David Schurk:
Welcome to this episode of the ASHRAE Journal Podcast, where today we'll be discussing dehumidification for the environment of care, hospital operating rooms. My name is David Schurk, and I'm director of applied engineering for Innovative Air Technologies out of Covington, Georgia. We manufacture solid desiccant dehumidification units that are used to remove moisture for the air and applied in all kinds of applications, one such being hospital operating rooms, one of my favorites, and our topic of conversation today.
I have over 40 years of experience in the HVACD industry. I'm an ASHRAE Life Member, an ASHRAE Distinguished Lecturer, and I co-instruct ASHRAE's Humidity Control I and II course with Mark Nunnelly, who's also my co-host on today's podcast. I've worked my entire career as a sales engineer for major equipment manufacturers, applying dehumidification solutions to help my clients produce comfortable and productive indoor environments.
You'll notice throughout this podcast that both Mark and I will be adding the D for dehumidification to HVAC. We feel it's such an important and oftentimes overlooked component of successful environmental design and it deserves its own rightful place in our industry's acronym. And it's our show today, so we can kind of do what we want. Right, Mark?
Mark Nunnelly:
That's right.
David Schurk:
Mark, with that, why don't you quickly introduce yourself and tell us a bit about you?
Mark Nunnelly:
Sure. Thanks, Dave. And first of all, thank you for letting me co-host this with you. Great opportunity. Dave, you and I, as we've talked before, we have a lot of similarities in our careers. I'm also a Life Member of ASHRAE, which means I'm on Medicare now, and I've served on several grassroot committees and technical committees over the years from local, regional and society levels.
I'm a professional engineer in my home state of Alabama, plus many others throughout the southeast primarily. I've got over 43 years of experience in our construction, engineering and energy world. I started off out of college with a large general contracting firm for industrial and commercial applications. And then from there I worked with a natural gas utility as a utility marketing engineer.
And it was during that time working with a number of large hospitals that really piqued my interest in humidity control, particularly within the hospitals. So 25 years ago, I founded Nunnelly & Associates primarily as a manufacturer rep agency specializing in dehumidification product, much like you, Dave, but we represent a desiccant as well as mechanical-based products for not only dehumidification, but also energy recovery and custom air handlers.
And we served the commercial and industrial applications. However, about eight years into the manufacturer rep world, I transitioned back to the engineering consulting role where we spend our time now mostly in forensics humidity control consulting or HVAC and decommissioning. I've been blessed like you have, Dave, with opportunities to give back to our industry throughout my involvement as a distinguished lecturer for probably about 20 years, and then also as a lecturer in the short courses for probably about 15 years.
David Schurk:
So Mark, can you give us a quick rundown of the temperature and relative humidity recommendations for hospital operating rooms based on the current ASHRAE Standard 170-2021 titled Ventilation of Healthcare Facilities?
Mark Nunnelly:
Sure. Standard 170 has given us guidelines for a long time with regard to the maximum and minimum temperature and relative humidity design conditions, as well as a number of other conditions that we need to have for our operating rooms, such as air change rates, filtration standards, pressurization standards, things like that. But two of the most important design guidelines that we'll talk about mostly today would be the space temperature and relative humidity guidelines.
For the longest time, the guidelines for space temperature has been 68 degrees to 75 degrees dry-bulb, and that's been for the operating rooms. Those are general guidelines, and that's worked well for most procedures such as years ago when my youngest child had tubes put in his ears, it was about a 30-minute procedure and there's no reason to drop the temperatures that low.
However, an often-overlooked footnote in these standards such as in Table 7.1, which is for inpatient spaces, and 8.1, which would be for the specialized outpatient spaces. That's what we refer to as Note O. Note O says, "Surgeons and surgical procedures may require room temperatures, ventilation rates, humidity ranges, and/or air distribution methods that exceed the minimum indicated ranges."
So in other words, paraphrasing that, what it's saying is that designers, you need to speak with your clients and find out exactly what the space needs are for their operating room. So in other words, it's a novel idea. Engineers, talk with your clients. That's especially true in what we refer to, Dave, as the deep cavity surgeries such as neurosurgeries, cardiovascular, orthopedic, things like that.
