Thermal power of the air extracted from a building

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Discussion Overview

The discussion centers on calculating the thermal power of air extracted from a building, specifically whether it exceeds 80kW. Participants explore the necessary parameters for this calculation, including air flow rates and temperature differences, within a technical context.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about calculating the thermal power of extracted air, given flow rates and temperature data.
  • Another participant suggests that knowledge of specific heat or enthalpy is necessary for the calculation, noting that enthalpy depends on temperature and humidity.
  • A participant proposes a formula for power: P = Cp·flow·DeltaT, seeking confirmation on the correctness of the units involved.
  • Concerns are raised about the use of flow in the formula, with a participant questioning whether it would yield power in watts.
  • A suggestion is made to convert volumetric flow to mass flow using the air's density, based on temperature.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the correct application of flow in the power calculation and whether the proposed formula is valid. There is no consensus on the final approach to calculating the thermal power.

Contextual Notes

Participants have not fully resolved the implications of using volumetric flow versus mass flow in the calculations, and there are dependencies on temperature for determining air density.

ricardo46
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Good afternoon.
I need to know if the thermal power of the air extracted from a building is greater than 80kW.
I know the flows entering and leaving the building. Also, I know the temperature of the air inside and outside the building.
How do I calculate the power of the air extracted?
thank you
 
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ricardo46 said:
Good afternoon.
I need to know if the thermal power of the air extracted from a building is greater than 80kW.
I know the flows entering and leaving the building. Also, I know the temperature of the air inside and outside the building.
How do I calculate the power of the air extracted?
thank you
You'll need to know either the specific heat of the air at the temperatures encountered or know the enthalpy of the air. The latter will depend on temperature and humidity.
 
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SteamKing said:
You'll need to know either the specific heat of the air at the temperatures encountered or know the enthalpy of the air. The latter will depend on temperature and humidity.
Ok. I know the specific heat. I have solved problems like this in the past. I am just a bit rusty.

I think I have found the solution:
P = Cp·flow·DeltaT

I am just not sure about the units:
P- power (W)
flow- m3/sec
DeltaT- temperature diference between the inside and the outside of the building
Cp- specific heat (J / (kg . ºK))

Is this correct?
 
ricardo46 said:
Ok. I know the specific heat. I have solved problems like this in the past. I am just a bit rusty.

I think I have found the solution:
P = Cp·flow·DeltaT

I am just not sure about the units:
P- power (W)
flow- m3/sec
DeltaT- temperature diference between the inside and the outside of the building
Cp- specific heat (J / (kg . ºK))

Is this correct?
Seems to be.
 
SteamKing said:
Seems to be.
I think I can't use the flow like this. If i use it like this i won't get the power in watts but this W.m3/kg.
What should i do to get watts?
 
ricardo46 said:
I think I can't use the flow like this. If i use it like this i won't get the power in watts but this W.m3/kg.
What should i do to get watts?
You can convert the volumetric flow to a mass flow, using the temperature of the air to find its density.
 
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