Operating Room HVAC Approach 2

[BronYrAur]

I have a chance to make improvements to an air handling system serving 4 operating rooms. Right now it is constant volume with a reheat coil for each OR. Is VAV a good option for Operating Rooms? I need to maintain 15 ACH during occupied hours, but I assume that requirement can be dropped while the room is not in use. I'm sure that I need to maintain a positive pressure at all times, which leads me to be concerned with VAV. I assume that I would need boxes on both supply and return ducts in order to keep a positive differential.

Any thoughts on how to best approach this? Any good design guides? I purchased the ASHRAE "HAVC Design Manual for Hospitals and Clinics", but it is very general. It doesn't go into enough detail on duct systems.

 

[hepa99]

if you want to pressurize the room, you need to introduce outside air to VAV or treated outside air directly to the room. You can dropped VAV air supply to the room (not outside air supply to pressure the room) based on room temp and humidity.

as simple as FCU (to control room condition) + OAC (to pressurize the room) + exhaust system will work on operating room.

 

[SAK9]

It is not clear whether each theatre has its own AHU.If that is the case,it is only a matter of setting down your VSD for reduced airflow in the night.You would not need any VAV boxes.

 

[tys90]

All operating rooms I've worked on design with have been VAV with reheat coils on the supply side and a VAV box on the return side to maintain positive pressure. I've never had just a single unit serving a single operating room though.

As a side note I've also been using 20 as my minimum ACH. Even though AIA guidelines are 15, the ASHRAE handbook recommends 20. Some have even been 25 ACH.

Hope that helps!

 

[BronYrAur]

What percent Outside air do you recommend? I probably have 20% right now. To answer a question raised, there is 1 AHU serving 4 ORs.

Tys90, did you have VAV boxes on the supply ducts or just reheat coils?

hepa99, are you suggesting a two duct system - one for recirculating air and one for dedicated outside air?

 

[cry22]

OA ACH is covered in the AIA guidelines at 2 or 3 ACH of OA. Don't have cod ein hnad.

Total ACH depends on which book you read. AIA says 15, VA says 25. In these days of sustainability, one has to make a choice. I'd go with 15 min and set temp. as teh doctors want.

VAV is good for not only prerssurization, but also for shutting down VAV to min. when is not in use.

We typically use a booster humidifer at each OR in addition to RH coil. With a central humidifier at the AHU.

The next thing is filtration, some guideliens such as the VA, recommend HEPA filters if you perform orthopedic surgery, if not they are OK with 95% filters. Don't forget that final filters in OR AHU's are placed after the fan with a diffuser plate

Some folks use HEPA filters at the laminar flow diffusers.

Recirculation units OK if provided with HEPA filters only and ducted to laminar flow diffuers, do not use regular FCU's in the room.

One last thing, do NOT trust ASHRAE when in comes to hospital design, use AIA healthcare design guidelines, which is considered a code as opposed to ASHRAE, which is only a recommendation.

http://www.va.gov

search for technical library, you will find a wealth of information ranging from specs, to cad details, controls equence of operation, and so on.

 

[Zesti]

Seems to me Dutch designs for OR-HVAC is a lot different from what you are discussing:

- 20 ACH OA with a minimum of 2000 m3/h (dilute anesthetics)
- a laminar flow area with HEPA of 3m x 3m is pretty much standard
- laminar flow velocity of 0,25 - 0,35 m/s
(total circulating airvolume ~9000 m3/h)
- air return grilles placed low and high, VAV for room pressure control

Outside air can of course be reduced outside "business"-hours. Seems to me supply needs to be 100% during "business-hours", no VAV-action.


Most systems have a make-up air unit for outside air and some form of recirculation unit for each OR.

Most new systems are made with two temperature zones to help laminar downflow: a 2 x 1 m area over the table surrounded by a zone completing the 3x3 m. The inner zone is at 1.5 K below the surrounding zone.

Even a third zone can be added to give a higher temperature zone over the person doing the anesthetics.

Heating can be added to the OR to help create a temperature difference for the laminar flow.

Extra AHU's can be added as spares.

 

[KiwiMace]

VAV controls on airflow, rather than pressure, where you have terminal hepa filters. Chasing a moving target otherwise.

 

[BronYrAur]

To expand into a somewhat related topic, is there a rule of thumb on the chilled water requirements for an OR? By that I mean that I have seen designs where there is a "low temp" chiller serving the OR's. Without knowing any actual load data, it would still seem to me that the number of air changes alone would be enough to provide adaquate cooling. Are there industry standards on cooling loads? I don't have any actual lighting or equipment heat rejections. Let's say for example that I have a 5,000 cuft OR with 25 ACH. So I have approx 2,100 CFM. Let's further assuem that the supply air temp is 55 deg F. Is there are standard approach temperature that I can expect for a typical OR? Can I maintain 70 degrees in here? How about 68?

I know that I only have half the pieces to this puzzle, but I would appreciate any comments.

 

[Zesti]

Over here, a coolingload of approx. 5 kW seems to be the norm at the moment.

Do you want to supply air at 55F to the room? Seems rather low to me. Are you making a turbulent system or a laminar downflow?

Given a cooling load of 5 kW, for a system with 25ACH in a room 7x7x3 metres you'd end up with a temperaturedifference of 4K between supply and return/room air. Therefore, there's no need to supply air at 55F.


PS: Haven't you guys still not discovered the elegance and beauty of the metric system...?