Oftentimes they require much cooler temperatures, oftentimes 60 degrees or even lower. I've had clients even down to 55 degrees was their desire. Years ago, for example, this is when I was a manufacturer's rep, I made my rounds to hospitals throughout the southeast. And I had been meeting with one hospital and they said, "It's a good time that you're here because we are about to build a new hospital and we need conditions better than what we have right here."
Our surgeons are often screaming because they want the temperatures below 60 degrees in these certain operating rooms, and we have the exact same problems that you've been talking about, Mark, which would be raining in the operating room and condensate issues like that. So I asked him if I could go speak with their design engineer there in town.
And so he invited me to do that and asked me to do that. And so when I met with the engineer, I said, "Look, I've been meeting with such-and-such hospital and they want temperatures much colder than that in some of the operating rooms." And the engineer said, "No, they don't need it that cold. Standard says 68 degrees, and we're not going to design it for anything colder than that."
So that's what I ran against quite often is that the design engineers were not speaking with their clients and finding out what they really wanted to have regarding the temperature. The next thing that we would look at would be not just the temperature, but also the relative humidity. So prior to the 2017 edition, Standard 170 required a minimum relative humidity of 35% in the operating rooms, primarily for fire safety reasons.
If you remember back then, many of the anesthetic gases were explosive and the relative humidity too low because you would have a chance of static electricity and then that could cause a serious issue. But in 2017 edition, they relaxed the minimum relative humidity requirement down to 20% from 35% while maintaining the maximum requirement allowed of 60% still. This was formally documented as Addendum D to the 2008 Standard, which was approved by the ASHRAE Standards Committee in 2020 and then published in 2021. So that's a little bit of a rundown on Standard 170.
David Schurk:
So Mark, to your point, that narrative Note O is very important. I'm not sure that anybody ever reads the fine print, but I kind of consider that to be that fine line in the sand between environmental success and failure. With that said, we've both spent our careers working in the dehumidification business and more specifically in hospital environments, either fixing ailing and failing operating rooms or trying our best to get their dehumidification systems aligned with the various stakeholders expectations from the start of initial design.
What are some of the key issues you see impacting the ability of the HVACD design to either maintain standards compliance or surgeon comfort, or both?
Mark Nunnelly:
Well, Dave, I think to me, the primary issue that I've encountered over the years, whether it be from a manufacturer rep years ago or more in the design field now, the primary issue has been encountering engineers that don't want to do anything different, that won't think outside the box. The technology is certainly proven and available, whether it be with desiccants or low temperature chillers, whatever the technology to meet whatever the demands are for that particular operating room.
But too often, I think from the design side, we just don't want to think outside the box. And Dave, another design issue that I see very frequently is that of not designing around the proper ambient conditions or the outside air conditions. I can remember past president of ASHRAE Dr. Don Colliver, many of our listeners will know Dr. Colliver, an engineering professor, University of Kentucky.
He shared with me back in 1996 very important and very informative climatic data that was soon to be published in the upcoming 1997 Fundamentals Handbook. And I remember telling him, I said, "Don, when this data is published, it is going to revolutionize the way that we design our HVAC and D systems." Well, here we are about 28 years later, and most of our design engineers don't look at that data.
I still refer to it as the new climatic data, but it's not new. Again, 28 years old. With that, we've got new data that's available in all these various cities, thousands of cities. And prior to this data coming out in 1997, we designed our air conditioning systems basically on the peak dry-bulb with the mean coincident wet-bulb time of the year. Basically, that's the hottest time during the summer that particular city might experience.
But it was realized in many of the cities that that was not necessarily the most humid time of the year or even humid time of the day that the systems were being designed to handle. So we took the same weather data, we being ASHRAE, took the same weather data, but put it in bins based on peak dew points as opposed to peak dry-bulbs and looked at the data from a different angle.
In other words, what is the peak dew point for that city and what is the mean coincident dry-bulb for that particular city for that bin of dew point temperatures? So that's what we refer to as the peak dehumidification ambient condition. Again, it was found that in many climatic regions, the peak dehumidification climatic design, when it's considered, that's considered for the peak dehumidification capacity and not necessarily the peak cooling capacity. So both of those design conditions need to be looked at for a particular client such as an operating room that has some very extreme conditions.
