kW equals BTUs...or does it?

[Nuclrchiller]

Really? I have been using 3.41 BTU's Per Watt for many years. Was I wrong all that time or has physics changed more recently?

PHM
-------

Lol!
Yep, like Icy said, good catch. Book actually says One Ton Of Refrigeration = 3.515 kW.

When you don't type and have to finger peck, it's so slow that the disconnect between brain and finger is magnified. That's my story...

[coolcoil]

An electric motor inside an AHU which is inside the airstream typically raises the air temperature 2 to 2.5 degrees F. This tells you that inside the unit most of the KW input is converting the power input to mechanical energy. What happens after that can be argued but the main thing to remember is in this application is the KW input is directly related to motor horse power.

It is not always true that most of the kW input of a fan is converted to mechanical energy. Wire-to-air efficiency of a small motor and fan is very low - 25% is not unusual (eg. a fan coil with a 75% efficient motor and 33% efficient fan). In that case 75% of the electrical power becomes heat. I know a lot of people are surprised that the efficiency of small fans is so low, but that's just physics. When I first started in VAV and fan coil design I was surprised myself.

[WAYNE3298]

icy I remember a conversation on another thread like you said but don't remember posting what you said. Someone on that thread did argue that all energy is converted to heat.

Coolcoil I was referring to the average temperature rise of most AHU'S. I am aware it can vary.

3412 and 3413 are both acceptable BTU per KW. Both can be found in reference books. I use 3412.

[pageyjim]

icy I remember a conversation on another thread like you said but don't remember posting what you said. Someone on that thread did argue that all energy is converted to heat.

Coolcoil I was referring to the average temperature rise of most AHU'S. I am aware it can vary.

3412 and 3413 are both acceptable BTU per KW. Both can be found in reference books. I use 3412.

Your post from the other thread linked below.

"If you Google the engineering tool box heat gain of electric motors in continuous operation there is a table that breaks down the distribution of the heat in kw. Notice with the motor and fan in the same enclosure all kw (heat) is dumped inside the enclosure. The table is set up to tell you how much heat is put into the airstream when the motor and fan are in separate enclosures. The heat of producing the energy needed is subtracted from the total motor input kw when calculating the heat added to the airstream when the motor and fan are in separate enclosures. I'm not going to start another subject by getting into that break down but with everything inside the fan enclosure the motor kw input is the total heat added to the air."

https://hvac-talk.com/vbb/showthread...tion+of+energy

I am reposting this again from engineering toolbox

"Heat Loss from an Electric Driven Device to a Room
The heat transferred to a room or enclosure depends on the location of the motor and the driven device related to the room.

1) motor and driven device in the room - all energy to the motor is at the end transferred to the room (heat loss from the motor and frictional energy from the driven device)
2) motor in the room and device outside the room - only heat loss from the motor is transferred to the room - frictional energy from the driven device is lost outside the room
3) motor outside the room and device inside the room - frictional loss from the driven device is transferred to the room - energy loss from the electric motor is lost outside the room"



https://www.engineeringtoolbox.com/e...oss-d_898.html

[icy78]

Your post from the other thread linked below.

"If you Google the engineering tool box heat gain of electric motors in continuous operation there is a table that breaks down the distribution of the heat in kw. Notice with the motor and fan in the same enclosure all kw (heat) is dumped inside the enclosure. The table is set up to tell you how much heat is put into the airstream when the motor and fan are in separate enclosures. The heat of producing the energy needed is subtracted from the total motor input kw when calculating the heat added to the airstream when the motor and fan are in separate enclosures. I'm not going to start another subject by getting into that break down but with everything inside the fan enclosure the motor kw input is the total heat added to the air."

https://hvac-talk.com/vbb/showthread...tion+of+energy

I am reposting this again from engineering toolbox

"Heat Loss from an Electric Driven Device to a Room
The heat transferred to a room or enclosure depends on the location of the motor and the driven device related to the room.

1) motor and driven device in the room - all energy to the motor is at the end transferred to the room (heat loss from the motor and frictional energy from the driven device)
2) motor in the room and device outside the room - only heat loss from the motor is transferred to the room - frictional energy from the driven device is lost outside the room
3) motor outside the room and device inside the room - frictional loss from the driven device is transferred to the room - energy loss from the electric motor is lost outside the room"



https://www.engineeringtoolbox.com/e...oss-d_898.html

Thanks pagey!
I mis-remembered again

Sent from my SM-G950U using Tapatalk

[WAYNE3298]

Thanks pageyjim.
I had forgotten about that post. I was actually thinking of another thread where we discussed conservation of energy. When I said most of the motor energy input was converted to mechanical energy I was hoping to get different comments than I got. You guys are too clever for that bit of bait. My out was going to be remember the motor KW.
If you have 800 CFM being moved by a 1/2 horsepower motor the air temperature rise calculates to right at 2 F. That indicates all the motor KW input results in heat.

[OHDataCenterGuy]

Well, it's been a couple weeks since I last visited this thread, and I'm happy to see you guys have continued the discussion. I look forward to reading your comments once I can come up for air at this new job, which has turned into everything I suspected it would.

On the plus side, I appear to have a good crew of guys, so that's a relief. And my boss, while extremely inexperienced, seems overjoyed to have someone with my abilities. On the negative, I can see WHY he's overjoyed...this place has been running on a wing and a prayer since its inception. No organization, no procedures, no documentation, guys are bringing in their own tools from home because nothing's here...it's a nightmare. Only the good work ethic of the guys on my crew seems to have held it together this long.

I'm slowly digging into everything, instituting positive changes as time permits, noting problems for tackling later otherwise. That list is growing larger by the hour.

[LoraMoser]

You can calculate them online, just google kW btu converter