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Old 01-10-16, 12:27 PM   #1
Trebleplink
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Default 12x12 building Enthalpy based economizer

Hi folks,

I'm a broadcast engineer looking to design an approximation of an Enthalpy based economizer controller for a local nonprofit radio station's transmitter building. It operates 24/7 and is unattended.

The building is about 12x12' inside, and I have two AC units, a filtered air intake fan (vari-speed) and an exhaust damper. All of these can be controlled by an Arduinio. Sensors are temp & RH inside & outside.

The transmitter exhausts about 1000 watts in heated air into the room. This and sun radiation are the dominant heat sources.

Many will simply throw the air-conditioning at it, full recirculation. But I want to use as much outside air as I can without risking condensation. Desired temp is 60-72 F.

I'm an electronics guy and never took chemistry, so I'm having some trouble beyond basic thermodynamics concepts.

Are there any "rules of thumb" or first order approximations for setting the outside air temp and RH% limit?

Chris Scott
Bowling Green, KY

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Old 01-10-16, 12:36 PM   #2
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Chris, what is the design temps and RH you are working against. I bet you (Kentucky) can get up to 100F with about a 75 dew point (75% RH at 100F).

Is this the location in Kentucky?

Secondly - any insulation in the walls and ceilings? Windows, etc?

Steve
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Old 01-10-16, 01:31 PM   #3
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Steve,

Well, I want to keep it 60-72 inside. Max 80% ? RH so as not to risk condensation?

It can get hot & humid on Summer days. Summer nights will often get below 70.

I'm thinking that most of my gains will be had Fall/Winter/Spring.

Location is Bowling Green, KY. No windows, uninsulated block walls, concrete floor, wood frame roof with 6" fiberglass ins.


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Chris, what is the design temps and RH you are working against. I bet you (Kentucky) can get up to 100F with about a 75 dew point (75% RH at 100F).

Is this the location in Kentucky?

Secondly - any insulation in the walls and ceilings? Windows, etc?

Steve
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Old 01-10-16, 03:00 PM   #4
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OK - let's first start by insulating all the walls. Right now, there is little use "generating cool" as it will immediately be sucked out on hot days.

I would suggest an easy fix - use two layers of 1 inch "blueboard" - aka closed cell foam. It has an R value of about 6 per inch so you get up to R12 or so - a huge improvement.

This stuff can be glued up to the walls and ceiling with construction adhesive. The boards come as 8 x 4 foot sheets at big box stores (Lowes, Home Depot, Menards, etc) for about $15 a sheet. Easy to cut to fit. Forget the floor unless it is above a crawl space.

This is a one afternoon job and it will do wonders.

Then we can tackle the cooling issue. I am leaning toward a small window AC unit as it also dehumidifies. You don't want humidity around transmitters . . . .


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Old 01-10-16, 03:29 PM   #5
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Steve,

The trouble with adding insulation inside is that with all of the surface-mounted conduits, electrical boxes, 4" grounding straps, wall outlets, etc, is that it would have to be a lot of smaller squares ...

The 30 year old shingle roof although well insulated, is starting to leak, so I was considering adding an inch of styrofoam to the top, and overlaying with white metal.

I also thought about outside insulation on the South wall...

There is now one 25K window AC, and I need another for backup. Each would be controlled by external contactors or SSRs.

Ideally, the transmitter would like 62 deg at 30% RH, but I don't think that's a practical goal. Outside RH here varies from about 40% to 90%

My main goal is maximizing outside air use while avoiding indoor condensation. Perhaps a max of 75% inside RH?


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OK - let's first start by insulating all the walls. Right now, there is little use "generating cool" as it will immediately be sucked out on hot days.

I would suggest an easy fix - use two layers of 1 inch "blueboard" - aka closed cell foam. It has an R value of about 6 per inch so you get up to R12 or so - a huge improvement.

This stuff can be glued up to the walls and ceiling with construction adhesive. The boards come as 8 x 4 foot sheets at big box stores (Lowes, Home Depot, Menards, etc) for about $15 a sheet. Easy to cut to fit. Forget the floor unless it is above a crawl space.

This is a one afternoon job and it will do wonders.

Then we can tackle the cooling issue. I am leaning toward a small window AC unit as it also dehumidifies. You don't want humidity around transmitters . . . .


Steve
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Old 01-10-16, 10:06 PM   #6
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I would suggest 3plink go online and pursue 'psychometric chart'.

That is the basis for the questions he is asking.

After becoming familiar with the ps chart, 3 plink can find the weather data for his area from NOAA, and design for either worst case conditons, or for nominal summer days.

Getting more fancy, he can look at the MTBF of the transmitter for other than 62F 30RH operation and see how much 12x12 space conditioning is most economical .

That is a standard engineering approach.
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Old 01-11-16, 11:56 AM   #7
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I would suggest 3plink go online and pursue 'psychometric chart'.
Yes, real engineering.

