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Estimating humidifier evaporative capacities

  1. Nov 17, 2008 #1
    I'm an HVAC technician trying to assemble a spreadsheet with formulas for different furnace humidifiers to analyze the outputs of different models in different scenarios but the factors are horrendous. All would be of the same basic type "flow through", bypass air models. - Water flows through a solenoid valve and enters the top of a "water panel" at the rate of 3 gallons per hour. The water panel is always a honeycomb shaped device with a "wetting agent" coating over it to make it slightly absorbent. About 120 CFM of hot air (120F) is forced through the cascading water going down the water panel. The water that trickles down the water panel via gravity is NOT recirculated. The water that doesn't evaporate merely goes down the unit’s drain located under the water panel. This lost volume of water typically amounts to between 66 & 80% of the inlet flow rate (of typically 3 GPM) depending on the air temps, water temps, flow rate of both the water and the air, and obviously also on the RH in the house. So even with the best-case scenario, only 1 of every 3 gallons of water consumed ends up the air.

    Unfortunately, the manufacturer's specs don't allow you to compare apples to apples, because they rate them at different conditions, ESPECIALLY the water temps (from 55F to 140F) and they often leave out required factors. So we often get caught with our pants down when we try to properly size these for given application or try to address customer’s capacity complaints for ones we didn’t install.

    What formulas would you recommend for creating a spreadsheet to estimate the evaporative capacities with the variables being:

    1. RH level & temp in the house
    2. Cubic inches of size of the water panels
    3. Inlet water temperatures
    4. Inlet water flow rate in gallons per hour
    5. Temps of the air blowing over the water panel

    The altitude would always be fixed for this area, i.e. at sea-level.
  2. jcsd
  3. Nov 17, 2008 #2


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    Staff: Mentor

    Formulas are tough for this. How strong are you with psychrometrics? Basically, you model one of these humidifiers as functioning like a cooling tower and you figure 50-75% efficiency at achieving saturation. But how you find out if it is 50% or 75% would be pretty tough.

    I think, though, what probably causes you to get caught with your pants down is the difficulty in estimating how "tight" the house is that you install it in. I've dealt with some old buildings converted to luxury apartments and they are rediculously loose. Tenants want humidifiers, but the humidity just goes straight out through the 80 year old window frames.

    Best advice: figure out what size you think you need....then pick the next bigger one.
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