 

[Zesti]

I forgot about your question about chilled water:

Depending on atmospheric conditions there can be a need to "dry" the outside air and therefore your chilled water needs to be cool enough to do that (combined with the appropriate cooling coils).

But, for systems with circulation AHU's for each OR (as I described before) you do not want those cooling coils to form condensate. Those coils would be run at a higher temperature chilled water to prevent condensation and thus dehumidification of the circulating air.

 

[hepa99]

BronyrAur,

> is there a rule of thumb on chilled water requirement for an O.R.?.... Do the water balance (Q = p x V x Cp x AT).
> are there industry standard on cooling loads?..... it's better to do basic cooling load calculation.
> if you have 2,100CFM (based on room volume and air change computation), you still need to verify if 2100CFM is enough for OR cooling load requirement. If the air volume requirement of cooling load is below 2100CFM, you need to adjust the supply air temperature difference between the supply and return air on your cooling load and used 2100CFM. If the air volume requirement of cooling load is more than 2100CFM, you need to used the computed air volume.

good luck!

 

[BronYrAur]

Yea, I know that a calculation is necessary to do it right. I just wondered why some OR's have "low temp" chillers and others do not.

In my particular case, I'm starting to look at exsiting OR's served by air handlers with probably 42 deg chilled water. DAT is probably 55 deg. I don't have enough information yet to do it properly, but is the supply air temp to OR's typically below 55 deg, thus making a low temp chiller necessary? or is the outside air dehumidification requirement driving the low water temp as Zesti suggests?

PS. The metric system is all we are taught in engineering school, then we get out and don't know what a BTU is. It took me a long time to get use to these units. Now it's been so long that I can't remember metric.

 

[mauricestoker]

The standards Zesti gives seem to mirror VA requirements. Current standard is 100% OA, 20/10 occupied/unoccupied. The VA standard requires pressure independent control valves for volumetric control. HEPA at final filter is required.

Try looking at the VA HVAC Design Manual, Chapter 6A. This includes chilled water requirements for central plant and the DX chiled water unit, as well as interlocks on the 3-way valve. Fairly detailed for CHW temp, dew point, and DB conditions. If doing a VA suite, I'd recommend full redundancy on the DX in case of failure/operation problem on central plant chillers. Standard for that is 44*F at design, won't cut it for OR suite.

VA also recommends considering dessicant on OA intake for energy. Looking at that now for possible usage (climate zone 4a). Heat exchange definitely needs consideration along with other available exhaust streams (existing is near another 100% OA, one or the other AHU is going to be run cheap).

Just starting on design for replacement of a single fan dual duct suite with 4 OR's, same time as replacement of the central plant.

VA is a very stringent standard; you may want to clarify if you are to use AIA or VA, or local variations, room by room in the suite. Operatories allow two settings, other areas of the suite require CV to maintain relative differential pressure/volume between clean/dirty sides. Since you can't predict simultaneity, a VFD with multiple frequency setpoints can be an energy saver. Using ABB's sold me on this.

The ASHRAE manual has good detail on thermal effects for surgical, but not sure if you want to get to the level of doing CFD. VA design provides very specific locations and flows. Had to do that with ABSL-3 design, and it did not add much value (position and size of personnel, local equipment cause large variation, just as with local exhuast ventilation-don't trust the comuter runs as much as snoke tests and mock-ups, when available).

Learned everything by metric, worked in Europe for 7 years, after a couple decades stateside, all I remember is that a liter is meant for beer size.

 

[Zesti]

Supply of 55F-air to the OR is too low.

However, it is spot-on as far as RH is concerned.

55F at 95% RH after the OA cooling coil gives just the right RH in the OR once the air is warmed up to OR conditions.

The correct RH (50%-65%) is very important in an OR because of drying out of wounds and static electricity issues.

 

[Zesti]

Out of curiosity: have any of you ever seen two-temperature laminar downflow designs in their part of the world ??

On the subject of CFD and full scale model testing:
Almost all of these tests model stationary situations which is of course not in accordance with the actual situation in a working OR.

There are all kinds of objects (light, monitors, gass-supply arms) and of course people influencing the airflow patterns.

More importantly, the behaviour of the OR-team can do more harm to the sterile environment than any HVAC-system can correct. Good behaviour of the team is a much more important factor in post-operation infections than HVAC.

For instance, idealy the doors of the OR should not open at all during a procedure: I have seen statistics where doors open 50 times or so during a relatively short procedure.
Not to mention the people of the team scratching their ear or nose...

 

[ordesign]

BronYrAur,

Not sure whether you are still working on the O/R issue, but thought I would chime in.

I'll give you the the design requirements for California, which are slightly more stringent than other states.

If recirc is used, total ACH = 20, 5 of which is OSA.
If 100% OSA, ACH = 12.
Temp range is 68F - 73F (although plan on surgeon wanting it mid 60s. This will effect dehumidification for OSA.)
RH range is 30% - 60%.
Pressure relative to adjacent must be positive.

Are you considering laminar flow? If so, through the ACH numbers out the window, as only the OSA number will be applicable to a laminar flow design.

 

[ordesign]

BronYrAur,

One more thing. I noticed in a prior remark, you wondered about the low temp chiller. This is in order to dehumidify to the level required for the lower temperatures that these rooms usually are operated at. (Mid 60s F) Dessicant is another method, especially if you have a heat source for regenerating the wheel. You should have some steam for humidification, so you can always use a steam coil in the dessicant unit for regen. Also, keep in mind that you will need reheat on your OSA.

 

[AbbyNormal]

there sure is a lot of info available from the VA cry22

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