David Schurk:
So Mark, your comments with regards to the importance of using the right outdoor design weather condition for the application cannot be overstated. Most certainly, dehumidification design day weather data is appropriate for hospital operating room environments. I wrote a engineering newsletter several years ago for one of the manufacturers that I was working for at the time, and I did a comparison of outdoor air systems processing about 10,000 CFM of air down to 52 degrees Fahrenheit saturated.
So your typical outdoor DOAS type air conditioning scenario, if you will. And I analyzed eight major cities across the US. If I remember right, Minneapolis, New York, I think Omaha, Charlotte, Orlando, Chicago, Los Angeles, places like that.
And the analysis came back that when using the cooling design day conditions for calculating the ventilation load or the latent load, the moisture removal requirements of the system for that outdoor air as compared to the dehumidification design day data, the system would fall anywhere from 20 to 30% short in delivering the right amount of dehumidification using cooling design versus dehumidification design day conditions. So really important, particularly on those days when you need dehumidification the most.
ASHRAE has been promoting the use of dedicated outdoor air systems or DOAS units for years, and there's no better application for that than hospital operating rooms. But there's more, of course. Oftentimes surgeons are heavily dressed. They're wearing two or three layers of garments. They're wearing the space helmets that we see in orthopedic surgeries along with sometimes the cool suits that NASCAR drivers might wear.
They're using an array of surgical equipment, things like hammers, saws, chisels. They're working their rear ends off under very stressful conditions. A big complaint they have is that they perspire profusely underneath all that gear and they become very hot. They request, or should I say deservingly demand colder temperatures in the operating room, and that's when the train can come off the tracks.
Docs need it both cool and dry, so the environment that surrounds them can evaporate perspiration from their skin, wicking it through multiple layers of clothing into the surrounding ambient. We know this removes somewhere around a thousand BTUs per pound of sweat evaporated. And without this evaporative cooling effect, they cannot maintain physiological comfort or reach that thermal equilibrium that's required for comfort. Mark, what kind of issues have you seen this create for HVACD systems struggling to achieve surgeon satisfaction?
Mark Nunnelly:
What I've seen is very similar to what you've seen. Some of the same issues, whether it be condensation dripping from the ceiling or doctors that were having to wear these very much tethered cool suits, like you mentioned, the ones that the NASCAR drivers wear, and they're just very cumbersome and very heavy, so very uncomfortable to wear.
Actually, the first hospital that I worked with to help install desiccant dehumidifier in to precondition the outside air was a hospital that I had been working with or trying to work with for a number of years, and the hospital engineer was on... He was on board with trying something new, but the hospital just didn't want to spend any money to make anything different.
Well, he had a call one day from the neurosurgeon, chief neurosurgeon of the hospital, and asked him to dress out and come down to the operating room where they had a procedure going on. And so the facility engineer dressed out in the gowning, walked in there with his hygrometer, and the doctor was yelling at him and saying, "It is too hot in here." And he looked at the hygrometer and said, "Doc, it's 55 degrees in here. It's not too hot. It's 70% relative humidity in here. It's too humid in here. That's the problem."
And the doctor said, "Well, you do whatever it takes to get it fixed." So things changed and we were able to put our first desiccant dehumidifier in that particular hospital. And so away we went from there.
David Schurk:
So 60 degrees Fahrenheit at 40% relative humidity might be the sweet spot for surgeons performing what's considered more invasive or complex surgeries, the orthopedics, the neurological surgeries, the heart surgeries and the like. That is a 36 degree Fahrenheit dew point temperature in the space requiring about 32 degree Fahrenheit dew point temperature supply air. This is air that's dry enough to help wick perspiration through multiple layers of clothing into the surrounding air and can help provide relief through the evaporative cooling of their body.
Mark Nunnelly:
I agree, Dave. I've had many of the surgeons and the hospitals that have asked for the same things. They want the temperatures colder. Most of the time it's around 60 degrees was their desire for these deep cavity surgeries. But I have had hospitals, such as the one that I just mentioned, that wanted the temperatures to be maintained at 55 degrees.
Now, I'm pretty certain once the systems were designed and we were certainly able to maintain 55 degrees and even 40% relative humidity, once we were able to make the rooms much drier, they didn't need the rooms as cold as that 55 degrees after all. So I believe they did begin elevating the temperature to closer to 60 degrees.
