Indoor package units?

[dblyle]

I have a customer that wants me to put package units (RTUs) on his control room which is inside a larger warehouse. The temperatures on the control room roof reach about 120 degrees and the roof of the larger warehouse is about 8 foot above. Everything in me says this won't work, but my client insist that if I enclose the roof (like an attic) and use a fan to draw in fresh air to pressurize the room, we can simulate outdoor conditions for the package units.

Can anyone give me a good reason why this won't work?

Thanks.

 

[NCDesign]

A couple of questions, what tonnage are we talking? 3-5 ton or something larger? Is the 8 ft clear from the top of the unit or the top of the control room?

Some things to keep in mind, if you continually pump outside air into this new plenum space you will need to exhaust either back outside or into the warehouse space. It you put it in a fake attic you will still need access to the unit. They will probably still need to reinforce the control room roof to handle the unit. You will still need to maintain all of the manufacturer's clearances to maintain proper airflow around the condenser coil, and you will still probably have to de-rate the capacity based on higher "ambient" temperatures.

All that being said you could probably cobble it together and make it work. However, I find it hard to believe that building a fake attic, installing supply and exhaust fans, and reinforcing the control room roof, would be better and cheaper than putting the unit on the roof and installing a couple of sections of supply and return duct.

 

[atlas06]

For your 120F roof. Consider roof mist cooling systems. They consists of PVC piping network laid out on the roof with spray nozzles over the roof surface, performs evaporative ccoling of large roofs in hot climates at a very small installation cost. (google it out)

Will reduce your roof load substantially. You will not need to build a fake attic over a space.

 

[MintJulep]

Well I guess it could work, but why run a condenser at 120 F if you don't need to?

Wouldn't it be easier and cheaper to put the units on the wearhouse roof and simply run supply and return ducts eight feet straight down?

 

[gerhardl]

You have not described your year-around climatic conditions and variations. Is it necessary to check for possible condensate problems and resulting additional building costs by colder at night or during winter?

Why the suggested solution?

It will probably cost more than putting it up on the existing floor, even if extra walls/roof for architectual visual reasons is needed. (Cost is reason enough in itself - direct placement on roof will be cheapest with best maintenance access)

If reason is total building height or aestethic, perhaps it would be sensible to use the area above the control room, but build it as an extra closed 'mezzanine' placed room, enclosed else with floor and walls, roofed over with new roof a bit above existing, with larger wall - vents above existing roof, and enforced circulation if needed.

 

[dblyle]

Sorry, I guess I could have given more specifics. My original proposal to the client was to use some split systems to cool the space. I planned on putting a 2000 hr salt spray coating on the condensing units and coating the coils with an electrofin coating. The facility is inside a plant in La Porte, Texas (outside of Houston) and the condensing units will be exposed to SO2, SO3, and sulfur (not to mention the high concentration of salt since we are so close to the bay).

The client thinks that by installing package units inside the warehouse, we can avoid the exposure to chemicals in the air, so he'll save money by not having to coat the coils, etc. However, by installing the package units inside, we now have to create an environment for them to operate properly.

The client wants to do exactly what gerhardl said: use the area above the control room and build it as a closed mezzanine and force circulation to keep the room around 105 degrees F. I still think it is a bad idea and, at a minimum, it is another system (the ventilation system), that has to be maintained.

 

[IRstuff]

There are two additional things to consider.

An A/C or heat pump operates because a temperature difference is created in two places, the cooled air and the waste heat. By forcing the waste heat to dump inside the facility, you still have to remove in from the building, which potentially means an additional air handler. This is additional power that must be consumed and results in a net inefficiency in the overall system.

Placing all this inside increases the noise level. The waste heat is only efficiently removed if there's sufficient air flow over the exchanger. The added noise from the fans might be worse than the original heat.

TTFN

FAQ731-376

 

[gerhardl]

... agree. And sitting near Houston you will probably have access to advice and producers supplying to offshore specifications.

Given your details, outside with proper corrosive protection and materials will be a commercially probably easily available 'standardized' solution (materials and protection for surface available).

