Operating Room AHU Freeze Protection 2

[JDengg]

Can somebody please assist me in coming to a conclusion on this argument between me and my new boss. Old boss left recently, new boss is young with minimal experience as a PE, and he's been fighting most of my design decisions since he started.

If you're answering this post, you already know the dangers freezing air can lead to and the damage that it can cause to an AHU. In my control sequences, I always have a unit shutdown if the freezestat is tripped. Recently, I've been developing controls with dual stage freeze protection to mitigate potential stratification in the unit. My new boss's argument was simply "what if somebody is having surgery when the freezestat is tripped? Unit cannot shutdown". I have concerns with this because I know the damage that can be caused to an AHU if freeze protection weren't utilized for the unit. If a cooling coil were to burst, it could cause millions in damage to the building, and I feel that is reason enough to shut the unit down if the freezestat is tripped. If the pumps were the point of failure, then the reheat coils wouldn't be able to operate either, which means <38 degF (freeze protection setpoint) air would be sent to the space, which I feel would be way more harmful to the patient than a shutdown.

Have any of you come across freeze protection requirements for an operating room? I have had difficulty finding any information on this. Alternatively, what measures have you taken for freeze protection in AHU's serving critical/sterile spaces? In the end, it isn't my stamp on the drawings, and if he is comfortable doing this, he can. However, I'd like to know that I'm being guided in the right direction for how to approach these situations, and don't feel that I am in this scenario.

Thank you,

 

[GT-EGR]

Would need to know more to evaluate the pros and cons of your two options

What type of cooling coil - is it chilled water, or glycol, and does it have a freeze protection pump setup

What type of heating coil - is it upstream or downstream of the cooling and does it have a freeze protection pump setup

Do you have redundant air handling units

Your freezestat is detecting air temperature across your coils correct? Not clear why your pumps are at the point of failure due to air temp.

Have you heard of or reviewed the facility guidelines institute (FGI) guidelines for your type of hospital - it may be specific about your redundancy requirements.

You are both correct and need to design a reliable system that won’t let airflow shutdown during critical hospital care, and also can’t let air handling units incur damage due to forced operation in unstable conditions. So it sounds like you are missing redundancy in your overall design.

 

[urgross]

This might be helpful, both OR and sterile process. OR's require a redundant chiller, using two coils is an option. Having propylene and a separate pump would be an option or addition. Using DDC warning of decrease in discharge temperature, further decrease shutdown OA damper, then shutdown. Sorry, I cannot e-mail you a copy of ASHRAE 170.

When as owner's rep for AFIP (BSL-3 and BSL-3 labs) I could make the decision-redundant labs and shutdown condition was going to minimum air flow at BSC flow for hot cabinets and supply air to maintain flow to 0.05 IWC relative diff pressure. The controls were Cylon based, which were excellent. I wish they covered where I am now (we have JCI).

If allowed to use redundancy, make sure the electrical to each unit (including exhaust) is separate and independent. Failure of the MCC has been more frequent than I've seen.

I've had a cooling coil burst and shut down the air handling unit. I was called out of what is called the DSB (wearing suit and tie required) and went to the mech room. Repair was about $10K and my best suit.

https://www.cfm.va.gov/til/dManual/dmHVAC.pdf

Above is the VA HVAC Design Manual. It makes a fairly good reference to start with.

 

[Zesti]

I don't know of specific requirements in your country.

That being said, is your AHU providing 100% of the air going through the sterile air filters above the operating table?

Common practice over here is that the air over the filters is 90% recirculated air and only 10% fresh outside air.
If the outside air was absent due to the MUA unit being shut down because of frost protection, then sterile air would still be available.

Most of the time MUA's are redundant 2x100%, but of course this is no help if the source of heat is down.
Also, if a heat recovery wheel is used, OA temperatures of -10°C can still be lifted above freezing and so the heating coil will be protected. With only 10% OA to the recirculating AHU, the resulting mixing temperature will probably still be alright because of the heatloads being present in the OR.

Where things might go a bit wrong is the required humidity of the air.

This setup might not be in accordance with your local codes and regulations, but it is energy efficient and somewhat resilient against the heating coil being out of service.

All that being said, you can never provide a 100% guarantee on sterile air being available. If the air is down the surgeons will perform/finish up the operation anyway: stopping isn't an option.