This article is meant to help clarify common misconceptions, and answer commonly asked questions about cleanroom temperature and humidity control systems:

Statement
1. Softwall single pass cleanrooms don’t need air conditioning.
Fact – That’s correct if your inlet air into your fan filter units is sufficiently cold to cool the heat load in the cleanroom and the mechanicals (FFU) and makeup for exhaust and positive pressurization. You need to calculate this. A general rule of thumb however is make sure your inlet temperature to fan filter units 3-4 degree “F” lower than desired room temperature. (See Cleanroom Talk #1 Comment #10)

Remember – your being in smocks, gowns in the cleanroom apparel makes the cleanroom appear warmer – so you should plan on cleanroom temperatures between 68-72 degree “F” while the support non-cleanroom areas can function between 70-75 degrees.

Statement
2. Re-circulating (chimney low wall returns) cleanrooms are a lot cleaner.
Fact – Absolutely true. If you re-circulate your air you tremendously improve your filtration performance. Much more so that if you switched from HEPAS to ULPA’s. Every time you re-circulate your air you challenge your fan filter unit filters with up to 100,000 pp cu. ft. air. If you challenge with outside air you easily can add 5 million particles to the challenge. That’s a 50 to 1 ratio. Upgrading to ULPA over HEPA filters is at best a 10 to 1 ratio (depending on challenge counts).

This also relates to pre-filter and final filter (HEPA or ULPA) life. You get up to 5 times the life of a final filter by re-circulating.

Statement
3. Re-circulating cleanrooms require air conditioning.
Fact – Absolutely. Every time the air passes into the room it adds heat load from thermal conduction, equipment, fan filter units, people, electrical stuff and all other heat generators. At LLNL, overnight, a Class 100 cleanroom re-circulating without functioning air conditioning got up to 145 degrees. The gel seals in the ceiling melted – not good.

Statement
4. You need a dedicated air conditioning system for your cleanroom.
Fact – Not necessarily. Many times we are able to share “house” air conditioning (existing facility air) by utilizing a drop next to or above the HEPA return and allow the draw of the fan filter unit to pull in cooling air. This is an unsophisticated by pass of air, but works in small, loosely temperature controlled environments. A better system is utilized on a re-circulating or single pass system.

The A/C supply is hooked up to a “Y” collar connector on the fan filter unit with a bypass supply to the remoted pre-filters and A/C to the other collar. One side has an adjustment damper. This allows you to introduce cooling air into as many of the fan filter units as you need to cool the room, balance the room and not have hot and cold spots in the room. You don’t generally need all of the fan filter units to be connected to A/C as this is expensive. Another good method is to dump A/C air into the top of a return air plenum. (See concept drawings)

Rough Estimate Only
Class 10 25%
Class 100 35%
Class 1000 40%
Class 10,000 50%
Class 100,000 50-60%

This of course, is layout dependant and changes as soon as you add extraordinary heat load or exhaust. Also – sharing A/C require thermostat in one place generally – not the best for tight control.

A dedicated air conditioner system is required when:

A. No A/C or insufficient A/C is available from facility.
B. More stringent controls are required that house A/C cannot supply – example humidity control.
    There is much more to discuss concerning tolerances later.
C. Special FDA or process requirements require dedicated air handlers to eliminate cross – contamination.

Question
5. What is dedicated air conditioning?
A dedicated air conditioning system must introduce cooling and or positive pressurization and makeup air for the cleanroom. This system generally consists of a pre-filter, heating, cooling elements, air handler/motor/blower and ducting to the fan filter unit or terminal filter. (non-motorized). With humidity control features a simple systems has a de-humidifier and a re-humidifier, with a more complicated and expensive control than a standard A/C system. There are hugely more complicated systems out there than what I just described above. O.K. – here’s the thing.

The mechanical system for temperature control is expensive – to buy and to install properly. The mechanical system for humidity control can get really expensive depending upon how tight you want control.

If you just want to be able to walk into your cleanroom and not get electrocuted by a bolt of static electricity ( you know Harrah’s in Tahoe on the carpet in the winter type stuff) and you don’t want it raining in your cleanroom – Then don’t get crazy and spec 68 + 2 with 45 + 5 RH. That’s a tight spec and its expensive. It’s a semiconductor processing spec especially around positive photo resist that’s hydroscopic.

