Museum ASHRAE class A ? 2

[Zesti]

We'are in Europe and are not very familiar with ASHRAE guidelines.

ASHRAE Class A for a museum specifies limits for temperature and relative humidity.

Question is: does it also specify maximum hourly changes for T & RH ?

 

[Shophound]

I'm the chief engineer of an art museum in the USA. I work closely with the conservationists and collection management staff to provide an effective preservation environment for our collection, which is widely varied between oil and acrylic paintings, water colors, photography, sculpture, works on paper, rare books, etc.

Conservationists like the preservation environment to be STABLE. Minimal fluctuations in temperature and humidity, and if you must choose between which parameter to hold more stable over the other, humidity stability wins every time.

Since you are in Europe, you should become familiar with the writings of Tim Padfield. I believe his website is

http://www.conservationphysics.org

. He understands the European climate very well, whereas I deal with a museum in a hot, humid climate, which requires a more intense level of sensible and latent heat control than perhaps a European museum might encounter.

 

[Drazen]

Interesting.

If I may step in, I would ask which control logic you prefer to use in museum, and what is approximate SHR at your place.

Is it multi-zone VAV system that you are dealing with?

 

[Shophound]

Drazen,

I will assume your question was directed at me, wherein I hope my answers benefit Zesti as well.

As for control strategy, early next year we're going to convert one of our air handlers to dew point control for humidification/dehumidification. Currently, all of them control humidity via return air relative humidity sensors. If the return RH becomes too high, the controller overrides the discharge air temperature setpoint and drives the chilled water valve open via its own PID in order to lower humidity. If the return RH is too low, a steam to steam humidifier kicks in and adds purified steam to the discharge air.

This works acceptably well a good part of the time, but an ongoing problem is when return air dry bulb temperatures are considerably higher or lower than the desired setpoint in the conditioned spaces. This can result in the system working too hard to remove moisture in summer, or add too much moisture in winter. It can also lead to unacceptable swings in humidity levels in the art spaces, which as I said in my last post, the more stable the moisture levels are maintained, the better the conservators (and the collection) like it. End result is too much energy used to deliver inadequate climate control.

Most of our air handlers are VFD feeding VAV boxes, with the majority of the VAV boxes either series or parallel fan powered with either electric or hydronic reheat. A few boxes are cooling only, depending on heat sources in the space for reheat (harder to do these days with lighting and computer monitors putting out much less heat than in bygone days). One air handler is constant speed multi-zone and another is VFD hot deck/cold deck (dual duct) feeding dual duct VAV boxes. This is the air handler we are replacing, going with fan wall design, UV lighting on the cooling coil, and dew point control.

As for SHR, I imagine in real time that varies with the load. A great deal of the sensible load in our building is from lighting, particularly in gallery spaces. I can hear my chiller load up in the morning when the lights come on in the galleries. They put out a lot of heat. Someday we hope to convert them to LED; that technology is edging closer to gallery level quality. Latent loads will vary with outdoor dew point levels and building occupancy loads. We use minimal outside air for ventilation to reduce latent loads and introduction of outdoor pollutants to the conditioned air. Our outdoor summer design is 100°F dry bulb with a 74°F coincident wet bulb, which in most cases yields a SHR of .90 +/- . Since the latent load can swing with occupancy and outdoor dew point, the control strategy mentioned above is designed to compensate in order to minimze humidity swings in the art spaces.

 

[Drazen]

thanks for comprehensive answer and i also hope zesti will find it interesting as well.

Once i was involved in consulting phase museum design concept, and there were some discussion which are pretty aligned with what could be summarized from your post: while VAV could be thought of as must-be solution for space with variable and unpredictable occupancy, there is serious issue with humidity control which comes in conflict with dry bulb control in many scenarios, and if we accept basic principle, which you also clearly mentioned, that humidity control is single most important parameter to deal with, than different solutions comes in mind.

i had no budget to develop that project to the end, but what remained in may mind that some solutions with fixed speed mixed air AHU may prove better for museum environment.

as regards to shr, i was mostly thinking about design shr. in reality, shr varies all the time, but designer must have had some referent point for equipment sizing, which will affect year-round operation.

with mentioned dry bulb and wet bulb temps, i can only assume that you are in texas, nevada or similar place

 

[Shophound]

Texas. Good guess. Nevada? Highest coincident wet bulb for that state I've found is 65°F at 106°F, so I'm not there.

