Forced Ventilation and Heater in Same Enclosure

[fifty60]

I have an approximately 50x50x10 Hoffman enclosure that already has a fan and vent for forced ventilation. I would like to add a heater for condensation control in the summer and to keepa PLC from freezing in the winter.

The hoffman enclosure itself is housed inside of a vented 10 ft by 10 ft enclosure. And is located in the Northern US, so mild to hot summers and extreme prolonged cold winters.

The heater thermostat will be set to 70F year round. In the summer it just has to keep thehoffman enclosure above the dew point, and in winter it just has to keep the hoffman enclosure above freezing.

Will this work or is it destined to hace problems? Is there anything I can do to improve the chances of success?

Sent from my LG-D855 using Tapatalk

 

[drcampbell]

Using a single thermostat isn't a good way to prevent condensation. Controlling the heater on temperature alone won't assure that it remains above the summertime dew point unless it's set quite high, and setting it that high will waste a lot of energy all the rest of the year.

For example, the ASHRAE 1% design wet-bulb temperature for Minneapolis is 77^oF; using a thermostat alone would require setting it for 87-97^oF.


The traditional approach is to size the heater for a 10-20^oF temperature rise above ambient and leave it on at all times. That's not enough heat to protect against freezing, but I'm not persuaded that electronic parts actually need freeze protection. And it will waste a lot of energy on dry days.


There are a lot of other variables to consider. What else is in the building? A dripping water pump or faucet could drive the humidity way up. So could a plain concrete floor slab and a high water table.

How much power does the PLC and everything else in the cabinet dissipate? If it's a lot, you could omit the heater and rely on self heating.


For a cabinet of this size, each watt of heat dissipated inside will increase the temperature about 0.1^oF if the fan's not running. Triple that if you insulate with a half inch of polyisocyanurate house sheathing. (despite the borderline-scary name, it's as readily available as a tuba foe) If the fan's running, you'll need to calculate how much heat the air's carrying away. Arrange it so the incoming air passes over the heater before it passes over the PLC.


If condensation and freeze protection are required, I think I would install two thermostats and a humidistat, turning the fan off when it's less than 80^oF inside the cabinet and turning the heat on when the relative humidity's above 50% or the temperature's below 40^oF.

 

[fifty60]

The 50"X50"X10" electrical enclosure is inside of larger enclosure. There are no RH producers inside of the larger enclosure. The PLC is a 10+ year old design, so I am sure it has electrolytic capacitors that do not want to operate when frozen...

Wasting energy is normally always high on my list, but for this particular project it is not really a consideration. The enclosure heater is a 400W heater with internal thermostat and fan. The other electrical components inside of the enclosure generate close to 100W.

My main concerns are condensation in the summer and the cold weather in the winter. The electrical enclosure already has forced air ventilation that comes on above 85F. The heaters are set to come on above 55F. Another thermostat is inside the electrical enclosure to turn off the PLC if the temperature inside the electrical enclosure reaches 34 F.

Is condensation really a concern inside the enclosure? I am trying to wrap my head around it. For example, I get condensation on top of my car but not inside of my car....is there enough volume inside the 50X50X10 enclosure to create condensation?

The winter concern is not only freezing, but also the fact that the openings for the forced air will still be there. The fan will be off, but the 6" X 6" vent opening for the exhaust will still be there, as well as the 6" by 6" opening for the fan itself. A lot of the heat generated by the heater will escape out of the vents. My concern is that this will damage the heater after about a month or two of this...

 

[fifty60]

My concern with adding a humidistat and having the heaters on above 50% RH is that I would potentially overheat the electrical enclosure.

How effective will the forced ventilation fan be at preventing condensation ithey are on above 80F? Could I then potentially ignore dew points above 80F since the fans are on above 80F?

 

[SG-1]

Have you considered a differential thermostat ? You only need to be a couple of degrees above the outside temperature to prevent condensation.

 

[kwired]

For example, I get condensation on top of my car but not inside of my car....is there enough volume inside the 50X50X10 enclosure to create condensation?

Go back to the physics of why the water condenses.

