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  1. #1

    Humidity issue in surgical units

    Good afternoon, I'm having issues controlling humidity throughout several operating rooms. It is a VAV system. AHU has a cooling coil only and we use VAV hot water reheat. AHU has 44 supply water temp and 53 degree return water temp. The supply water pressure is 66PSI and return pressure is 55PSI. Discharge air is 52 degrees leaving the AHU (I dropped the disharge air setting to 50 hoping to remove more moisture). AHU has a common return (shares a mechanical room with another AHU). Humidity level in the mechanical room is 48%. I climbed in to the mixed air section of AHU before filter section and humidity is 49%. I climb in to the coil section, after filters but before cooling coil and it's 53%. I climbed into the blower section of AHU, after the cooling coil and it's gaining 20 to 25% humidity. The supply air has anywhere from 70 to 75% humidity. Drain pans are clear and there is a steady flow of water coming out the drain of the AHU. Inlet guide vanes on the AHU are not working and are stuck open. Is it possible the air is moving too fast over the coil to remove all the moisture? I'm thinking of working on the vanes in order to slow the air down to test my theory.
    Most of the rooms are set between 62 and 65 degrees. I noticed if I turn the thermosat up, the humidity drops. It's in direct relation to how much air is supplied from the AHU. Any thoughts or ideas would be greatly appreciated. Thanks for your time.

  2. #2
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    The air coming off a properly performing cooling coil should be at, or nearly at, 100% RH.

    PHM
    ------





    Quote Originally Posted by Usafwhite View Post
    Good afternoon, I'm having issues controlling humidity throughout several operating rooms. It is a VAV system. AHU has a cooling coil only and we use VAV hot water reheat. AHU has 44 supply water temp and 53 degree return water temp. The supply water pressure is 66PSI and return pressure is 55PSI. Discharge air is 52 degrees leaving the AHU (I dropped the disharge air setting to 50 hoping to remove more moisture). AHU has a common return (shares a mechanical room with another AHU). Humidity level in the mechanical room is 48%. I climbed in to the mixed air section of AHU before filter section and humidity is 49%. I climb in to the coil section, after filters but before cooling coil and it's 53%. I climbed into the blower section of AHU, after the cooling coil and it's gaining 20 to 25% humidity. The supply air has anywhere from 70 to 75% humidity. Drain pans are clear and there is a steady flow of water coming out the drain of the AHU. Inlet guide vanes on the AHU are not working and are stuck open. Is it possible the air is moving too fast over the coil to remove all the moisture? I'm thinking of working on the vanes in order to slow the air down to test my theory.
    Most of the rooms are set between 62 and 65 degrees. I noticed if I turn the thermosat up, the humidity drops. It's in direct relation to how much air is supplied from the AHU. Any thoughts or ideas would be greatly appreciated. Thanks for your time.
    Last edited by Poodle Head Mikey; 06-16-2013 at 09:11 PM. Reason: it originally seemed less understandable than I prefer
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  3. #3
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    65^F, 50%RH is 46^F dew point air. The cooling coil needs to be <41^F or colder.
    The air leaving the AH needs to be colder. Colder coil and slower air flow.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
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    Quote Originally Posted by teddy bear View Post
    65^F, 50%RH is 46^F dew point air. The cooling coil needs to be <41^F or colder.
    The air leaving the AH needs to be colder. Colder coil and slower air flow.
    Regards TB
    Yup. He'll need colder chilled water to accomplish this. Right now the system is doing very little dehumidifying.

    Most of the rooms are set between 62 and 65 degrees. I noticed if I turn the thermosat up, the humidity drops. It's in direct relation to how much air is supplied from the AHU.
    Two things could be happening here. First, raising the temperature in the rooms with everything else remaining the same decreases the relative humidity. The actual moisture content in the air remains the same. Second thing is if the amount of air going through the unit decreases then the coil may be extracting more humidity. I'm guessing it's more likely the first. Warmer air holds more moisture so the relative humidity reduces.
    Plot both points on a psychrometric chart (or tell us the temps and %RH so we can do it), if the line created by connecting the dots goes down as it gets warmer then the coil is taking out more humidity. If the line stays horizontal then it's merely a change in the relative humidity and the same amount of moisture is in the air.