The fact remains though, Note O needs to be considered when designing operating rooms, whether it be the design engineers, the service contractors, even the hospital engineers, it's imperative that they look at that Note O and know that it's okay to get beyond the standard of 68 degrees to 75 degrees.
David Schurk:
That Note O is kind of a pay me now or pay me later kind of thing. You can either read it upfront and make adjustments initially in the design, or you can go ahead and design, run head first into a wall and have narrative Note O kind of sort of put in front of you after the fact based on the fact that surgeons are going to complain. And it's just a omnipresent issue in most healthcare facilities, at least the ones that I'm involved with.
What I've found is that it's difficult, if not impossible to both decrease the sensible temperature in the OR while also maintaining a low enough relative humidity to provide compliance at a maximum of 60% relative humidity and achieve the comfort derived from drier air.
The 42 degrees supply chilled water temperature used in most US hospitals results in supply air conditions delivered to the OR that can easily maintain 60 degrees Fahrenheit space temperature, but with relative humidities that rise over 70%. Have you seen this problem too, Mark?
Mark Nunnelly:
Yes, same thing. They oftentimes maintain 44 degrees for the patient rooms and all that, but they have to either put in a lower temperature chiller or reduce the temperature to 42 degrees just to be able to maintain that particular temperature of 60 degrees that they would like in the ORs.
David Schurk:
I do a lot of presentations for ASHE groups and ASHRAE chapters, ASHRAE meetings, et cetera. A lot of design engineers and facilities managers in the healthcare industry, those particularly catering to hospital operating rooms. And I always ask, why do we use 42 degree Fahrenheit chilled water in hospital design when we know that 42 degrees chilled water just doesn't cut it from a standpoint of drying the air, at least many times?
And the only answer that comes back to me is just simply that's the way we've always done it. It doesn't mean it's right, but that's the way we've always done it. Perhaps that's something we most certainly need to look at changing moving forward. The issues we're talking about happen because the 42 degree chilled water temperature can't make the cooling coil surface cold enough to lower the supply air's dew point below about 47 degrees Fahrenheit.
They simply can't condense enough moisture from the air. With the HVACD loads in the OR, this results in the temperature and relative humidity conditions that we've been discussing.
Mark Nunnelly:
Going back a little bit, the ASHRAE 170 design, I see many of the design engineers that are designing for that design of 68 degrees and the peak of 60% relative humidity thinking that they've got that covered. Well, at 68 degrees and 60% relative humidity, that's a coincident dew point of 53.6. So now what happens if the neurosurgeon or the CV doctor comes in and says, "I want this room colder," and they drop that temperature down to 60 degrees, that 60 degrees and also 53.6 degree dew point, that's 79.5% relative humidity. So that's a real problem.
So in addition to needing to drop the chill water temperature much lower in order to get the temperatures where they want this in the air handler, particularly from the design standpoint, those coils are going to be much deeper. They may be 10 rows or 12 rows deep. And with that, in order to get that much dehumidification to have 10 to 12 rows deep, that's a maintenance nightmare each year of having to clean the coils and get rid of all algae, anything else that builds up in the coils. So that's a real problem.
David Schurk:
So these higher humidities that we're talking about, which are very typical in hospital operating rooms, drastically limit the evaporation of perspiration from the surgeon's skin and also results in spaces that are out of compliance with ASHRAE Standard 170's maximum limit of 60% relative humidity. It is a dilemma.
Mark, as I mentioned already, you and I co-instruct ASHRAE's Humidity I and II professional development courses at the AHR each year, as well as some other venues. I think we have one coming up virtually in the next month or so. Let's talk about some of the system design options that we discuss in class that are geared to produce lower dew point temperatures, which then allow for both cooler space temperatures and reduced relative humidity.
Mark Nunnelly:
Dave, sure. As we've mentioned earlier, some of the technologies that are used in hospitals to try to achieve the lower temperatures would be a low temperature chiller. A low temperature chiller could be that chiller that is for the operating rooms only, and that separates the load that the operating rooms need from the patient care rooms, admin rooms and all the others that could run off of the chillers at 44, 45 degree chilled water.
But as we know, as we drop the temperature of a chiller down from 44 degrees to 42 even, or even down to 40 degrees, we're going to increase the energy cost. We're also derating the chiller so that you don't have the full impact of the tonnage that maybe you thought you had for that chiller. And that could be at the expense of temperature control elsewhere in the hospital if you were taking your existing 44 degree chiller and just driving that down to a lower temperature.
