Wind effect on heat loss/gain through walls

[ktgottfr]

Greetings all,
I have been working on an HVAC calculation for a shelter made from a standard sea container. I have been using ASHRAE 2001 as a guide. However, I was asked by the clients to take a 50km/hr wind into account in the heat loss and/or gain through the walls.
My problem is that ASHRAE Fundamentals 2001 does not seem to mention any effect due to wind speed in any of its simplified methods. It seems to say that heat loss through the walls is treated as conduction only.
What I actually did, was use the Canadian Windchill Index to find the apparent outdoor temperature at a 50km/hr wind, and used this as my outdoor air temperature in the calculations. I thought that this would probably be a conservative estimate.
But, if I use the windchill index for cold ambient temperatures, what should I be using for warm ambient temperatures? I had assumed that even when the outside temperature is warmer than the shelter interior temperature, the effect of the wind would be to cool the shelter, and therefore I ignored wind effects for the shelter cooling load. But a colleague here has taken issue with this assumption, suggesting that increased windspeed would increase the convective heat transfer rate, and therefore actually heat the shelter if the outdoor temp is higher than the interior temp.
As I mentioned before, I cannot seem to find anything in ASHRAE fundamentals that helps me. Does anyone have any experience with this? Thanks a bundle!!
-Kristjan

 

[AbbyNormal]

weather strip the doors and I think a container will be prety tight

just worry about the ventialtion load

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

you were worried about air films, going to be insignificant assuming you will be insulating

In IP units you get an R.17 for a 15 mph wind, 50 kmh is about twice the speed so if you ignore the air film you will be fine

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

cooling it is all about the sun beating down on the surface, not driven by air temperature

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ktgottfr]

Hmmm. According to ASHRAE's simplified methods, the outside air temperature is very significant. The effect of the sun would be to raise the apparent outdoor temperature, but so long as the temp outdoors is warmer than the temp indoors, you will get conductive heat transfer due to that temperature difference.

However, while thinking about your answers I think I may have come up with the answer to my own question. The wind speed will not increase the heat transfer rate, because I am already calculating the heat transfer rate based on pure conduction - which assumes that the heat transfer to and from the air is already perfect.

So, this would mean that my use of the windchill index to lower the apparent outdoor temperature might have been unnecessary. Really, the wind would only affect infiltration rates as you meantioned in your other posts.

Thanks for the help!

-Kristjan

 

[AbbyNormal]

what kind of heat gain do you get from a north wall in cooling calculations?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ktgottfr]

According to Ashrae fundamentals 2001 radiant time series method, you use a somewhat involved formula to calculate the "Sol-air" temperature (for cooling loads only. heating loads ignore solar heat gain). If this temperature is higher than the interior temperature of the shelter, then you get conductive heat transfer through the wall.

For a north facing wall, the Ashrae formula still gives a small bit of solar temperature increase due to light reflected off the ground and surroundings. However, even if you ignore the solar temperature increase (such as in the case that the wall is completely shadowed all day) then you would still get some conductive heat transfer through the wall if the outside air temperature is still higher than the inside temperature.

The radiant time series method has a few factors that spread the heat gain from a given hour over time depending on the wall construction and interiorlayout. But it seems that the heat transfer through the wall is entirely based on the temperature difference between inside and outside using pure conduction. Any energy transferred to the air is considered to do so instantaneously.

So, I don't know if this is absolutely correct due my limited experience. However this seems to be the way the method is laid out in ASHRAE Fundamentals 2001.

I was initially confused, as I had been asked to take windspeed into account for the shelter cooling/heating calcs, and assumed a windchill effect would be involved. However, since the wall heat transfer is purely conductive, and takes the outside temp of the wall to be equal to the outside air temp (same for interior temp) then obviously the convective heat transfer coefficent does not come into play. And so for this method increasing the windspeed would not alter the heat transfer through the walls.

If I had used ASHRAE's "Heat Balance" method - which is really really involved - then I would have used some sort of finite difference method on the wall heat transfer that takes the convective heat transfer into account. But that method is not practical for the type of shelter I was analysing.

 

[coldmaster]

Wind speed and convection coefficient are taken into account using ashrae. Although I've never been able to make any sense of the discussion regarding RTS or heat balance...I believe those methods are innappropriate for hand calculations...I believe these are for the authors of computer programs that can stand to do these sums and iterations and you only have to actually understand that chapter once...then you get to use the program...

convection is accounted for in ASHRAE by considereing an equivalent thermal resistance of air "film." There is a different value for air inside the structure, vs. the wind outside a structure. There is also a value you use when there is an air gap in your conduction (wall) assembly. Take a look at the chapter preceding the complicated ones..."for residential use only."

And look for the simplified CLTD method. This gives you a modified temperature difference to use in your conduction calculation, based on solar heat gain...I believe this was last published in Fundamentals 1997. Also these tables are in the AHSRAE pocket guide...

 

[AbbyNormal]

my point kt is that in heating calculations you worry about the differential between indoor and outdoor temperatures driving heat through a resistance of R-values including 'outdoor air flims'.

In cooling the difference between indoor and outdoor temperature factors in mainly on infiltration and ventilation. The heat gain through walls is more about the heat from the sun beating on the wall, than it is the temperature differential between indoor air and outdoor air.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ktgottfr]

Ok, thanks Coldmaster - that's what I needed to know! It is also good, because I did not include the insulating value of airfilms in my calculation so I will be nicely on the conservative side.

I can see now what Abbynormal meant when referencing air films...

I'm suprised the discussion of airfilms is not more detailed in Fundamentals. Perhaps I just have not found the correct chapter yet. Although in heinsight it is the obvious way to deal with wind speed in a simplified method.

As for using the CLTD method -too late I already made a computer program to perform the radiant time series analysis.

Thanks everyone for your help on this!
-Kristjan