  5. #5
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    If the Docs truly want 62F in the space, the ONLY way to achieve that and still keep humidity below 60%RH, is to keep the dewpoint of the incoming air <47F. IF chilled water is your only method of dehumidification, then that means a very large chilled water coil to get as low an approach as possible. This is the reason many hospitals throughout the humid south have started using dessicant dryer systems to dehumidify for the OR's.

    Most likely, your OR's were designed for 68-73F, and can control humidity fine within that band, however, with the doctors now wearing much more PPE than in the past, many want the space a lot colder (62-65, as you are hearing). This is not going to be possible with your equipment that was sized for 68-73.

    If you cannot up the chilled water coil size, the other option to 'virtually' raise the coil size is to pre-cool the incoming air. An additional coil on the inlet of the air handler, even if it is fed with the 53F return chilled water, is going to make the effective coil larger, and allow a closer approach to chilled water supply temp. You can also, as previously mentioned, lower chilled water supply temp, but that ends up needing the entire site to run with colder temp, just for a few problem areas - dropping overall efficiency considerably.

  6. #6
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    You may be able to retrofit the chilled water coil with a wraparound heatpipe. It does like CraziFuzzy suggested and precools the incoming air to further drive out humidity then wraps around to the outgoing side of the chilled water coil to reheat the leaving air. It is self contained refrigerant based and works off phase change. The incoming air gives up its heat to the refrigerant changing it to a vapor, the vapor moves via cross connected piping to the leaving coil side where it gets cooled (reheats the leaving air) and changes back to liquid. No electric, controls, or additional external piping required. But you do need coil space in the unit.

    http://www.heatpipe.com/HomePage/abo...%28DHP%29.html
    Last edited by vangoghsear; 06-18-2013 at 03:13 PM. Reason: Left out a detail

  7. #7
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    Quote Originally Posted by vangoghsear View Post
    You may be able to retrofit the chilled water coil with a wraparound heatpipe. It does like CraziFuzzy suggested and precools the incoming air to further drive out humidity then wraps around to the outgoing side of the chilled water coil to reheat the leaving air. It is self contained refrigerant based and works off phase change. The incoming air gives up its heat to the refrigerant changing it to a vapor, the vapor moves via cross connected piping to the leaving coil side where it gets cooled (reheats the leaving air) and changes back to liquid. No electric, controls, or additional external piping required. But you do need coil space in the unit.

    http://www.heatpipe.com/HomePage/abo...%28DHP%29.html
    This system works well if you have a need for humidity removal, without the subsequent need for heat removal. depends on the climate, and the incoming air conditions.

  8. #8
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    Quote Originally Posted by CraziFuzzy View Post
    This system works well if you have a need for humidity removal, without the subsequent need for heat removal. depends on the climate, and the incoming air conditions.
    That's a good point. I didn't notice mixed air temps in his post.

    He did state that they were doing reheat through the VAV boxes, but I couldn't see the need for any at the discharge air conditions he mentioned. If the approach was lowered then it may be needed.

  9. #9
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    Quote Originally Posted by vangoghsear View Post
    That's a good point. I didn't notice mixed air temps in his post.

    He did state that they were doing reheat through the VAV boxes, but I couldn't see the need for any at the discharge air conditions he mentioned. If the approach was lowered then it may be needed.
    A modern operating room will almost always be running in reheat mode, unless outside air is incredibly dry. This is because desired room conditions are in the lower to mid sixties, with dewpoints in the high 40's/low 50's. So you are right, a heat recovering dehumidifier does work well in most cases. The extremely high air change in the space means that in most cases, discharge air into the space is going to be within a degree or two of space temp. The lowest discharge my or's usually see is 60F, but I need to run the air handlers 47-49F to maintain the humidity control during humid seasons. We do not use the heat recovering dehumid's here, though, because we are only dehumidifying about 2-3 months total of the year, so the added equipment doesn't really pay for itself, and instead, we just use REALLY big chilled water coils.

