Humidity with Mini Split

Do you know how to use psychrometric charts, Mildred?

If you want the room to be drier, you need to use something else in addition to something that uses refrigeration only. So what else do you plan on using apart from your existing dehumidifiers? Do tell!

A well installed split system circulates the air in the room indeed. But also a lot of moisture will be trapped in the evaporator.

If you let this water drain in the evaporator pan and drain it out of the room, all the moisture drained off is not in the room anymore.

The lower you set the cooling temperature, the more liquids you will produce.

Your dehumidifiers work about similar, if they have a drain out of the room. The difference is that in these units, the produced heat in the condenser (sometimes a combined heat exchanger with the evaporator) stays in the room and can make the room warmer due to all losses that produce heat.

A real split unit (small one) has no fresh air (from the outside) inlet.

A through wall or window unit has one.

The condensate need to be exhausted outside the room to make it work good.

Define, "humidity as low as possible".

Why would you need lower than 30% RH? Iron and steel WILL NOT rust at this level.

Anything lower would probably not be healthy.

Bigg - you are correct. I was simply going off of the humidity reading on the thermostat. I turn the dehumidifiers on and the moisture collects in a holding tank. Then I turn the mini split on. The condensate drain does go outside by the condenser. I do remove substantial amounts of moisture because the holding tanks on all three dehumidifiers will fill up. This is a room that I have set up for tinkering and playing with different refrigerant oil. Because the new synthetic oil's are so hygroscopic I need the humidity as low as possible.

One of the biggest challenges you will face is moisture migration. The moisture "moves" due to pressure differential between the gasses. Going back to the earlier mention of psych charts- they contain the "pressure" of the moist air mix along with a lot of other data. It is VERY hard to keep a normal room "dry".

Using your dehumidifiers with the pan tapped so the moisture will drain outside the room along with the split system with its separate drain will help but- the hygroscopic oils will act as a desiccant and scavenge whatever moisture is available.

If this room has any true "value"- and your post indicates that is does- I would suggest using a product from a company called Kathabar which uses a liquid desiccant that is heated or cooled and passes through a packed fill- much like a cooling tower- where the air also goes through. This system can produce air as cold as -60F or as hot as 200F with the RH of the supply air between 20% and 90%- controlled by the concentration of the desiccant. For a very dry room, set it up to supply cool (40-50F) air at 20% then add heat to the air to maintain room temp. Their engineers can help you size the system- they are available pretty small to really big- to meet your goals. You will also need to supply heat and power for the desiccant regenerator system.

When you are occupying the room, I suggest using a breathing "mask" connected to a "large" flexible hose connected to the exterior of the room to protect YOU from the dry air (and keep your body moisture from adding to the room's moisture load). Also- either use goggles or periodically apply eye drops to protect your eyes.

Maybe desiccant wheels could help. As for refrigeration schemes, go for low evaporator temperature (which will need periodic defrosting) and reheat. Most "mini-splits" may not offer that, but AHUs (air handling units) often do.

With the advent of fiber optics for communication, A large manufacturer of service tents for telephone line repair asked if I would run tests on a portable air conditioner/tent arraignment to see if the humidity could be reduced to the level required for fiber optic splicing. A sling psychrometer showed it was possible, but took too long. The phone companies went on to adopt the trailer type units, when methods for reducing or isolating generator vibration were developed.

A slightly smaller a/c unit would have worked better....If the a/c unit is too large for a space it cools the air quickly and doesn't stay on long enough to drop the humidity to the appropriate comfortable level....You can reduce this effect by slowing the evaporator air flow, by either lowering the fan speed to low or restricting the air flow with a restrictive filter to somewhat mitigate the effect....to further lower the humidity run the dehumidifiers but either empty the water often or drain it to the outside....below 30% RH will dry out your soft membranes, so hydrate often...some air handlers have a delayed fan switch that will continue to run for 30 to 45 seconds or so, after the compressor has shut off, if you have this in yours you can switch it to a direct acting type of fan relay that shuts the fan off with the condenser unit...this will eliminate some of the moisture from being evaporated off the coil back into the room....seal any air leaks around the windows and doors and electrical outlets....

If you want to drop the relative humidity without heating the air, look into using desiccant which will bring the RH down to 8% or under if the room is tight. It will take a number of pounds to get you there but we do that in 60-80 cu ft chambers regularly. You can buy on-line from ULine and find it in bulk for your purposes, not small packages. We buy in 50# bags.

the obvious: The hot air being expelled is being replaced by humid air from under the door. You need some throw cats.

BTW 100 F is definitely not hotter than hell. Hell will essentially vaporize rock you are familiar with. Hell is somewhere in Iraq, or maybe Saudi Arabia.

Are you planning on occupying said low humidity room? Are you wanting to dehydrate food goods? Store you mother (mother-in-law) while preventing that ugly fungus?

You failed to mention celing height, and total make-up of construction. Did you know that your walls are not air-tight, unless you are on the International Spacestation.

Even if this room were made out of conrete, will rubber seals on the door, concrete still breathes, albeit not much. I know that for example a concrete bunker is useless as a de-aerating water storage tank.

