That’s Humidifying

A typical humidification need in the HVAC discipline may require the addition of moisture to maintain a space condition of perhaps 72° and 50% relative humidity when some 2,000 CFM of dry, winter, outdoor air is added to the space.

This results in the need to add some 65 pounds of water per hour using 22 kilowatts of electric power to convert the make up water to steam. The use of city water is acceptable when proper blow down techniques are applied.

A duct handling the 2,000 CFM may have a height of 24″ in which 7 steam distribution tubes on 3″ centers can be installed. This 65 pounds of steam per hour can readily be delivered through the 7 distribution tubes. The absorption into the air stream is rapid and excellent results are obtained.

Now take this same example and apply it to a food processing oven that requires the same 2,000 CFM of cold, dry, outdoor air delivered at 50% relative humidity but the temperature must be 200° and the water must be absolutely pure.

The project now requires some 2,400 pounds per hour of reverse osmosis water delivered to the same 2,000 CFM in the same 24″ high duct. The reverse osmosis system must treat 5 GPM of city water by passing it through a softener, then through odor removing charcoal and finally through several pads of osmosis filtration. The inlet air must be filtered through high efficiency particulate filters.

The pure water is now ready to have its state changed from water to steam, requiring some 800 kilowatts of energy. Such a high level of electrical power is too costly to purchase and is unacceptable to management. Where electrical steam generation was fine for the HVAC application, natural gas becomes the energy choice to make.

The high steam output requirement dictates the use of gas fired units supplying the steam to forty two distribution tubes still in the 24″ duct height and 3″ on center constraints. The forty two tubes would be in six banks of seven tubes each with the banks in series air flow.

An application of this sort requires an ability to respond to the changes of the year round seasons and constant weather front activity. In the HVAC electrical system, the use of SCR control allowed for 100% modulation of the steam generation and full response to changes in load. In the oven process system, the convenience of full SCR modulation is lost and gas turn down ratios become the control method of choice. A 3 to 1 turn down ratio in a single unit is rather insensitive while the 18 to 1 ratio available when three double burner units are used is far more desirable.

The control concern that becomes apparent occurs when the gas fired units have stepped down to the lowest level (150 lbs/hr) and cycle off. It is at this point that a 150 lb/hr humidifier cycles on and is modulated between 150 lbs/hr and zero pounds per hour using the silenium control rectifier. This method allows for predictable control through the wide range from 2,400 pounds/hr to zero pounds per hour.

The need for many distribution tubes in a 24″ duct height requires the use of several rows of seven tubes each in air flow series. Each row of seven can be fed from one generator and excellent response to load change can be obtained.

The fact that preheating of the inlet outdoor air to 200° and humidity sensing at the 200° and 50% level is required must not be ignored.

Obviously this process system requirement is a far cry from the simple HVAC humidification system and concern for the details is most important.

Engineering Bulletin -Volume 2, Issue #4
by: Kenneth W. Wicks, M.E. – ASHRAE Fellow


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