (conservation of energy) I'm assuming ineffiency of motor goes to space, and inefficiency of pump mostly goes to water (say 90/10 or 80/20 split), but does the remaining energy of the pumped water transfer to heat within the pool? Any tips on how to convey this to an owner so they understand and buy off on it?
Final thought: Other half of lab is similar but instead of driving pumps the motors drive fans all contained in test chamber/room. Does this behave the same way with the fans? (energy of moving air within room all converts to heat)
Sorry for the long post. Long time follower, first time 'poster'.
Thanks in advance





RE: Complete Heat Gain Analysis of Motor / Pump
Take a look at h
Electrical Motors and Heat Loss
Heat gain from electrical motors to the surroundings
RE: Complete Heat Gain Analysis of Motor / Pump
GPM x ht of head x specific gravity / ( 3960 x impeller efficiency)
Probably if you could measure the power draw into the pumps, you could break out the heat into the water with that estimation and the reminder of the heat goes to the air.
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Complete Heat Gain Analysis of Motor / Pump
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Complete Heat Gain Analysis of Motor / Pump
The theoretical temperature rise of the water within the pump at steady state conditions assuming that all the heat generated remains in the fluid.
dT = H / (778*Cp*n)
dT = Temperature rise of fluid, deg-F
H = Pump head, Feet
Cp = Specific Heat of the liquid, Btu/(lb-deg F)
n = Pump efficiency, decimal value
This is the theoretical temperature rise of the liquid within the pump at steady state conditions assuming that all the heat generated remains in the fluid.
The heat loss through the pump casing and insulated piping will be minimal(given the low values of dT you get).So allocating 100 % work out put from the pump to water is a fair assumption in my opinion.
RE: Complete Heat Gain Analysis of Motor / Pump
RE: Complete Heat Gain Analysis of Motor / Pump
SAK9 - Thanks, yes I had used your equation (same one I found in Cameron Hyd. Data) to find temp rise in pump but still wasn't sure where the "left over" power/energy went from the moving water. The energy of the moving water in the pool converts to heat, right? I don't know how else to account for it in an energy balance in a closed system like this. Is this what you mean by, "So allocating 100 % work out put from the pump to water is a fair assumption in my opinion."
Thanks again
RE: Complete Heat Gain Analysis of Motor / Pump
Where the pump impeller inefficiency energy goes I am not sure, but would assume it goes into the water?
But then you need to start considering heat transfer from the pipes in/out of the plantroom. If the water is colder than the plantroom the water in the pipe will absorb plantroom heat and it all too complicated and I would ignore for my plant sizing.
RE: Complete Heat Gain Analysis of Motor / Pump
Similarly if you examine the computer heat load calculations you will find that all the fan power is added in as sensible heat and is used in arriving at room air flow rates.In the olden days the motor was mounted outside the air handling unit(using a belt drive)and therefore the heat rejected by the motor was not accounted for in the cooling load calculations.
RE: Complete Heat Gain Analysis of Motor / Pump
RE: Complete Heat Gain Analysis of Motor / Pump
RE: Complete Heat Gain Analysis of Motor / Pump
The clouded paragraph is against the laws of Physics.Imagine a closed pipe loop with a 5 hp pump driving water through it, pump head being equal to frictional losses.According to Carrier there will no temperature rise and 5 Hp will be vanishing into thin air .The situation I have described above is very similar to churning in a centrifugal pump(pump operation with a closed discharge valve).If you can try this out somewhere,it will be a good learning experience for you.
RE: Complete Heat Gain Analysis of Motor / Pump