Heater sizing for stainless steel enclosure.

[kamikhan]

Hey guys!

I've just stepped into the field of engineering and am instantly faced with a problem that I'll need your help with:

Our electrical dept. has just bought a transformer that would be placed outside in the open. It has a relay(?) mounted on the outside that is subject to temperatures close to -40C in the winters. In order to maintain the temperature of the relay at around +20C, it has been decided that a stainless steel enclosure of 1m x 1m x 0.6m will be put around the relay on the outside of the transformer. The wall thickness of the enclosure is 1.5mm.

My task is to size a small heater that would enable the temperature inside the enclosure to be maintained at around +20C, to protect the relay.

I have tried to put some numbers together, but none of the results make much sense. My heater sizing appears to be way too big.

Please help me with this problem, as I have now almost given up hope trying to figure it out myself. Any help will be greatly appreciated.

Cheers,

 

[MintJulep]

What happens when the relay gets cold? Does it fail? Does its failure result in the failure of the entire transformer?

So what happens when the heater fails, or the thermostat that controls the heater fails, or the squirrel that decides that your enclosure is a nice place to spend the winter chews through the wires?

The entire transformer fails?

1.5 mm thick stainless steel offers rather little insulation, so a big amount of heat will be required to maintain an 80K delta-T.

It would very likely make far more sense from a life-cycle perspective to purchase equipment properly rated for the environment than to cobble something together out of inappropriate equipment.

 

[kamikhan]

Transformer performance or failure is not a concern under my scope of work.

My job is to size the heater based on the requirements handed down to me. The risk and life-cycle analysis of this design is the baby of the elec dept, and I need not be concerned with it.

As far as the equipment rating and classification is concerned, this is a fairly new design that is being tested on this project, so some of the things (like the enclosure that the squirrel decides is nice place to spend the winter) are design development changes.

Help!

 

[MintJulep]

Ok.

I get about 3.7 megawatts required.

Good luck.

 

[kamikhan]

Good.

I got the same number.

Now I want a solution.

Help.

 

[MintJulep]

Sorry, it's only a 60K delta isn't it?

2.8 megawatt then.

 

[hollandhvac]

To get it back to serious; I do agree first with MintJulep, to purchase equipment properly rated for the environment. Other but lot less solution would be to insulate the enclosure and put a heater with temp. control on/off to matain the correct temp.

But still, beter is to talk to your E dep. and purchase properly rated equipment.

This was my 1 cent contribution

 

[gerhardl]

In my mind you have to talk to the electrical department anyway. This seems to be a typical example of an oversized requirement formulated by someone not realizing the cost/size and practical problem impact on the solution.

As the transformer itself is placed outside (minus 40 degree design) and only the relay has to be ensured at a certain temperature in the box, something doesn't rhyme.

Is worst/actual case really needed to be minus 40 deg C outside and real required designed operating temperature for the relay plus 20 degres? (How is the relay operated?)

When transformer is operating wouldn't there be some heat leakage from the transformer?

Could it be simplified to a start up problem (when does the relay have to operate)?

Might a simple electrical tracing/heating line (at lower temperature?) more directly attached to the ralay be sufficient?

In addition, if not placed in South- or North-pole area or in very high altitude, minus 40 degrees winter could be up to 30 deg centigrade or more during summer.

Temperature control needed! How hot will it be inside the box with direct sunsine during summer and transformer running? OK?

Sometimes it is necessary to recheck the design basis to design sensible solutions. Be concerned!

 

[zerosum]

Could you not glue some suitable fireproof insulation to the inside of the compartment? Check hottest and coldest days to determine the inside environment. The assumed wind will be very important to establish the outside film coefficient. Radiation may also be important on hot days.

A side benefit is that the squirrel will be more comfortable.

 

[kamikhan]

Thank you all for your help, so far.

So the way things stand now, the box is covered with 1 in insulation (R4) that would take care of the squirrel during the freezing winter up north, while not having us put up a multi-megawatt powerplant to achieve the ultimate comfort level for the animal. The heater would indeed be triggered by a thermostat

Now, with PETA in hindsight, what would be the best way to avoid overcooking our pet during the summer? Ambient temperatures can go as high as +40C, and we're still trying to achieve +20C inside the enclosure.

Motor actuated dampers?

 

[DRWeig]

Kamikhan,

You're not going to get +20C with +40C ventilation... I can't say I've even seen a venturi-style cooler that can move enough to do that.

You'll need an air conditioner. Don't forget to handle the condensate drain appropriately...

Let's see what the other HVAC folks come up with, though...

Good on ya,

Goober Dave

 

[IRstuff]

The OP say -40ºC, hence the need for a heater.

TTFN

FAQ731-376

 

[zerosum]

Use a Peltier cooling device closely coupled thermodynamically to the relay for the summer. A good one should be able to develop a differential temperature of 20 deg C