The metal outside of your car cools off faster then the air around it, when it falls below the dew point of the air that is when the condensation will form.

Same applies to the inside of your car or the inside of your cabinet in question. You need to keep the walls of the cabinet from reaching temperatures below the dew point of the air inside the cabinet, if you succeed with that you should never get any condensation on the walls, same for any equipment located inside.

If you are constantly changing air in the cabinet it will take a lot more energy to keep temp vs dew point stable enough to prevent condensation, especially if using an air to air heat exchanger. Something that more directly heats the cabinet walls will be more effective.

 

[fifty60]

I am limited with what I can do at this point...

I can still add a din mounted electronic hygrotherm. This can turn on my heater at below 45 F or above 50% RH. The heater itself is made by Hoffman (pentair) for the enclosure. It has an internal thermostat so the hygrotherm will be redundant to this. I am going to set the internal thermostat on the actual enclosure heater to 90F, so that the Hygrotherm will only work at temperatures below 90 F...

At 90F, the forced air ventilation will turn on and there will be no more heat. I have to have ventilation because the electronics inside the enclosure cannot overheat on the hot summer days, though the enclosure itself is out of any direct sunlight it is still a concern.

So below 90F, i can turn on heat if the RH goes above 50%. Above 90F, I only have ventilation fans. I do not have any ventilation fans below 90F.

Actually, I don't like the fact that the heater will shut off at 90F and the fan turns on at 90F. If i have the fans kick on at 80 F, and the heater RH control turn off at 90 F, the heater should still be able to drive the RH down between 80 and 90 F even if the ventilation fans are on?

Is it reasonable to think that this conditioning scheme will work?

 

[kwired]

I am limited with what I can do at this point...

I can still add a din mounted electronic hygrotherm. This can turn on my heater at below 45 F or above 50% RH. The heater itself is made by Hoffman (pentair) for the enclosure. It has an internal thermostat so the hygrotherm will be redundant to this. I am going to set the internal thermostat on the actual enclosure heater to 90F, so that the Hygrotherm will only work at temperatures below 90 F...

At 90F, the forced air ventilation will turn on and there will be no more heat. I have to have ventilation because the electronics inside the enclosure cannot overheat on the hot summer days, though the enclosure itself is out of any direct sunlight it is still a concern.

So below 90F, i can turn on heat if the RH goes above 50%. Above 90F, I only have ventilation fans. I do not have any ventilation fans below 90F.

Actually, I don't like the fact that the heater will shut off at 90F and the fan turns on at 90F. If i have the fans kick on at 80 F, and the heater RH control turn off at 90 F, the heater should still be able to drive the RH down between 80 and 90 F even if the ventilation fans are on?

Is it reasonable to think that this conditioning scheme will work?

Have you looked into "cabinet coolers"? There are some that are cooling only as well as some that have heating options.

 

[fifty60]

A cabinet cooler would have been ideal, but at this point I just have the ventilation fans and heaters. So I am hoping to make the hygrotherm work.

With the above scheme, and I think this what Kwired was trying to point out about the fans destabilizing the RH and Temperature inside the enclosure, I am really concerned about the crossover point where the heaters shut off and the ventilation fans come on.

If I make the fan and heater point both 90 F, then I can see the heaters coming on because of the RH, heating the cabinet until 90 F, then the heater shuts off and the fans come on at 90 F, which drops the temperature below 90 F which raises the RH% and turns off the ventilation fans, which causes the heater to come back on, which raises the temp above 90 F....

And there it would be resonating at 90 F cycling the fan on for a moment, then the the heaters on for a moment, then the fans back on....

How can I prevent this from happening? Should I let the ventilation fans overlap with the heaters? For example, the ventilation fans come on at 80 F, and the heater shuts off at 90 F. Would this overlap help, or would I have the same potential for short cycling?

 

[mgookin]

... I am sure it has electrolytic capacitors that do not want to operate when frozen...

....

-40F is a common rating on electrolytic caps.

Are you having failures?

 

[kwired]

-40F is a common rating on electrolytic caps.

Are you having failures?

