BrentEubanks
Mechanical
- May 23, 2019
- 3
This thread - thread403-416011 - discussed how much of the fan's energy input became heat in the air stream. There was wide-ranging discussion but no clear conclusion was reached.
Having had to research and resolve this issue for my current project, I decided to share the conclusions here for the sake of those who come afterwards.
Short answer: 100% of the fan input energy (brake HP) becomes heat in the airstream, except for the motor losses which are included only if the motor is in the airstream.
Sources:
[ol 1]
[li]ASHRAE Fundamentals Handbook. As was noted in the original discussion, the ASHRAE Handbook shows the heat to the fluid as 100% of the input fan energy, minus the motor losses if the motors are not in the airstream.[/li]
[li] Philip Haves, researcher at LBNL: "1. Heat from fan inefficiencies and motor inefficiencies, if the motor is in the air stream, appear immediately in the discharge air (no surprise). More surprisingly, the fluid work done by the fan on the air stream also appears as heat immediately in the discharge air. This is because air behaves pretty much as a perfect gas (apparently); by contrast, fluid work performed by a pump appears as heat at each pressure drop around the circuit, rather than at the pump outlet. This is because water is an essentially incompressible fluid. "[/li]
[li] Steve Taylor, Taylor Engineering: "Because air at typical HVAC conditions behaves almost as a perfect gas, all fan energy is seen as an enthalpy (temperature) rise at the fan. It does not dissipate in the duct system, i.e. there is no gradual temperature rise in air due to pressure drops."[/li]
[/ol]
Three distinct, authoritative sources all say the same thing, so I think we're on solid ground here. And it makes sense once you think about air as a perfect gas. Pressure goes up and volume does not change (assuming constant duct area, typical in an AHU) so temperature must increase at the same time.
Having had to research and resolve this issue for my current project, I decided to share the conclusions here for the sake of those who come afterwards.
Short answer: 100% of the fan input energy (brake HP) becomes heat in the airstream, except for the motor losses which are included only if the motor is in the airstream.
Sources:
[ol 1]
[li]ASHRAE Fundamentals Handbook. As was noted in the original discussion, the ASHRAE Handbook shows the heat to the fluid as 100% of the input fan energy, minus the motor losses if the motors are not in the airstream.[/li]
[li] Philip Haves, researcher at LBNL: "1. Heat from fan inefficiencies and motor inefficiencies, if the motor is in the air stream, appear immediately in the discharge air (no surprise). More surprisingly, the fluid work done by the fan on the air stream also appears as heat immediately in the discharge air. This is because air behaves pretty much as a perfect gas (apparently); by contrast, fluid work performed by a pump appears as heat at each pressure drop around the circuit, rather than at the pump outlet. This is because water is an essentially incompressible fluid. "[/li]
[li] Steve Taylor, Taylor Engineering: "Because air at typical HVAC conditions behaves almost as a perfect gas, all fan energy is seen as an enthalpy (temperature) rise at the fan. It does not dissipate in the duct system, i.e. there is no gradual temperature rise in air due to pressure drops."[/li]
[/ol]
Three distinct, authoritative sources all say the same thing, so I think we're on solid ground here. And it makes sense once you think about air as a perfect gas. Pressure goes up and volume does not change (assuming constant duct area, typical in an AHU) so temperature must increase at the same time.