-AC
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Old 01-11-16, 08:36 PM   #8
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Concrete block, in addition to near zero insulation value, is extremely porous. Wind, and the moisture in the air, goes right through it. Controlling humidity in such a building is an exercise in futility.

I suggest 4" of foam on the outside of the building. Two layers of 2" foam, seams staggered and taped. Add vertical furring strips and vinyl siding nailed to the furring strips. Add at least 2" foam to the roof, then roof over that. Tape the seams.

With those improvements, your largest source of heat gain will be the 1000 watts of heat from the transmitter. The existing 25,000 BTUH window unit will be too oversized to properly control humidity, so put in a smaller unit and leave that one for a backup. With that much insulation and internal heat, plus a properly sized air conditioner, the internal humidity should stay low under all conditions.
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Old 01-12-16, 03:35 PM   #9
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Just paint the walls and add a 1T backup AC.

Bowling Green KY, winter design temp 10F, summer 95F.
Just for kicks, looking a physcho chart and off top of head calculations.

1 kW heat.
....continued from end of text below (Houston type day, 95F, 90% RH), somehow message block keeps scrolling back to top.
anyway, roughly 0.03# h20 per #/ air needs to be reduced to around 0.01 #H2) per #/air.
Say 1/4 air change per hour = 20# air/ hour; or 3 pints of water per hour dehumidified by AC
Dropping from 60 BTU/# dry air down to 25 BTU/# dry air on the Enthalpy scale.
35 BTU *20#/hr = only 700 BTU/hr for dehumidification.

No need for all kinds of added insulation.


Electronics actually perform better at lower temperatures, some devices may have problems below 20 F, but let's put winter inside temp at 50F just for maintenance comfort.
For 12x12 building, Roof has say R-23 insulation -
Ceiling loss = 144*40F/R23 = 250 BTU/hr
Wall loss = 12*4*8*40/R3 = 5120 BTu/hr, Simple fill of block cores wit;h vermiculite or similar cut that down to 2500 BTU/hr, so no added heat ever needed.

OP says 72F in summer is OK. White paint on South wall = about 205 BTU hr gain
12*4*8*(95-72)/R6 = 981 BTU
Internal transmitter loss = 3412 BTU
Total summer gain = 4643 BTU/hr or under 1/2T AC minimum
Paint the outside walls with 3 coats of vinyl acrylic white (or thermally reflective) paint to control humidity infiltration, fill block with insulation if not done so earlier. No need for 'heroic' insulation.

3plink only needs say a 1T for backup AC. Colder is better, so only needs to control for humidity. Use a dehumidifier sensor for primary AC control vs. temp

KY summer is 95 dry bulb, 77 wet bulb equates to only 40% typical RH.
Say it is a Huston summer day, 95F 90% RH.
The AC already cooling down to 72F, which means AC coil is probably below 60F worst case.
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Old 01-12-16, 03:38 PM   #10
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Just paint the walls and add a 1T backup AC.

Bowling Green KY, winter design temp 10F, summer 95F.
Just for kicks, looking a physcho chart and off top of head calculations.

1 kW heat.
Electronics actually perform better at lower temperatures, some devices may have problems below 20 F, but let's put winter inside temp at 50F just for maintenance comfort.
For 12x12 building, Roof has say R-23 insulation -
Ceiling loss = 144*40F/R23 = 250 BTU/hr
Wall loss = 12*4*8*40/R3 = 5120 BTu/hr, Simple fill of block cores wit;h vermiculite or similar cut that down to 2500 BTU/hr, so no added heat ever needed.

OP says 72F in summer is OK. White paint on South wall = about 205 BTU hr gain
12*4*8*(95-72)/R6 = 981 BTU
Internal transmitter loss = 3412 BTU
Total summer gain = 4643 BTU/hr or under 1/2T AC minimum
Paint the outside walls with 3 coats of vinyl acrylic white (or thermally reflective) paint to control humidity infiltration, fill block with insulation if not done so earlier. No need for 'heroic' insulation.

3plink only needs say a 1T for backup AC. Colder is better, so only needs to control for humidity. Use a dehumidifier sensor for primary AC control vs. temp

KY summer is 95 dry bulb, 77 wet bulb equates to only 40% typical RH.
Say it is a Huston summer day, 95F 90% RH.
The AC already cooling down to 72F, which means AC coil is probably below 60F worst case.

anyway, roughly 0.03# H20 per #/ air needs to be reduced to around 0.01 #H2O per #/air.
Say 1/4 air change per hour = 20# air/ hour; or 3 pints of water per hour dehumidified by AC
Dropping from 60 BTU/# dry air down to 25 BTU/# dry air on the Enthalpy scale.
35 BTU *20#/hr = only 700 BTU/hr for dehumidification.

No need for all kinds of added insulation. Paint the inside too where easy to do if not already painted.

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