Oftentimes too, to get a much lower supply temperature for that chiller, we would add a glycol solution to make sure we're not freezing up water inside the coils. And we can get a few more degrees lower in that, maybe even 38 to 40 degree delivery out of the chillers from that. But the problem we oftentimes see is that we're not building up ice inside of the coils, but maybe on the outside of the coils on the tubes and the fins, and that's going to result in blocked airflow.
So we have to be very careful not to let ice build up on the outside of the coils if we're getting that chiller too cold. But most importantly, I think the design team must have the frank discussions with the hospital engineering staff and hopefully the representatives from the surgical team as well. It's always great to have somebody from the surgical staff to sit in on those meetings with the design engineer and the hospital facilities engineer.
So obviously other technologies besides just the chillers, the low temp chillers, would be desiccants, whether it be solid or liquid desiccants. And I've had experience in those as well as Dave. So Dave, I'll let you talk about the desiccant dehumidification systems.
David Schurk:
And Mark, to your point, glycol chilled water systems can definitely do a better job of dehumidifying the air beyond that of what I'd call a traditional 42 degree chilled water hospital system. But the problem is you can make as cold of water as you want. Once the surface temperature of that cooling coil either approaches is at or below 32 degrees Fahrenheit, the condensate freezes. You've created a block of ice. Air doesn't move very well through a block of ice, and the train comes off the tracks.
That's where systems such as desiccant dehumidification systems come into play. We talk about this in class. There's a couple of different types, generally speaking, solid desiccant dehumidification systems, which adsorb moisture, A-D, adsorb, or liquid desiccant dehumidification, which absorb moisture. Typically, solid desiccant dehumidification systems have a wheel or a rotor that's placed between two air streams. The rotor is fluted. And within the substrate of the wheel or rotor material has been impregnated silica gel.
Silica gel can be maintained at a very low vapor pressure. So as moisture enters one side of the wheel and flows through the flutes, its vapor pressure is higher because of the moisture in it. And as it comes in contact with a lower vapor pressure silica gel, the silica gel adsorbs extracts or removes, sucks the moisture out of the air as a molecule, holding it in the desiccant material as a molecule, never condensing, never getting wet.
Liquid desiccant systems work similarly. Typically, liquid desiccant systems use lithium chloride or salt water. Salt water is a desiccant. And in similar fashion, that liquid desiccant solution can be maintained at a low enough vapor pressure to absorb or suck the moisture out of air that comes in contact with the desiccant and make that moisture load a part of the desiccant reservoir, if you will. And in both systems, they can be regenerated. So it's a process that's ongoing.
You are literally removing moisture based on low vapor pressure, regenerating whatever your desiccant material is with high temperature to increase the vapor pressure and blowing the moisture off of the wheel or the liquid desiccant solution to dry it out. The beauty of these systems is that there is no condensation that occurs. There is no cold coil that we need to be concerned with. And because of that, we can garner considerably lower dew point temperatures.
In fact, solid desiccant dehumidification systems can give you dew point temperatures leaving as low as negative 80 degrees Fahrenheit or around negative 62 degrees Fahrenheit. And when we're working in hospital operating room environments where maybe we want to try to hit that 60 degree Fahrenheit, 40% relative humidity mark, which is a 36 degree dew point temperature in the space and a 32 degree dew point temperature of supply air entering the space, it's easy to see a desiccant solution is the only way to achieve those conditions.
Mark Nunnelly:
You mentioned the lower dew point air that you can deliver with the desiccant systems, and this is something that we found in all of our hospital projects that we worked with. When we preconditioned the outside air with the desiccant systems and delivered air at a much drier dew point than the coil temperature, such as the 36 degree dew point that you mentioned, the downstream coils from the air handler that we were serving that air to, they would have dry coils now.
So they could oftentimes elevate the temperature of the chilled water going into those coils because they no longer needed to try to drive that down to 42 degrees or even 44 degrees. Oftentimes, they could elevate that to 45, 46 degree supply water temperature and still maintain all of the sensible load that they need to in the hospital.