    You CAN, believe it or not, used chilled water return for the reheating as well, if that is something you need, and it is a simpler proposition. After your chilled water coil, you run through another coil, that is taking the return from the main chilled water coil, with a 3-way valve to control how much reheat to use. How much reheat you use can be controlled by zone demand. Ideally, you'd want a couple of the reheat valves closed, so that the air handler is putting out as warm air as possible for the zone that needs the least cooling, and then only using hot water for the other zones. This, to be effective, still requires rather large cooling coils, so that you can run a pretty high CHWR temp out of the coil, to maximize the amount of heat you recover into the air stream. As i mentioned, though, I don't have a lot of direct experience in my climate with more advanced humidity removal methods, just aware of the thermodynamics behind the various methods.

  10. #10
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    I like the thought of using chilled water return as reheat. It removes a bit of the heat from the return so you get some recovery, but still allows for cooling. Cool idea.

    I've used CHWR for units that were just tempering air before, using a pair of T's and a decoupler off the return with a circulator on the circuit, but I've never tried it for reheat.

  11. #11
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    Another note to the OP. Any coils added will have some impact on the external static pressure the fan is handling and could result in reduced airflow rates. Being as this is a multistation unit, I doubt it will have a great effect on airflow rates, but you should be aware of the possibility.

  12. #12
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    The problem is that chillers are unable to provide cooling near freezing like a DX coil can. Chilled water warms 1^F per 1^F btus of cooling provided. DX is able to do the majority of their cooling at the refrigerants evaporating temp. You have a coil that is 33^F providing 35^F dew point air exiting the coil.
    Dessicants are energy intensive compared to refrigerant DX coils with heat exchangers by a factor of 2-4.
    Units like the Ultra-Aire SD12 use a plate heat exchanger that uses the 35^F cold air through a plate heat exchange to precool the incoming air before entering the cooling coil. The results are 60^F, 50%RH, 41^F dew point return air entering the plate heat exchanger counter flowing cold air from the evaporater.
    The cold air from the cooling coil precools the return air before the return enters the cooling coil. The results are that a ton of cooling removes 2-3Xs more moisture straight DX cooling.
    Most of the Ultra-Aire dehumidifiers use this technology to remove more moisture per KW than another dehumidifying technology. This is slightly more efficient than heat pipe technology.
    This is viable technology for +36^F dew points.
    Off the shelf units are 6-8 lbs. of dehumidification. Large custom units are possible.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
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    Quote Originally Posted by teddy bear View Post
    The problem is that chillers are unable to provide cooling near freezing like a DX coil can. Chilled water warms 1^F per 1^F btus of cooling provided. DX is able to do the majority of their cooling at the refrigerants evaporating temp. You have a coil that is 33^F providing 35^F dew point air exiting the coil.
    Dessicants are energy intensive compared to refrigerant DX coils with heat exchangers by a factor of 2-4.
    Units like the Ultra-Aire SD12 use a plate heat exchanger that uses the 35^F cold air through a plate heat exchange to precool the incoming air before entering the cooling coil. The results are 60^F, 50%RH, 41^F dew point return air entering the plate heat exchanger counter flowing cold air from the evaporater.
    The cold air from the cooling coil precools the return air before the return enters the cooling coil. The results are that a ton of cooling removes 2-3Xs more moisture straight DX cooling.
    Most of the Ultra-Aire dehumidifiers use this technology to remove more moisture per KW than another dehumidifying technology. This is slightly more efficient than heat pipe technology.
    This is viable technology for +36^F dew points.
    Off the shelf units are 6-8 lbs. of dehumidification. Large custom units are possible.
    Regards TB
    The thing is, you don't NEED to remove that much moisture from teh air in an operating room environment. Those units are great for whole house dehumidification, because they can remove a set amount of moisture with a smaller amount of air flow, so you NEED to ultra low dewpoint. An operating room, on the other hand, running 20+ air changes an hour, and in most newer hospitals, using 100% outside air, is already moving so much air, that you only have to drop that air down to 45-47 dew point minimum, so chilled water is still a very viable tech for it.

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