Try 50% + 15% awesome – now we can look at your area cyclometric chart and figure a system which will be affordable to buy, affordable to install, affordable to operate and give you the max. positive design days possible – What does that mean?

It means if your willing to go slightly out of spec when the weather gets totally wacky a few days a year, then the design engineer doesn’t have to design for 365 days a year (worst case scenario). You know what engineers design for when you give them a worst case scenario. Now – temperature relates to humidity hugely again. A 1 degree shift in temperature can affect RH 5%. That’s why its called relative humidity. So guess what – heat sources in the cleanroom going on and off can screw up the whole deal, if not designed properly. And guess what – now comes dreaded exhaust, because that air has to be made up with a controlled air. Dust off your engineering book!

O.K. – so – a fairly loose RH spec with a reasonable temperature spec gives you the best of all worlds if you need reasonably controlled humidity.

A really tight humidity control situation requires a lot of planning unless you own a piece of PG&E.

You need as air tight a facility as possible. Even conduit with plugs in the walls must be sealed – Ceilings must be absolutely tight. Humidity leaks like beer through a drunk.

This brings up recovery time – No not the drunk. Recovery time is the amount of time you’re A/C system requires to bring your cleanroom back into spec after it has been challenged by a breach or change. Example – You open the door from the gown change room and walk in – you’ve done two things – you’ve brought in tons of particulate and you’ve let in or out all your precious humidity. On weak control systems it can take 15 minutes to ½ hour to recover. On strong systems a minute or so. On a tight process this recovery time is important.

Generally – a good strong humidity control system for tight control consists of chilled water/chiller system coupled to an air handler with deep row coils. This system will probably have a re-heat system and a re-humidifying system. How this works is – you get your air, cool it to the point where you wring out enough moisture to get below spec. Then you re-heat the air and re-introduce moisture to get right amount. This would be like emptying your car gas tank on the ground and then putting five gallons in at a time cause you didn’t know how much you had in the tank. Big bucks to run. Minimize the area you got to control like this (micro environments) if you can. Anyway – these issues require a very experienced engineer. These type of guys – I hire when I need ‘em I’m not that smart.

Question
6. Can I just add a Wal Mart wall mounted A/C system on the wall?

Answer
Sure – but you better dump the air through a HEPA filter and the A/C system better be able
to handle the static pressure requirements of a HEPA filter generally .4”-.6” initial .8”-1.2”
final. Also – where’s the heat going? – in the main room. Also – condensate – bucket or
drain. Sometimes you spend more money to save money.

Question
7. Can I get an A/C system which doesn’t need to go on my roof?

Answer
Yes – you can buy one specifically designed to mount on your cleanroom – side walls or top.
Your condensate can gravity feed to the drain or be pumped out. These systems are available
3 ton (36000 BTU’s or 2 x biggest Wal Mart unit) up to whatever.

Also there are commercially available heat pumps that roll around and provide cooling air.
They work best in dry climates. And there is updraft air conditioning, stand alone systems
that have been used for years in the computer industry available.

Question
8. When planning my facility, how can I help this design process along?

Answer
Damn good question – I’m really glad you asked it?

Get the right information.
1. Establish design parameters – real ones – temperature and humidity.
2. Get insulative valves of building. (R factor)
3. Get real mechanical plans as to existing units and stuff on the roof.
4. Get a survey from an A/C contractor as to serviceability of existing equipment.
5. Evaluate building for accessibility and structure to handle additional equipment on roof or establish local pad area for outside mounting of equipment. All kinds of codes and regs lerking here so watch out. We usually get a structural engineer to look at the building early.
6. List all equipment going into cleanroom get – name plate voltage and amps,
Running amps (if possible), exhaust – cfm, heat load in room.
7. Establish service factor ;
That means how much of this stuff is gonna be on at once.
Four fume hoods at once is a lot different than one on at a time.
8. While you’re at it, plan – exiting and material flow. That way you don’t have to go back and forth 20 times. Give this stuff to your engineer and you just saved 3-4 weeks of heart-rending e-mails.