Heat loss through an open door

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Discussion Overview

The discussion centers around the heat loss through an open door, particularly in commercial settings during winter and summer. Participants explore the implications of keeping doors open on energy consumption and potential solutions to mitigate heat loss.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the average door size and temperature assumptions, seeking to quantify energy waste due to open doors.
  • Another participant provides a back-of-the-envelope calculation estimating heat loss through air leaving the store, suggesting a significant energy loss of about 9000 Watts based on specific assumptions.
  • A third participant agrees with the estimated energy loss and contextualizes it in terms of cost, suggesting that business owners might find the expense acceptable for customer experience.
  • A later reply questions the wind velocity assumption used in the calculations, proposing that the situation may involve free convection rather than forced convection, and introduces the idea of using air screens to reduce heat loss while keeping doors open.

Areas of Agreement / Disagreement

Participants express varying views on the assumptions made in the calculations, particularly regarding air velocity and the nature of heat loss. There is no consensus on the accuracy of the initial estimates or the best approach to mitigate heat loss.

Contextual Notes

Participants highlight potential limitations in the assumptions made, such as the estimated wind velocity and the nature of convection, which may affect the accuracy of the energy loss calculations.

Who May Find This Useful

Store owners, energy efficiency advocates, and individuals interested in the practical implications of heat loss in commercial buildings may find this discussion relevant.

James Allsopp
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All the info I could obtain talks about heat loss through windows and doors. I am trying to get
several store owners in my town to close there doors in the middle of winter and summer.
The average door is 28 sq ft. I am assuming the inside temperature is 68 degrees and the outside temperate is 34 degrees. How much energy are they wasting? This is not a homework question. I am 65 years old. Too old to remember any of my physics.
 
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Interesting application- here's a back of the envelope estimation:

Assume all the heat loss is through air loss: heated air leaving the store. The specific heat of air is about 0.001 J/(cm^3*K) (from wikipedia). What's the amount of air leaving through the doorway? 28 ft^2 is about 27000 cm^2, estimate a 'wind velocity' of 1 m/s with the door open gives about 3*10^6 cm^3/s air leaving the store. The air is heated 30 degrees, this gives 9*10^4 J/s (9000 Watt) energy loss.

That seems like a lot, so double-check my calculations- I did them in my head.
 
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That seems reasonable to me. At 10 cents/kWh that is only about $1/hour, so I could see how a business owner might think that an open and inviting door is worth that cost.
 
Andy Resnick said:
Interesting application- here's a back of the envelope estimation:

Assume all the heat loss is through air loss: heated air leaving the store. The specific heat of air is about 0.001 J/(cm^3*K) (from wikipedia). What's the amount of air leaving through the doorway? 28 ft^2 is about 27000 cm^2, estimate a 'wind velocity' of 1 m/s with the door open gives about 3*10^6 cm^3/s air leaving the store. The air is heated 30 degrees, this gives 9*10^4 J/s (9000 Watt) energy loss.

That seems like a lot, so double-check my calculations- I did them in my head.

1 m/s sounds quite high velocity-it's 36 KM/Hr, I don't think I feel such strong wind when I'm standing at the door.It sounds more like a free convection problem then a forced convection. But there are some other solutions,in some malls I saw a blower installed above the door, the air flow from the blower creates an "air screen" which reduces the heat losses, on that way they can keep their door open and still reduce the energy wasting.
 

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