Humidification of air is an operation concerned with an interphase transfer of mass and energy that occurs when air is brought into contact with water in which the air is essentially insoluble. Depending on whether the water is in a form of a liquid or a vapor, there are two air humidification processes: a) An adiabatic process, in which the treated air is brought into a direct contact with water and the required evaporation heat is extracted from the air that is being humidified, and b) An isothermic process, in which a water vapor at atmospheric pressure is added to the treated air to increase its moisture, in which the heat energy of the treated air is unaffected.
The isothermic humidification process is usually carried out in a central air conditioning air duct system or in an open space, by distributing and mixing of a stream of atmospheric steam with a stream of air. The amount of steam that can be added to a stream of air is limited and depends on the dry bulb temperature and the absolute moisture content of the treated air. The steam for humidifying the air may be produced either at the location of the steam distributor in a compact humidifier, or it can be delivered to the steam distributor from a central steam boiler.
Technical and commercial literature indicate, that the current art compact isothermic humidifiers are producing the atmospheric steam in a sealed water tank by boiling and evaporating the incoming feed water at atmospheric pressure in a single stage evaporation process. The required heat is provided either by electric power via two or more electrodes or electric resistance heating elements submerged in the boiling water, or by a pressure steam delivered from a central steam boiler in a submerged heat exchanger.
An electric steam generating humidifier is disclosed in U.S. Pat. No. 4,239,956 issued Dec. 16, 1980 and reissued Oct. 30, 1990 as RE 33,414. Referred to in this patent are U.S. Pat. No. 3,386,659 and U.S. Pat. No. 3,436,967 as disclosing a steam generator in combination with duct work of forced air heating system.
Disclosed in U.S. Pat. No. 4,564,746, issued Jan. 14, 1986 to B. W. Morton et. Al, is a cabinet type steam generating room humidifier.
The feed water used in compact isothermic humidifiers may be a city water, softened water, deionized water (DI) or reverse osmosis treated water (RO). Regardless of the feed water quality, all compact isothermic humidifiers are provided with a method to control the flow of the feed water into the water tank, a method to control the volume and the water level in the water tank, and a method to control the operating pressure in the water tank.
As the feed water is converted into steam, impurities which enter with feed water are concentrated in the water tank. Of concern are mainly the inorganic compounds of hard scale forming substances such as calcium and magnesium. Each substance has its own solubility limit in water solution. When its concentration exceeds the solubility limit, the excess substance precipitates and builds up a hard scale on the submerged electric resistance heating elements, electrodes, heat exchanger, and the water tank walls. The build up reduces the overall heat transfer rate. To maintain the performance and the efficiency of the humidifier, the water tank, the submerged heating elements and the heat exchanger are regularly cleaned, and the water tanks provided with the electrodes are regularly replaced at a considerable maintenance and replacement material cost.
To extend the operating period of the water tank, all isothermic humidifiers operating with feed water containing dissolved solids (DS) are provided with a method to control the concentration of total solids TS in the boiling water to reduce the hard scale build up rate. Most of them control the TS in the boiling water by a regular periodic blowdown of a mixture of feed water and boiling water which results in excessive consumption of feed water and with excessive heat loss with the blowdown water.
The steam generation process used in current art compact isothermic humidifiers involves steps including transfer of heat from the heat source into the water to cause it to boil, evaporation of the water, concentration of TS in the boiling water, separation of the produced steam from the boiling water, discharge of the produced steam from the water tank, replacement of the evaporated water with feed water, blowdown of boiling water to maintain an acceptable TS concentration in boiling water, and replacement of the blowdown water with feed water. All these process steps occur in a water tank at atmospheric pressure.
In current art compact isothermic humidifiers operating with feed water containing DS the flow of the feed water into the water tank is periodic and the steam generation process is cyclic and includes two operating periods.
The first operating period involves steps including transfer of heat from the heat source into the boiling water, evaporation of the boiling water, precipitation of DS and concentration of TS in the boiling water, separation of the produced steam from the boiling water, and discharge of the produced steam from the water tank.
The following second operating period, in addition to the above steps, involves process steps including filling up of the water tank with make up water, draining of a small portion of the boiling water, and heating of the feed water up to its boiling temperature. During the second operating period the concentration of DS and TS in the boiling water drops in proportion to the flow of the feed water. After a number of steam cycles the water tank is drained and flushed to minimize build up of the hard scale.
Because of the periodic intake of cool feed water into the water tank and use of a portion of the process heat in heating up of the feed water to boiling temperature, the rate of production of steam during the second operating period is reduced. Depending on the flow rate of feed water, temperature of the water in the water tank may fall below the boiling point. Under such conditions the steam is not produced and the treated air stream is not humidified.
All current art isothermic humidifiers use process controls to keep the process equipment and the integrated humidification and steam generation processes operating and carried out as required to maintain a preset humidity of the air being humidified. There are four groups of controls that are integrated to maintain the operation of the humidifying system fully automatic. They include: a) the humidification process controls regulating the humidification of a stream of treated air, b) the electric power and safety controls regulating the power input in response to a humidity load, or steam controls for regulating the steam input to the heat exchanger in response to a humidity load, c) the water controls regulating the flow of feed water, water level, concentration of TS, blowdown of the boiling water, and draining of the water tank, and d) the monitoring and display of selected performance parameters.
A major concern with the current art compact isothermic humidifiers are the very high operation energy cost and operation maintenance cost. The high operation energy cost is the result of use of the electric power as the source of the heat required in the production of the steam. The high operation maintenance cost is due to the required regular cleaning of the water tank and of the submerged electric resistance heating elements or heat exchanger, or the regular replacement of the water tank operating with electrodes due to the excessive build up of hard scale.
Therefore, to overcome the shortcomings of the current art humidifiers, the object of the present invention is to provide a compact apparatus that produces steam from feed water containing dissolved solids using a gaseous or a liquid fuel and at minimum capital, operating and maintenance costs.