Sadly, budget constraints are often the case when museums are built or remodeled. My museum seems to be an exception; we've learned that "value engineering" does not always yield great value if we end up spending more money down the road to do what should have been absorbed as first cost.

Also, I can't stress enough how important proper and and accurate commissioning of the controls and components are after installation. Good maintenance closely follows.

 

[DRWeig]

Throw in carbon-dioxide based demand controlled ventilation with your humidity and temperature loops, and it can become quite complex. It can be successful, though -- we've done it in hospital settings to meet the needs of some cutting-edge medical equipment that are sensitive to humidity levels. The rooms in question were designed for hands-on teaching so the occupant load can vary all over the place (thus affecting load, shr, and ventilation needs all at once).

Good on ya,

Goober Dave

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[Shophound]

With the dew point control proposed for our new air handler, the cooling coil will be split, with more than one chilled water valve. One coil dedicated to sensible cooling and the other for dehu via dew point. If the supply air has the correct dew point, the spaces served by this air will have the correct RH once the air is reheated via the ambient heat gain of the spaces. That's the theory, anyway.

 

[Zesti]

Thanks Shophound and others, a lot of intereseting stuff!

The basic design for the museum we are working on as a HVAC-contractor is made by a consultancy firm. Their design contains some aspects that we think are questionable.

We were also thinking about control strategies and locations/types of the sensors, something that is also mentioned here.

Shophound, do you use "special" RV-sensors or just the "regular" ones from just any control system vendor?

Do you use multiple sensors per room and average their values?

 

[DRWeig]

I'd like to hear Shophound's experience with RH sensors too.

In museums, they need to be very unobtrusive if not completely hidden. Often, that leads to a flush-mount installation that can really mess with both sampling and accuracy.

A recent project I had to analyze had humidity transmitters installed flush, with only the sensing element sticking out front of a wall plate a few millimeters. The electrical box in which they were mounted was surrounded on all sides by insulation.

What a lot of folks don't take into account is that a 4-20 mA transmitter dissipates an appreciable amount of power. The most common sensors do 4-20 mA over 0-100 %RH and are powered with 24 VDC supply. So at 50% RH (12 mA), if the controller has a 250Ω input impedance, the voltage drop across the transmitter electronics must be 24 - (0.012 x 250) or 21 VDC. So the transmitter is dissipating 21V x 0.012A or 0.25W. That's a lot of heat inside a 2" x 2" x 4" insulated box. Temperature rise in the box was measured at 6° to 8°F, and was enough to offset the RH sensing element's own temprature sensor by about half that amount. With an elevated (false) temperature measurement, the sensor reports an artificially lower (inaccurate) RH.

Our solution was to separate the loop electronics from the sensor, put them in separate boxes with insulation between. Worked much better.

This probably happens often but is not often noticed, in my opinion. The only reason it came to light for me to study was the particular museum project. It had temporary loan of some Egyptian antiquities, and the Egyptian authorities came periodically to measure temperature and humidity around their treasures. They had very expensive, tightly calibrated instruments that showed the error in the wall sensor measurements.

We had other issues with air circulation to the sensors since they were so tight against the walls as well, but the museum staff came up with a number of creative and very effective solutions for those too.

I decided to throw all this in here to see if anyone else ever ran into such? Self-heating of humidity transmitters doesn't get a lot of discussion...

Good on ya,

Goober Dave

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[Shophound]

Our humidity sensors are in the return air stream, prior to this air stream mixing with any air from outdoors. They consist of a thick probe in the air stream, with the electronics mounted in a box outside the return ducting. Johnson Controls is the manufacturer.

The control strategy is to maintain a 55 degree F discharge air temp (DAT). Should return RH rise above the maximum setpoint, the controller overrides the DAT and controls the chilled water valve via return air RH. As I said above this works acceptably well a fair amount of the time. Where it starts to go south is when the return air temperature is inflated or depressed notably away from the conditioned space setpoint. This depresses or raises the RH level sensed in the return air stream, even if the absolute moisture content is the same (and is at or near target)as in the conditioned space. Since our building uses open (plenum) returns everywhere, heat gain or loss is easy to understand.

We do not have RH sensors in the galleries, largely because of the aesthetic concerns mentioned DRWeig above. The temperature sensors are button sensors either high on the wall or in the ceiling tile. We do have RH sensors in art storage areas, but do not use them to control RH. Only so we can monitor the space with the BMS.

 

[DRWeig]

Thanks Shophound!

Good on ya,

Goober Dave

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