And they probably don't even reach that temp if they are in use, so turning equipment on when they are already cold would be the biggest issue from that perspective.

 

[fifty60]

The storage temp for the PLC is -40F, but the operating temp is 32 F. It is not supposed to be frozen before at the moment it is powered...

This is a new installation. No problems yet, but it is way too early to tell. I am trying to do what I can to avoid issues.

I think the best way to keep them from from short cycling/oscillating/ resonating would be to actually seperate the temperatures. So, instead of trying to overlap them (fan on at 80F, heater off at 90 F), I would need to seperate the temperatures.

For example. I can have the ventilation fan come on at 95 F, and the heater disabled at 90 F. This would cover me for pretty high Temp/RH combinations (90F and 95%)...and still give me some overheating protection. I believe all the electronics are rated up to 45C (113F).

Ideally I would have the fans come on earlier, but I have to make a trade off somewhere....

Does this seem reasonable? It just seems odd to want the heaters to come on when the enclosure is 80 F, but it is not the temperature I am looking at it is the %RH....So the heaters can and will come on when the temperature is 80 F if the %RH peaks above 50%.

If anyone can think of better %RH and temperature setpoint combinations, please suggest them.

 

[fifty60]

What is going to happen to the moisture after I warm the air. I know that warming the air will make the relative humidity (RH) go down since the warmer air can physically hold more air, but will I just be adding more moisture to the air over time? Raising the temperature doesn't actually get rid of the moisture, right?

Since I have a vent in the enclosure, one intake vent at the bottom (with fan) and an exhaust vent at the top, will the warm moist air simply travel out the top exhaust by convection alone, even though the intake fan is off?

If the moist air can somehow go away, then I like this solution. Basically the hygrometer is acting as a choke when the temperature drops. If it see's the temperature dropping in the enclosure, then it will monitor the %RH and if it see's it creep above 50% then it will turn on the heater and stop the enclosure from cooling down.

If the hygrostat + heater solution only lets the enclosure cool down if the %RH below 50%, but doesn't remove any water, will it ever let the enclosure cool down?

 

[kwired]

What is going to happen to the moisture after I warm the air. I know that warming the air will make the relative humidity (RH) go down since the warmer air can physically hold more air, but will I just be adding more moisture to the air over time? Raising the temperature doesn't actually get rid of the moisture, right?

Since I have a vent in the enclosure, one intake vent at the bottom (with fan) and an exhaust vent at the top, will the warm moist air simply travel out the top exhaust by convection alone, even though the intake fan is off?

If the moist air can somehow go away, then I like this solution. Basically the hygrometer is acting as a choke when the temperature drops. If it see's the temperature dropping in the enclosure, then it will monitor the %RH and if it see's it creep above 50% then it will turn on the heater and stop the enclosure from cooling down.

If the hygrostat + heater solution only lets the enclosure cool down if the %RH below 50%, but doesn't remove any water, will it ever let the enclosure cool down?

Raising the temp will mean the same volume of air is capable of holding more moisture. If enough cooler air is pulled in with moisture it sort of keeps bringing moisture in, of course if you are pulling air in you have to either let some out or keep building pressure, which higher pressure holds less moisture but we are talking about much more pressure then you will develop with your fan.

If same amount of moisture remains but temp increases it will not condense until there is a temp drop that falls below dew point or you introduce a cold surface for moisture to condense on, which is what happens in the evaporator coil of an air contitioner, and is really about the only way you will actually remove moisture other then to bring in air with lower moisture levels.

 

[fifty60]

It would be a bad idea then to simply raise the temperature without removing the moisture? Raising the temperature will simply mask the problem, but in actuality is making the problem incrementally worse, since raising the temperature will raise the dew point when more moisture is added over time, meaning I have to raise the temperature even more...which raises the dew point over time...and so on?

 

[mgookin]

I suggest you call the PLC manufacturer and ask them how others mitigate this issue. Or source a PLC with better specs.

All specs say 95% RH non-condensing and that's to cover their end. If it comes back full of water, don't ask for warranty replacement.

Fact is, when electronics are powered up, they are warmer than ambient and nothing is going to condense on them; condensation would only be on the outside of the enclosure.

