Sensible heat gain of electric motors

[tmartin125]

ASHRAE and SNAME T&R 4-16 (2015) use a formula for sensible heat gain from equipment operated by electric motors as follows: q_em = 2545*(P/E_M)*F_UM*F_LM

where
q_em = heat equivalent of equipment operation, Btu/h
P = motor power rating, hp
E_M = motor efficiency, decimal fraction <1.0
F_UM = motor use factor, 1.0 or decimal fraction <1.0
F_LM = motor load factor, 1.0 or decimal fraction <1.0
2545 = conversion factor, Btu/h·hp

The attached exampled 15 hp ballast pump, using SNAME or ASHREA and a UF of 1 the result is 41951 bth/h.

Using actual heat gain the result is 2950 btulh.

The energy going into the motor is 36722 btu/h.

How can the energy from motor heat gain to the space be more than the energy going into the motor?

 

[Antifed]

Hello,

How did you obtain the actual heat gain? Is that from the manufacturer's catalogue?

My old (2001) ASHRAE Fundamentals Book shows the instantaneous heat gain from a 15-hp motor as 44,400-btuh based on F_um=1.0 and F_lm=1.0, with the motor and driven equipment being located within the conditioned space. This is also based on a 3-Phase, 1750-rpm, 86% efficient open-drip-proof motor. If your system is known to run infrequently or under-loaded, you can take some reduction via F_um and F_lm (which it looks like you did in one of your calcs where you came up with approx 1-ton as a load using a F_UM=0.3).

I had a project with a fire pump where, based on fire pump testing standards, was able to reduce the instantaneous heat gain from the 60-hp motor (172-MBH) to something much less since the pumps are tested weekly, only for a certain amount of time, and at dead head. I was originally going to provide a cooling system for the fire pump room before a senior engineer talked me off of the ledge.

Also see:

https://www.engineeringtoolbox.com/electrical-motor-heat-loss-d_898.html

https://www.eng-tips.com/viewthread.cfm?qid=301417

Cheers!