Design Problem

[frank]

Went to a meeting yesterday where a client wanted some cooling into 4 Bio-reasearch labs.

The labs at present are Class 2 rated (slightly positive pressure) and have a set full fresh air ventilation rate with a slightly higher extract rate, mostly through the fume cupboards.

The existing set up supplies full fresh air into a sealed ceiling void from a central AHU and then individual fan coils supply the fresh air into the lab via ceiling grilles. The fan coils have LPHW coils for winter tempering.

The labs have been calculated as having total heat gains of, 12, 20, 27 and 35kw. they also have set fresh air input rates of 400l/s, 520l/s, 680l/s and 1250l/s.

The client stipulates that NO recirculation air will be allowed as this may lead to bio contamination.

The central air plant does not have any cooling for summer ambients but has LPHW pre-heat for winter. There is NO facility to incorporate cooling into the central plant and all cooling has to be done in each lab.

How would you approach this problem?

[Makeit go Right]

The general approach must be to replace the existing fan coils with units having a similar LPHW coil and a cooling coil, sized for the straight through 100% fresh air.

Maybe you will need a couple of extra FCUs to deal with the cooling load.

[Peter_1]

Frank, some possible ways I see:
1. a CCU in each room which cools the room with an independant unit and with special filters in it (TROX http://www.trox.nl/nl/products/filters_and_filter_units/terminal_filters_ceiling_wall_mounted/index.php or something similar)

2. After the individual FCU, installing an inline cooling coil which cools 1 lab.

3. is there no possibility to make the cooling battery in the AHU in summer a cooling battery via a 3 way valve (in case it's a water coil)? And cool it a little bit lower then necessary and reheat it electrical to the desired temperature? Or adjust the airflow (VAV) to deliver just as much air to cool the room.

Are the heat gains included in the energy supply from the fresh air if it's outside 35°C.

Do you have large margins for temperature and/or humidity?

[Brian_UK]

I think that you have to look at fitting cooling coils to the fan coil units.

If you could alter the main AHU to provide cooling is it likely to generate a problem in the ceiling spaces with condensation etc.

Regarding the fan coils, can you not replace them with (Daikin:rolleyes: ) void units utilising heat pumps; that way you would be able to provide boost heating if required but handle the cooling load directly.

You wouldn't be recirculating air as the void unit would take it's intake directly from the plenum.

[Abby Normal]

They have a hunidity condition to maintain as well?

Those heat loads account for outside air as well?

What is a worst case ambinet temperature there as well as a worst case ambient dewpoint there?

[winfred.dela]

Wanted to fully understand the existing condition:

Four Bio-research Labs:

Rm 1 - 12 Kw FCU, 400Lps airflow, heating only
Rm 2 - 20 Kw FCU, 520Lps airflow, heating only
Rm 3 - 27 Kw FCU, 680Lps airflow, heating only
Rm 4 - 35 Kw FCU, 1,250Lps airflow, heating only

Fresh Air Supply Blower to the ceiling - 2,850Lps

Have to ask the ff:

1) Required room temp = ______C +/- ___C
2) Required Humidity = _____%RH +/- ___%
3) Where are the filters installed?
I assumed this is a clean room with 3 stage air filtration.
Are the filters located in the Fresh Air Blower?
Is this right? - From fresh air blower the filtered air goes to the ceiling, then thru 4 units FCUs then to the rooms via ceiling grilles.
4) At present, how do they control the slightly positive room? do they speed up the fan coil fan motor? What sensor do they have in the rooms to assure the slightly positive room?
5) What are the capacity of the exhaust system (fume cupboards)?
6) Was the fresh air cooling load considered in the calculation?

My last projects for Laboratory Aircon units with supply fans, exhaust fans & 3 stage filtration (for micro lab, tissue culturing labs) was in late 90s.

Maybe, I can help on this given the right data.

Please note that we do not have heating in our part of Asia, so i might miss something.
Would appreciate if corrected/supplied the above data and assumption.

[Renato RR]

What kind of building configuration do you have on site.I would put one chiller,and replace old 2 pipe Fcc with 4 pipe Fcc.Each room should have central system maybe Honeyvell or Siemens with 3way valve.

Where are the experience guys?

Best regards,
Renato

[Samarjit Sen]

Since this is a class 2 Clean Room, you would require 3 stage filtration. There should be a Central AHU suitable for 100 % fresh air intake. There has to be a fine filter at the air intake, then another set of pre filters at the air outlet of the air from the AHU. It would be better if you can provide air tight air ducts and the air discharge to the individual rooms through ceiling fan-filter modeules with HEPA filters, other wise the cooled air could be supplied to the ceiling void and the conditioned air to the individual rooms through ceiling mounting fan-filter modules with HEPA filters. The Fan - Filter modules has to be selected properly. Trox may be the right source as they have suitable modules.

For detail assessment Winfred. Dela's opinion is most suitable. Clean Room Air Conditioning is a very sensitive technology and you have to be careful with the filtration to avoid contamination specially in the Class at which you are working.

[winfred.dela]

In addition to previous questions:

Will the customer required some sort of test after the completion of the project, like:
- a particulate counter
- temp/humidity log that will assure the condition are within the acceptable range.
- room pressure log to assure that the room is at slightly positive pressure at all times.
- an airflow evaluation to assure that the air passes in a laminar flow from the ceiling to the floor.

Refrigeration may look easy compared to this particular aircon application.

Maybe a better approach is to ask the customer if he is willing and have the capacity to pay for a REAL clean room condition.

I always find end users that just wanted to call their Lab as clean room but don't really care how many particulate or what temperature or humidity they are having especially . . .
- if the price will be quite high when all the above parameters will be attained.