 

[kwired]

It would be a bad idea then to simply raise the temperature without removing the moisture? Raising the temperature will simply mask the problem, but in actuality is making the problem incrementally worse, since raising the temperature will raise the dew point when more moisture is added over time, meaning I have to raise the temperature even more...which raises the dew point over time...and so on?

You are usually fine because the temp will usually stay above the dew point, but when you do have a cool down for some reason the risk of condensation is higher. That condensation is more likely to happen on a cooler cabinet wall because outside temp dropped then on warmed up operating electronics as was mentioned.

 

[drcampbell]

... Is condensation really a concern inside the enclosure?... I get condensation on top of my car but not inside of my car....is there enough volume inside the 50X50X10 enclosure to create condensation? ...

The question is not "inside" vs. "outside" but whether a surface's temperature falls below the air's dewpoint. In this example, the car's roof cools to a temperature below the ambient air temperature (and dewpoint) because it's radiating heat to the deep night sky. On nights with heavy, low cloud cover, or if you park in the shade overnight, the deep sky isn't visible; the radiant heat transfer isn't available and condensation/frost will not occur.
(yes, "parking in the shade at night" sounds goofy, but the physics works even if the language doesn't)

The winter concern is not only freezing, but also the fact that ... a lot of the heat generated by the heater will escape out of the vents. My concern is that this will damage the heater after about a month or two of this...

Not a concern. That's what heaters do; it's no different than a baseboard heater putting heat into a room, which then escapes out the walls, roof, windows & doors.

It's possible to eliminate much of this (and unnecessary heater-energy consumption) by placing both the fans and exhaust vents at the bottom and adding a baffle between the two, so that the air must be forced up & over the baffle before being exhausted.

Something else I forgot to mention: The air path should pass over the heater first, then the PLC, then the humidistat & thermostat(s).

How effective will the forced ventilation fan be at preventing condensation if they are on above 80F? Could I then potentially ignore dew points above 80F since the fans are on above 80F?

Forced ventilation will potentially make things worse, not better, by bringing in unconditioned air from outside and lowering the temperature. But if the fan remains off while the in-cabinet temperature is maybe 10^oF higher then the highest dewpoint that will ever be encountered, it's not a concern.

... How can I prevent [short-cycling oscillation] from happening? Should I let the ventilation fans overlap with the heaters? For example, the ventilation fans come on at 80 F, and the heater shuts off at 90 F. Would this overlap help, or would I have the same potential for short cycling?

You're on the right track, but in the wrong lane. The way to prevent it is with a dead zone in which neither one turns on. For example, heater off at (or below) 80^oF; fans on at (or above) 90^oF.

... This would cover me for pretty high Temp/RH combinations (90F and 95%) ...

The highest temperature and the highest relative humidity do not occur at the same time. You can design for each condition independently.

... If the hygrostat + heater solution only lets the enclosure cool down if the %RH below 50%, but doesn't remove any water, will it ever let the enclosure cool down?

1) Raising the temperature will lower the relative humidity, even if it doesn't remove any water. When the RH is under control, the temperature will find a corresponding equilibrium, not continue rising.
2) Do you care? As long as the PLC temperature is within limits, what's the harm in it being warmer than ambient?

 

[drcampbell]

... What is going to happen to the moisture after I warm the air? I know that warming the air will make the relative humidity (RH) go down since the warmer air can physically hold more air, but will I just be adding more moisture to the air over time? Raising the temperature doesn't actually get rid of the moisture, right?

Raising the temperature will lower the RH. But it won't change over time unless there's a source of water. If the ambient air arrives at, say, 99% "dry air" and 1% water vapor, that proportion of water vapor & air won't change when the mixture is heated or cooled.

If you're a chemist, looking at it from the perspective of "air" being a solution of water vapor in "dry air", and looking at unsaturated, saturated and over-saturated solutions, might make it clearer.

Don't feel bad. Relative humidity is one of the most-difficult concepts to explain, on par with quantum physics, the simultaneous existence of god and evil, and why arithmetic never seems to function correctly in a restaurant.