Excess BTU required for parked equipment

Rocky_3

are going to be parked on this 9" slab. Five bays, one fire truck per bay. Heat load on the building is approx 220,000 BTU. Radiant slab, but engineer is spec-ing one unit heater over each bay as well for "fast-recovery". Do you guys add some excess BTU's in your heat loss calc to cover such large heat sinks? Also, what is the prevailing attitude on sizing the boiler for such a scenario? I am inclined to size the boiler based on the radiant heat load, allowig nothing for the unit heaters. I mean, the heat loss is the heat loss right? The heat loss will either be satisfied by 100% radiant, or 100% unit heaters, or some combination equalling 100%, but not 100% each, right? If the heat load is 35 BTU/ft, do I add in another "X" amount to cover the heat sink associated with the large fire trucks? Any thoughts would be appreciated.
Rocky

J.C.A._3

Mix and High temp. zones.

Good question Rocky.

I'm thinking that the heat loss when the doors get opened is being WAY over thought.

It's nice to have some air exchanger kick on to keep the chill off, but having to have 2 temps. becomes a PITA if the whole joint is heated at lower temps.

If they already have some high temp. zones, just pipe in another... to a Modine type, and be done with it.

It won't kick on too often if the doors stay closed when they're supposed to...and the guy who over thought the design will probably feel good knowing his keister is covered.

Or maybe some radiant type overhead heaters like the big box stores do for the entry... will keep from having to oversize the heating plant? Thinking out loud. Chris

Brad White_185

Siggy once spoke about this

when I first met him in 1997 at one of his seminars. I mention that in order to "date-stamp" the thinking should it have evolved. His take (which for vehicle bays with sporadic exits but fast closings) was elemental and I agree with it.

Specifically that BTU output is a function of Delta-T between the radiant surface and the ambient. When the doors open and cold air spills in, the delta-T widens dramatically thus increasing output for the duration.

The recovery takes place over time and the slab- even your 9-inch slab in Alaska Rocky, is not likely to get anywhere near ambient in that amount of time. The "door open time" is relatively small. The recovery times between cycles tends to be long.

Then there is the aspect that, if everyone is out fighting a fire, who is left to feel cold, not that they would?

I can see the desire to install unit heaters for fast recovery, but let's face it, how long are we talking about? Also, that high air heat may stratify things a bit and require higher water temperatures to boot. Not sure if you are dealing with a low-temperature boiler, but if you are fixed at 140F maximum supply temperatures, those heaters may not leave room for the ladder trucks

BTW: If I were to install a propeller UH, I would use the down-blast type as a form of destratification.

Radiant in fire stations, aircraft hangers and ambulance or DPW garages are much appreciated by the occupants. The mechanics love it.

My $0.02 with homage paid to Siggy.

J.C.A._3

Hey Brad...

The Dalmatians love it too.

I have yet to see a radiant floored Fire station where the dogs aren't sleeping on the same floor as the trucks.

I think the guy designing is worried about the "door open/door close" losses...and we all know with a slab temp. designed right....it usually won't be a problem.

Worrywarts.What do you do?

Follow the engineers plans..and reap the spoils.JMHO. JCBSVFGF

Boilerpro_5

I think you guys missed his question...

It wasn't about the doors being open, it was about the 70,000 pound trucks at 0F entering the structure and having to warm them up.

Boilerpro

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Brad White_185

Ah, I see

You are correct, I passed over that part. Heck, throw in a unit heater and stage the boilers (have a spare that can kick in and not tax core efficiency.

Pat the dog.

Done.

Glen

nope

I think Siggy's solution was a little more complex - like figuring in the mass of the trucks, delta t back to ambient etc. He has spoken on it several times.

Brad White_185

Thanks-

Of course there is more to it and your point makes sense.

hot_rod

Unit heaters are nice

for the shoulder season however. Quick warm up in the morning without ramping up the slab.

I did a fire truck manufactuting shop last fall, about 10,000 sq ft. They installed unit heaters in four corners, and I must admit they really add to the radiant system. They also help move moisture from the melt waters on the slab.

I suppose you could calculate the amount of BTUs needed to warm 70,000 lbs of steel from 0F to 60F? I think radiant from the top and bottom would be the quickest way to heat a load like that.

What are their expectations for warming the trucks, or maintaining the space temperature? I guess it would depend on how many cold trucks were pulled onto the slab at once.

hr

Jerry Boulanger_2

Heat balance

If the power train in the truck is at operating temperature when it pulls into the space there will be a lot of heat given up to the space, perhaps enough to balance the heat required to bring the rest of the truck up to indoor ambient.

I used to have a Chrysler with a 440 - when I parked it in my then unheated garage with a warm engine in cold weather the temperature in the garage would go up!

[Deleted User]

Cold fire engines suck...

heat out of any space, at a fairly constant rate.

My thought process is this.

How many "extra" btu's do you suppose the slab is holding at say 75 degrees F. Lets say that its delta T is 10 degrees. The square footage of the slab, times the depth, times the density, times the specific capacity of concrete to hold or give up heat = a BUTT load of btu's.

Is it enough btu's to maintain reasonable conditions in the apparatus room? My gut says it is, because as someone else pointed out, the drive train is going to be rather hot upon return as well.

If it were me, I would rough in the necessary high temp lines between load and source, but not set or hook up the FCU's. Let the system run for a season and talk to the occupants and see if they were comfortable with its operation.

Stop and think about jet hangars. They don't add additional capacity to those systems, and when the airplanes come in, they are towed in by tugs. I think your engineer is overly concerned at someone else's expense.

Look to the airline hanger industry for guidance.

ME

Chris S

If you're going to go through all of those calculations ( weight of steel etc) keep in mind a single axle fire truck ( engine or pumper ) will weigh about 40-45K . A tandem tanker truck carrying 3000 gallons will weigh 50-55k. Only an aerial device, will have the type of mass you describe.
Unless you're working at an airport, or refinery with specialty equpipment, then those weights would be appropriate.
My other comment is that in radiant heated fire stations I have been in where radiant is the only source of heat, the occupants are very happy. As soon as the bay doors close & the cold air on the skin is stopped- one feels almost immediatly "comfortable". Design temp however where I am is 0*.
Additional air movement if desired can be accomplished by a system such as AirVac911 system which is marketed to emergency services.


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tim smith

another option

If gas available, what about a couple long radiant tube heaters that kick on based upon air temp, w/ interlocks so doors have to be closed. This would heat up the truck surfaces and also give fast recovery for the times when needed to supplement rfh.

Rocky_3

Air force base

where our nation's missile defense interceptors are located. These trucks will be 70,ooo pounds. Design temp is -47, and the wind howls like a banshee through that area. Very real possibility of lots of cold steel sitting on that slab. However, slab is 9" thick. My gut feeling is size the boiler according to ADS heat loss calcs, and let the excess mass of slab take care of any cold metal.

Regards,
Rocky

Lurker_2

re

Of course the truck will have around 1000 lbs of mass that will be up about 160f , since it was running when it came in there, so I think the point is unimportant. Also, few expect garage space to be +or- 1deg F even when working there.