[Samarjit Sen]

In case of a Clean Room application a Validation is carried out by a third party in which all the test as stated are carried out. The job is considered to be complete after the Validation by the authorized party.

[US Iceman]

Hi Frank,

The several projects I have been aware of typically used a very expensive central station air handling unit. Double wall constructtion, stainless steel, etc.

They were all 100% OA (outdoor air) ventilation. The air handling units were equipped with something like 20% pre-filters followed by 80% filters in a single filter rack.

Then a heating and cooling coil, then the fan, then some more pre-filters for the HEPA filters. The air was always discharged through duct work, never through the interstitial space above the room.

The cooling coils were typcially chiller water or chilled glycol and LPHW for heating.

When the fume hoods are off, how is the ventilation maintained? Are they using exhuast fans, which then have additional requirements for proper discharge.

A secondary consider may also be heat recovery, since these types of systems can be quite expensive to operate.

[frank]

Some more information after reading your replies.

The labs are existing, on the top floor of a 2 storey building with a flat roof over, which, in our "summer" months :D leads to high room temps especially as some of the labs contain low temp fridges, bunsen burners etc.

At present they are provided with filtered fresh air from a central air plant with all the necessary filtration in place.

The air input volumes are fixed, via the existing void mounted FCU's and the extract volumes are fixed via fume cupboards. The FCU's draw the input air from the void where it is discharged from the central plant. Secondary filtration is applied at the FCU's. The central plant serves other areas in addition.

Any cooling - for comfort purposes only - will have to be applied to these fixed input volumes, no increase or decrease allowed as this would upset the balance in the room. No recirculation in-room fan coils are allowed as the filters etc my get contaminated or cause contamination (picking up bio spores from one area and spreading them to another).

Due to the restriced size of the roof plant room there is not any room available to install a new cooling coil in the AHU, which would have been my first choice. So I am being asked to provide cooling to each room based on full fresh air (from the ceiling void) with an ambient of 30C. i. e. assume that the ceiling void air will be at 30C.

Using DX ducted units is out of the question as these work on re-circulation and will only generally give a TD of 7 - 8C per pass. This would give an air off temp of 22/23C which is not sufficient for cooling the room.

Also, when I did a few calcs, the fixed air volumes cause problems, such as the room which requires 27kw of cooling but only has an air flow of 0.68m3/s (680L/s) will give a supply temp of 6.7C - way too low :eek:

The room loadings have been calculated by the customer (the maintenance engineer) via HEVACOMP software http://www.hevacomp.com/ and include the fresh air loadings. I've worked with this guy before and he knows what he is doing so I've no reason to doubt his figures.

I've even thought along the lines of using 2 fan coils in the ceiling void, the first pre-cooling to serve the one connected to the ceiling grilles :confused:

[Andy]

Hi Frank:)

Sound like a consultant designed the ventilation :D

If they won't change their flow rates, either start again or add a new intake and extract to ensure the room air balance.

If they are not being sensible I would advise them of that and if they won't listen, that their problem:eek:

Me I would stick to my guns, if they want a good job done they will listen, if not it's their loss, I'm sure you have plenty of work:)

Kind Regards Andy:)

[Makeit go Right]

A couple of comments that may have some run-value in your final scheme, reducing the chiller size, maybe (assuming there is no existing chilled water on site).

1) There will be a great deal of heat gain through that flat roof and it might be worth looking at ways to improve the roof insulation in order to lessen the cooling plant sizes (not to mention the heat/cool running costs).

You'd have to take care of creating interstitial condensation problems, but e.g. you look at maybe 4-inches of polystyrene with slabs on top - rather simple. Or there are some lighter solutions about too. That HEVACOMP program allows you to check for interstitial condensation in the proposed roof structure. {There's a job for the maint engineer.}

(If slabs, you might also need to get the roof structure checked out for the extra weight, too, to be on the safe side.)

2) On the main plant, they should look at some form or recuperator to achieve further cooling plant size-reductions (and annual running costs). A run-around coil would not have any contamination issues and is very flexible regarding the physical locations of supply & extract plant.

3) If you win the install, you might want to check/upgrade your business insurance to cover potential design flaps.

[Abby Normal]

As you have pointed out there is to be no recirculation and it will then be cooling via 100% outdoor air.

Anyways the sensible load can be handled by supplying the fresh air to the space at a temperature lower than the space is being maintained at by the same calculation procedure used for systems that recirculate.

I would take the following approach to work out how much moisture would have to be removed from the outside air. Keep in mind "What goes in must come out"

Work out how much moisture per unit mass is in the lab air at the design condition you are to maintain. Use this to figure out how much moisture is leaving in the exhaust air. Call this "moisture out".

Work out how much mositure is added by untreated outside air. The total amount of mositure in the outside air. Add to this, the amount of moisture that is generated inside of the labs. Call this mositure in.

Subtract "moisture out" from "moisture in". What you are left with is the moisture that must be removed from the outside air, the condensate rate of the cooling coil. You use your ventilation flow and work out the required supply wet bulb.

It probably is not all that humid over there, some places the outside air is so humid it would have to be overcooled and reheated, other places it is so dry out to begin with, you would be running a humidifier in the summer.

So it sounds like you will be retrofitting cooling coils, if you need reheat then add the cooling coils upstream of your little fan coils. If there is a big difference betweeen the supply dry bulb and wet bulb, it could result in an "ADP" being out of range, and you would have to over cool and reheat. The cooling/dehumidifying process line would be steep if this was the case.

May have to bump up the motor or blower on the central ventilation unit.