Without knowing room volume and air-exchange rate, we are all just flapping our wings here with no chance of flying.

Mr Stewart got the question before I did. I was going to ask if the room was like the medical examination room on the Andromeda Strain or some type of home built mini shed ?

A few people skirted the issue of how the room is constructed, and of course what the ambient conditions outside the room are. What may be missing is an effective vapor barrier, without an effective one the outside moisture will most certainly migrate in.

Worst yet, if the interior walls/floor are a hygroscopic material like plywood, concrete, sheetrock, etc. there will always be a source of moisture unless there is something impenetrable like well sealed FRP panels covering everything. A lot of this moisture ingression was masked/stopped by the excess heat from the dehumidififers which may have even pushed the moisture back to the ambient.

The room is a 15'x15' sheetrock room that sits inside of a climate controlled 3000 sq ft warehouse. The duct work has been removed and there is ceiling tiles with rolled insulation on top of the entire warehouse with 10' ceilings. The space above the ceiling is 20' and it spaces the entire 10,000 sq ft warehouse.

I am not looking to make it a complete clean room for the CDC. But just trying to lower the humidity as much as financially possible with some slight redneck engineering.

The question that baffeled me was that with 3 dehumidifiers running, my thermostat would show non detect to 3% humidity on the screen. But the temp would rise to over 100F. So I thought by adding a Mitsubishi Mini-Split with a humidity control function, I would not only remove the heat from the room, but add some extra humidity removal. But when I run the 3 units and get the humidity to non detect, then turn on the mini-split, my humidity readings go from 0%-3% all the way back up to 50%. The room does cool and the mini split stays running because the set point is turned to as low as possible.

I have experimented with desiccant bags for humidity removal, but they are designed for low temp freezers. When the temp rises above 85F they begin to off gas. I did purposely oversize the unit. It is an 18,000 btu, but because of the heat load, it seems that it just barely maintains the room temp at 78F.

I am absorbing all the replies and I take no offense by the comments. Because this was a piece by piece addition. The statement made about Relative vs. Absolute humidity is the area where I think I missed in my "Assumptions".

It is an entirely true statement that a room at 100 F with 3% R.H. will exhibit a much higher humidity (assumption no transfer of air or water vapor in or out, i.e. a closed environment) as the room is cooled to a temperature near the dew point. I think you should review the Mollier diagrams, and psychrometric charts and try again. Here is a link for the latter charts:

http://www.truetex.com/psychrometric_chart.htm

Also, try this: www.coolerado.com for a large energy savings in cooling without adding moisture content to air. You might even get your idea to work if you do this: (1) test for air exchange between the room and the warehouse area, and eliminate free and open exchange of air.

(2) install an air-lock door system (consisting of double doors with extra cold space between the doors).

(3) use primary cooling/dehumidification (Coolerado perhaps followed by VCC to sub-cool the air), then really sub-cool the air with another VCC refrigerator system and use the condensate removal combined with dessicants, then re-heat the air to the proper temperature needed in the dry room, by use of another Coolerado (working air and product air reversed to heat the room air). This should give you air so dry, there will not be any tears. (And your mother-in-law will no longer grow fungi).

Most of the above suggestions deal with removing the moisture from the air without consideration of how it continues to get into the room. The envelope (walls, ceiling, and floor) materials of construction, fit, and finish can be a major source of continued moisture ingress into the conditioned space.

Basically it's a partial/vapor pressure problem, as the conditioned space gets cooler and drier the vapor pressure difference between the ambient and interior conditions takes on a steeper gradient, literally forcing the ambient moisture to find its way into the space. Even though your room is contained in a climate controlled space (whose conditions you didn't specify), it's a safe bet that the ambient is warmer and moister; i.e., has a higher vapor pressure, than your cooler and drier room. Thus water vapor will find its way through the drywall itself, any poorly made-up seams, and especially the (unsealed, porous) ceiling tiles/insulation combination.

You have approximately 1,050 ft² of surface area including the floor (concrete releases moisture too). A surface area that size has a permeance of about 1/7 of a pound of water per hour per inch of Hg Vapor (not barometric) pressure differential, so in 24 hours about 3.5 lbs of water gets forced into your room, all multiplied by the Vapor Pressure differential in inches.

Just in case anyone is interested, and in the interest of completeness (perhaps for future reference) I am posting this link to The Dehumidification Handbook put out by the Munters Corporation.

Thanks Bigg. This Dehumidification Handbook will really help me. I have a free weekend all to myself at the deer lease and I have feeling I am going to be reading this the whole time.

Hey buddy: If you are headed to the deer lease, the last thing you should do is spend the whole weekend reading/working. Rest, sleep, play, but do give your mind a change of pace, and a break. I only work on my own projects and/or lawn maintenance on the weekends, to let the mind de-pressurize. Otherwise, the internal cranial pressure may cause a rift in the family politic (if that is an issue in your case).

Also, most folks need to take a real vacation when they take one, and totally de-phase the mind from concerns at primary job. Spend time during vacation on what makes you happy, even give scientific talks, but on a topic not related to normal business routine. You will find a change of scenery, and a change of mental focus very refreshing, like the desert zephyr in the morning.