Can you achieve/maintain 55F at 90%RH?

Question

I am trying to build a vegetable/fruit cooler/humidifier that will keep specific food groups at 55-65F while being 85-95%RH. I can acheive either but not both simultaneosuly. Using Thermoelectric AC and handmade humidifier.

Answer 1

At 55F, and 90% humidity, you are above the dew point, you would need a supersaturated system

See this graph: http://en.wikipedia.org/wiki/Image:Dewpo...

Answer 2

So, I take it that you are ending up too low in RH?

If you were to take a sealed volume of air, and cool it SLOWLY, the RH should actually increase.  According to a psychrometric chart (which shows how these things work), if you started at room temp, around 70F, at 50% RH, then you would end up with around 85% at 55F.  

But, what is probably happening, is that your TEC cooler is running much colder than air temp.  The TEC needs to stay above dewpoint (wet bulb temp), or water will condense out of the air in your cooler onto the TEC, especially if you're pumping water in using a humidifer.

At 95% humidity, the coldest part of your TE cooler can only be about 1 degree below air temp -- so as you cool down it will take more and more time to cool without condensation.

A few thoughts ... first, use a thermal insulator to create a sort of seperate air volume that seperates the air holding the fruit from any cold walls of your TEC.  This obviously will make it take longer to cool, but you could have some sort of valve that opens to allow the two volumes to mix, then closes when you just want to maintain humidity.  You would then pump your humidified air directly into this closed volume -- so that the moist air cannot come into contact with surfaces below the dew point.  (That's how the fruit section in your refridgerator works -- you keep the fruit somewhat sealed seperate from colder walls of fridge)

If you're doing bang-bang thermostat control on your cooler, that also might not work ... you might want to consider a simple proportional control, or PID type controller.  Finally, you didn't mention how your humidifier works, but if it involves heating the water at all, then that won't work.  The water and air involved should be cooled to the same temp as inside your quasi-sealed volume.  As someone else mentioned, a water mist might be what you're after.  

But again, if you can avoid drying the air in the first place, you should need very little humidity at all -- ambient air contains almost all the water you need, as long as you don't lose water to condensation.

Answer 3

Yes, but possibly not maintain it as precisely as you would like with the devices you are using. As saturated air (100%rh) at 55f increases in temperature the amount of moisture it can hold will also increase thus decreasing the rh%. You may want to approach this backwards and use a cooled water mist as your medium.