David Schurk:
And as well, since you've desiccated the air upstream of the cooling coil, upstream of that cold surface, those cooling coils run dry. So there are no issues with regards to the proliferation of mold or fungus or bacteria, right? You don't have to worry about moisture carryover or blow off into the air stream. Notoriously final filters that are positioned immediately downstream of a cooling coil are always wet. And you get away from that issue as well.
So there's an array of benefits to using a desiccant dehumidification system in a hospital operating room environment beyond just maintaining surgeon comfort, maintaining compliance with standards. You're going to have a cleaner system. You're going to have a more efficient system, and most likely a more maintainable system overall. So Mark, if there was just one thing you'd like to see hospital design engineers get right from the start, what would it be?
Mark Nunnelly:
Well, there's not one thing. There are two things. Those two things would be to absolutely take into account the peak dehumidification climatic data that we mentioned earlier from the ASHRAE Fundamentals Handbook. We have got to look at the particular city, look at the weather for that city and design appropriately. As you mentioned earlier, Dave, there are many numbers that we have run as well on various cities when you compare the peak dry bulb with the peak dew point.
And if they did not design for the peak dew point condition also, they're oftentimes anywhere from 15 to 30% shy of the dehumidification capacity that would be required for that air handler. I have seen a few that were even as high as 50%. But it really is important for that design engineer to take into account the correct ambient conditions that that particular city may encounter.
The second thing would be for the design engineer to sit down with the client, the hospital, the facility engineer, and somebody from the operating team to sit down and find out exactly the conditions that they're trying to achieve in their operating room. Because if we don't have that particular dialogue between the client and the engineer, then we're going to be off the mark from the very beginning.
David Schurk:
So Mark, I'll wholeheartedly agree with everything that you said, and I'll add to that, that I think it's especially important for designers and engineers to have a working proficiency in psychrometrics and understand the importance that dew point temperature plays in system selection. That's what's going to allow the HVACD system to hit the mark environmentally providing both compliance and comfort. I always use the expression, it's psychrometrics, baby, and it's also why adding the D in HVACD is so important.
Mark, any closing thoughts?
Mark Nunnelly:
Right along with what you're saying there, I've had the opportunity as you have as a DL to speak to many ASHRAE chapters as well as the short courses that I've taught for the years, and I've been surprised at the lack of understanding of psychrometrics. I also teach a lot of courses to engineering firms just for continuing ed hours and things like that. And when we get into the psychrometrics, it's like deer in the headlights. It's just amazing to me the lack of understanding of psychrometrics.
I think as an engineering profession, we have a great handle on understanding temperature control, but for some reason we really have a poor understanding of dehumidification control. And it's really not that difficult. It's just a matter of understanding the psych chart and understand how that works.
One other closing comment that I would say, and I've shared this with you before, Dave, but like you, I've had an opportunity over the years to speak at a lot of hospital engineer conferences, the ASHE conferences. And sometimes I would start off with this comment, with this question. I say, "How many of you would like for your hospital to advertise as we provide the minimum care necessary for your health needs?" And of course, nobody raises their hand on that. And I follow up with it with, "Then why do we design our buildings that way to the minimum standards? We design to just get by."
For example, we've been talking about let's design this chiller so that we can just barely get to the bottom of that envelope of 68 degrees and 60%. So we design to just get by, to just satisfy the codes. Why don't we actually design it for what the surgeons need, the hospital engineers need and we're going to well exceed the minimum standards by just doing it right.
David Schurk:
Mark, I don't think that could have been stated any more eloquently. I love the analogy that you used of just getting by. And I'm telling you right now, I'm going to steal that. Just so you know. You'll hear me using it somewhere along the line.
Thank you for joining both Mark and me on this episode of ASHRAE Journal Podcast. It's been our pleasure hosting this event.
Mark Nunnelly:
Thank you, Dave.
ASHRAE Journal:
The ASHRAE Journal Podcast team is editor, Drew Champlin; managing editor, Kelly Barraza; producer and assistant editor, Allison Hambrick, assistant editor, Mary Sims; associate editor, Tani Palefski; and technical editor, Rebecca Matyasovski. Copyright ASHRAE. The views expressed in this podcast are those of individuals only, and not of ASHRAE, its sponsors or advertisers. Please refer to ASHRAE.org/podcast for the full disclaimer.