And probably at much lower cost than construction of a complete new room and probably extra protected ducts, fans etc. and still having to protect the rest of your original equipment in much the same way anyway.

' Clima-control' of a room with a huge amount of outside air are a 'not so standard' solution with more uncertain parameters, and not, perhaps, so readily commercial avilable solutions? More untried and uncertain result.

A rough cost spreadsheet (plus/minus 30%) for the two alternatives should convince. Include in spreadsheet all uncertain aspects, even if you cant put an exact cost on them.

 

[NCDesign]

dblyle putting the unit in its own space is not going to keep it from being exposed to the SO2, SO3, sulfur, and salt.

but my client insist that if I enclose the roof (like an attic) and use a fan to draw in fresh air to pressurize the room

...condensing units will be exposed to SO2, SO3, and sulfur (not to mention the high concentration of salt since we are so close to the bay).

The client thinks that by installing package units inside the warehouse, we can avoid the exposure to chemicals in the air, so he'll save money by not having to coat the coils, etc.

You are going to be pumping the same air you are trying to avoid directly into the space if you use outside air to try and temper the space. The only way around that would be if you tried to move air from the warehouse space around and not get any outside air. From what you have said I would guess that the warehouse air is probably too warm and more than likely has the same contaminates.

Given this information I would say putting the units inside is an even bigger waste of money because you are probably still going to need to apply the appropriate coatings with all the additional costs associated with coming up with some custom solution and additional equipment to make them work inside.

 

[willard3]

It's plain the guy doesn't understand thermodynamics if he wants to reject heat to an already hot space. The machinery will not operate as he envisions.

As the delta t decreases in the warehouse between the condenser and ambient air, cooling performance will approach zero.

Buy copper coils or get coated coils if there are corrosion problems....they are cheap and readily available

 

[AbbyNormal]

with more budget you could get a true indoor packaged unit,it should have forward curved condenser fans and ducted condenser air

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

 

[lilliput1]

Don't forget you also need to provide outdoor air for ventilation. There has to be a place in the plant where there is a source of clean air for ventilation. Otherwize all the workers should be dead by now!

 

[DrRTU]

I have done this a few times before, not for dirty environments but from a line of site issue. We have used small tonnage RTUs (less than 3 tons) and small ACCUs to provide conditioning for remote warehouse offices. This application is typically a shipping and receiving office. The RTUs were electric heat. We duct the OA to the roof and terminate with a low profile hood. The same holds with the split systems. We were forced by local building parks to screen all mechanical equipment. This application loads the condenser with dirt from the plant air. The unit never gets rained on to clean the coils. We hang he unit above a 4” deep drain pan with a large drain line. We install the drain pan under ACCUs. I have had a few code issues. IMC requires the system to be installed as per manufactures instructions. I could not find installation instructions depicting a detail of a RTU or ACCU installed under a roof. One inspector used the logic, the unit is UL rated for outdoor use and it does not state indoor AND outdoor. Addison, and Liebert make condensers / ACCUs designed for indoor installations.

http://www.addison-hvac.com/product...ONS Cool/Product Specifications 3-25 Tons.pdf

With the centrifugal ACCUs you can duct to the plant or to the roof for a heat reclaim process. In my region, we normally deign to 94 OAT so a normal condenser would be 105 designs. For in plant I use the 120 design if I can get the design data on the ACCU.

 

[BronYrAur]

This isn't the best approach, but can you go with a split system and put the condenser near the warehouse floor so that ambient won't be as hot? As long as it can get good air circulation and not be in danger of damage, temperature may be the same as outside.

Either way your whole space contains a certain degree of the corrosive elements that you are worried about outside.

 

[dblyle]

Thanks for your comments on this topic. It is nice to have a community in which to "bounce" ideas off. I have convinced my client that although we could probably design the system he is asking for, the method is not common and we could potentially be looking at some uncertain results. On the other hand, we could install traditional split systems and guarentee the system would work. He opted for the guarentee.