Heating an temporarily enclosed area

[CGarfield]

Hello everybody;
I'm hoping you guys can help me with a little problem I'm having. I'm trying to determine the heating requirements for a construction site. The space consists of a temporary scaffold enclosed with insulated construction tarps. We are going to use Herman Nelson industrial heaters to heat the enclosure.

I'm able to calculate the amount of heat being transferred to the enclosed area (using air flow from the heater and the temperature rise claimed by the manufacturer of the heater). I can also calculate the heat loss through the tarps (conduction). I am also considering convection and radiation losses. What I'm struggling with is the air that is lost. The tarps will be anything but air tight. Any air pumped in will also be exhausted. I've gone back to my HVAC book and am scratching my head. My text includes a discussion on infiltration. I know the number of air changes per hour. But do I need to consider this? The whole premise behind the infiltration (air change method) is to calculate the amount of heat required to heat the air needed to replace air lost. In my opinion, this calculation is not required. I do not need to know how much heat is required to make up for the lost heat because the amount of heat entering is a fixed value.

I'm going to use an iterative process (5 minute iterations):

1. Calculate the amount of heat being pumped into the enclosed area
2. Using the heat calculated in 1, mass of air, mass of steel, I determine the temperature of the enclosed area
3. Using the new temperature of the enclosed area, I calculate the heat loss through the tarps due to conduction, convection and radiation.
4. I repeat step 1.
5. I deduct the amount of heat lost calculated in step 3 from the heat pumped into the enclosed area calculated in step 4.
6. Using the net heat input, I calculate the new temperature in the enclosed area.
7. Repeat steps 3 through 6 until target temperature is reached.

There is still this little voice in the back of my mind that says I need to consider the amount of air being pushed out, but for the life of me, I can't figure out how to address this.

Can anyone provide some insight? I'd appreciate any help you could provide.

Regards,

Brian

 

[MintJulep]

The ratings of the heaters should be net.

So, you should have:

[ol 1]
[li]Capacity of heaters.[/li]
[li]Air flow of heaters.[/li]
[li]Temp rise through heater.[/li]
[li]Design outside temperature.[/li]
[li]Design (desired) inside temperature.[/li]
[/ol]

From From 1, 2 3 and 4 you calculate the heater discharge temperature.
With the discharge temperature, 2 and 5 you calculate the sensible heat capacity in the delivered air that is available for useful heating.

You need a useful heating capacity that is larger than your calculated losses.

 

[zekeman]

Since your central heat loss problem is air escaping the enclosure, you need to accurately get that data first. No need for iteration since you assume the equilibrium conditions you want (say Ti=65 deg F inside enclosure) and from there add the enclosure envelope loss, UA(Ti-T0) plus the escape loss thermal mass flow loss W'c(Ti-T0)through the tarp.
U overall conductance
A envelope area
Ti, T0 inside and outside temperatures
W' rate of air loss ( this may be tricky since you will need an accurate calculation of the inside pressure)
c specific heat of air

 

[IRstuff]

You have to have makeup air to maintain oxygen levels and remove carbon dioxide, that makeup air must be heated up from the external ambient temperature.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[zekeman]

Following up, since your transport heat loss is central, you will need fan data from the heater mfr as part of the air flow problem which I casually mentioned.
So, basically, you have a fan blowing hot air into an enclosed volume which is permeable.

 

[MintJulep]

Herman Nelson has direct and indirect fired models. Select an indirect fired flavor and the carbon dioxide issue doesn't exist.

The heater is a big fan blowing hot air. So it or they will pressurize the tent, and it should be safe to assume that infiltration is negligible. Unless of course you are in a windy area. Then things get more complicated if you are going to try to calculate.

Not that it really matters, because effectively your only options are to try to tighten the envelope as much as possible and add more heat.

 

[racookpe1978]

What is your minimum outside air temperature? What is your maximum?

How long will you need to use the enclosure?

Why are you heating the enclosure? For "comfort" of the workers? For a adhesive or curing temperature? For a weld or material temperature? Do you need a constant temperature or can it vary as people enter and exit and material is carried in and out?

Are you working 24x7 or just one shift? Can you allow it to cool down (or heat up!) during shift turnovers or during lunch hours or weekends?

Not that I mind getting heat when working outside! It's just not done very often on my jobs unless the steel needs it or the epoxy or concrete needs it .....

 

[CGarfield]

Hi everybody, thanks very much for all your input. I think I have it sorted out now.

racookpe1978: we are required by contract to heat the vessels for the refractory subcontractor. We are anticipating outdoor ambient temperature anywhere from -15 to +5C. We are required to maintain vessel temperature between +15 and 30C. We have to maintain this temperature for a minimum of two weeks prior to the subcontractor starting work. Once the subcontractor starts, they will be in the vessel for 4 weeks. When they are done, we have to hold the temperature for another 2 weeks.

 

[racookpe1978]

Do you see how much more restrictive your requirements have become with the requirement to protect the refractory for 2 months? 8<)

1) Double wrap your plastic with two layers. Not just for insulation "effectiveness - which will be substantial with the trapped air between the two layers of plastic - but for the physical protection of your enclosure temperature when the outside layer gets torn or ripped. And it will.

2) Add an "airlock" of sorts so no one can enter the enclosure directly, but has to pass into a double closure or through a double layer of heavy plastic/fiberglass layers that will automatically close around each person as they enter. That works for me when I have to wrzp outside scaffolding against 50 to 70 mph west Texas or Idaho winds when we are welding outside. Temperature is not as great a concern, but if the wind gets through the plastic and canvas, the weld shield gas is blown away and the weld fails. Same type of problem though: Shielding the work from the environment.

3) Use as much plywood or rigid walls as possible to protect your actual (plastic) heat shield wrap: construction is rough on "temporary wraps! Watch your "waterproof" details of the roof: ANY catch basin ANYWHERE will trap water or ice and cause your wraps to collapse into your enclosure ---> This will lose your heat AND get your refractory wet. And, of course, the worse rain and wind will happen on a holiday weekend on night shift.

 

[chicopee]

I don't know the specs on your vessels, however, I would say study the possibility at blowing hot air in the vessels and insulate them while they are being heated. As the refractory is to be installed, some of these blankets can be removed by the subcontractor; still build a temporary unheated enclosure for the sub's crew while installing the refractory.

 

[waross]

The Herman Nelsons that I have learned to hate were the indirect fired type. They are typically installed outside the enclosure and "Elephant Trunk" flexible ducting used to direct the air into the heated enclosure. This is a once through system. Outside air enters the heater and the temperature is raised. The colder the air going in, the colder the air coming out.
Hint; You can't get the heated area hotter than the air coming out of the heater.
Hint; In extremely cold weather the workers like to return one of the Elephant trunks back to direct heated air on the back of the Herman Nelson. This serves a two-fold purpose.
1> The air entering the heater has been preheated. The discharge into the heated area is less volume but higher temperature.
2> When plagued with a high moisture content in the fuel, the heat on the back of the heater avoids many of the fuel freeze-up issues.
The colder it got, the fewer heaters would operate.
Not much fun at -20 or -30, but the pay was great.



Bill
--------------------
"Why not the best?"
Jimmy Carter