How do I size the HVAC system?

The AC part of HVAC is for personnel comfort control.

It's ventilation that is needed here. Simply arrange a throughput of air and keep the pressure inside the room a little (0.5in w.g.?) higher than the outside.

In this day of rising energy costs, I'd want to consider first if it is necessary / appropriate to cool the room, and to what extent. There are a variety of things to consider:

• is the room manned? continuously (or, even, let's say, 8 hours /5 days a week?), or is it only entered for troubleshooting (while some equipment is still running), or maintenance--and is normal maintenance done with all equipment shut down). Is normal maintenance done at night (when it is cooler)?
• Is the equipment experiencing temperatures higher than their rated temperatures? We used to use a rule of thumb--we assumed (I know) that the temperature in a closed (but with ventilation louvers) cabinet was 10 degrees C higher than the ambient temperature in the room, so if the ambient in the room is 40 degrees C, we'd look for equipment rated at 50 degrees C installed in cabinets (or provide extra cooling).
• even if the equipment is operating within its temperature rating, there is a rule of thumb that the life of (electronic) equipment is halved for every additional 10 degree C temperature rise, so even if your equipment is operating within its ratings, it may make economic sense to cool it to achieve a longer life
• again, due to rising energy costs, if there is not a need to coole the entire room 24/7 for personnel comfort, one of the more economical approaches is using ducting to blow cooled air directly into the cabinets / equipment--cool the equipment, not the room.
• If cooling of the room is also required occasionally for maintenance personnel, I'd consider addtionally (to the cooled air ducted to the cabinets) putting something like a window air conditioner in the room--because of the pressurization and dust issues, I'd actually look for a split unit with a separate compressor unit outside the room.

Now, as to calculating the amount of cooling required, I haven't addressed that. I would find the nameplate power requirements for all the equipment in the room and convert that from watts to BTUs, then consider the target temperature I want, then talk to an HVAC engineer. (He will consider additional things, like the ambient temperature around the room, the insulation of the room (which you might want to improve if you have thoughts of cooling the entire room, not just the cabinets), etc.)

Thanks for the information.
The room is entered only for troubleshooting and maintenance. I was thinking of cooling as a way to lower the operating temperature of the electrical components and as a way to moderate the temperature and humidity levels of the air coming in from outside that I would be using to pressurize the room.
We were planning on increasing the insulation level in the room.
My concern (perhaps unfounded) was that simply pressurizing the room with outside air could bring in cold (-30C) air into contact with electrical components running at 15 or 20C or in a rainstorm that I would be bringing in too much moisture for the electrical components.
Thanks again
Mike

A well-designed fan and ducting arrangement would leave the raindrops outside.

Is it time to look to local suppliers to see what they recommend, perhaps?

Re: Thanks for the information.

You're welcome!

Re: My concern (perhaps unfounded) was that simply pressurizing the room with outside air could bring in cold (-30C) air into contact with electrical components running at 15 or 20C or in a rainstorm that I would be bringing in too much moisture for the electrical components.

Hmm, there are some factors working in two directions:

• if you're just pressurizing (in winter), it should require very little air flow (thus not bring much rain)--and, as somebody else mentioned, the ducting and such should minimize rain brought in that way
• when it is not raining, you might have a different problem (iff you use a lot of ventilation in winter--but I'm trying to also imply you don't want to freeze the room)--that -30 degree C air will not carry much humidity into the room at all, and will absorb the humidity in the room as that air heats up--you might start to have static electricity concerns, especially with respect to doing maintenance on electronic equipment in those conditions

I would minimize the outside intake air in winter. If not, you might have to consider steps to deal with or safely dissipate static electricity.

Yes I will be bringing in local suppliers shortly, I am just trying to increase my knowledge so I do not change the specs three times on them during the design and quoting phase.

Thanks

Mike

"How do I size the HVAC system?" = you don't, the load and environment will size it for you.....but if you must write your own specs be very vague.

I had success with a "RFP" request for proposals and then the winning design (that I would modify at my discretion based on what I had learned from all the proposals) would become the spec that all the companies that took part in the initial design phase would now bid on "RFQ" request for quotes.

I managed many projects that I knew nothing about the details except what the overall scope was "what must happen -or- stop happening". I have also paid a company to design a project and then allow them to bid on it. Someone always gets pissed but the issue must get addressed on time and on budget.

Do a load calculation on the room. From what information you provided, your best options might be a ductless a/c unit (these are less costly to install and maintanin than roof top units) that serves that specific room only. It would cool and recirc the air in the room as to eliminate any humidity issues. If you needed to have that room pressurized, no reason to by the way, you could add an ERV or intake fan.

I have done this in a few electrical rooms in Markham over the past few years.

Before I take a stab at this I have a question. If this room is rarely occupied and houses some expensive equipment where is all this dirt and dust coming from? I've worked on many many of these types of rooms and they don't call out for positive pressure? Wouldn't it be easier to just keep the dirt out in the 1st place?

Mike: You are right on in knowing the room shold be pressurized to minimize the intrusion of air borne dist.

Find two or three very reputable HVAC design/build contractors and have them submit proposals to design/install the necessary system. Have them also include a comprehensive maintenance agreement in their offering. Good luck.

Lou Bindner

I am with Fredski on this one. Where is the dust & dirt coming from? If you dont have a door to this control room you will have a heck of a time pressurizing it.

You have mentioned a "dusty" room, so I assume you have a moderately dusty environment.

Then, to pressurize the room, compute the total volume of the room, multiply it by 50% and then divide the product by 60. That will be the air flow required to produce a 1/2 air-change per hour flow rate. Select a direct-driven utility set- top horizontal discharge- that is rated for that volume per minute at 1/2 inch water column (or equivalent SI units).

House the utility set in a simple frame box with sheet metal at the discharge duct and the other three sides made of 25" x 20" x 2" Farr 30-30 (or equivalent) disposable filters and cover the box with a 47" x 47" cross-broken piece of heavy gage aluminum with a 1/2" hem on bottom that is cross-broken to form an "X" to provide drainage. Change the filters when they are noticeably dirty.

Seal the door to the room with high grade weatherstripping.

Feed the room with a rectangular aluminum duct from the fan discharge through the wall of the room. Use a "simple" sizing for the duct- like the size of a concrete block or whatever your room is built of. Seal the annular space between the wall surface and the duct with silicone. Have the duct turn vertical and discharge toward the roof, with the discharge at about 5 feet/ 1-1/2 meter below the roof. This will provide adequate distribution of the air without blowing directly on any devices.

An earlier suggestion was a recirculating cooling system- easily sized if you contact the manufacturer's of your VFDs and similar devices and get the maximum heat rejection loads in BTU per hour, then ADD 5% of the rated power consumption (in kW) of any "pure" electrical devices (MCCs, transformers, etc.) in the room multiplied by 3400 per kW of load (which will yield an electric-related cooling load in BTU per hour) PLUS add the flow rate of the utility set multiplied by 150 to cover the peak cooling load (in BTU per hour) of the ventilation air. The total of those values will be a "safe" maximum load sizing- that may require more than one split system. The cooling system will cycle as needed- if you do need more than one split system, use a multi-stage cooling thermostat to cycle the units "ON" as required.

The room relative humidity will fluctuate, but virtually ALL devices in the room are likely designed for such a range of humidity so that will not be a problem. Set the thermostat for the cooling system at about 80F (25C). During cold weather, the pressurizing air will totally offset any room cooling loads but it should not get too cold due to the relatively constant electrical loads.