Considerations in Selecting Steam Humidifiers
Electronic Steam Humidifiers The humidifier must condition the steam Steam cup humidifiers receive steam so that it’s completely dry and free of from the side of the cup which theoretically When steam is not available, self- significant particulate matter. Response permits the condensate to fall by gravity contained electronic humidifiers can meet to control signals must be immediate, to the steam trap. However, in practice low-capacity requirements. The primary and modulation of output must be precise. a great deal of the liquid moisture in the consideration in selecting this type Distribution of steam into the air must steam goes into the air flow, and the of humidifier is its ability to work with be as uniform as possible. Inadequate steam itself is poorly distributed. wide ranges in water quality. Ionic bed performance in any of these areas electronic humidifiers are frequently means the humidifier will not meet the The steam separator is a more selected for this capability. basic humidification requirements. sophisticated device which, when Direct Injection properly designed, meets essential Direct injection steam humidifiers are performance criteria. Steam Humidifiers available in three basic types: specially An evaluation of three performance designed steam grids, steam cups and characteristics is essential to understand the steam separator. the advantages steam holds over other humidification media: Specially designed steam grid systems incorporate advanced engineering in ■ Conditioning addressing unique applications where ■ Control vapor trail is of prime concern. ■ Distribution
Figure 18-1. Steam Grid Humidifier
From Boiler Control Valve
Strainer Figure 18-3. Steam Separator Type Humidifier
Trap Pneumatic or Electric Drip Leg Operator Header Integral
Control Valve Steam Jacketed
Distribution Manifold
Drying Chamber Drain
1"/12" Slope Air Flow 1" Air Gap Drain Separating NOTE: Condensate cannot be lifted or discharged Chamber into pressurized return. Condensate Deflected
Downward KEY Figure 18-2. Cup Type Steam Humidifier
Inverted STEAM SUPPLY AT SUPPLY Bucket PRESSURE Steam STEAM AT ATMOSPHERIC Trap PRESSURE
CONDENSATE
18 Steam Conditioning condensate carries a substantial portion of Humidifier control must provide immediate As steam moves through supply lines, the significant micronic particulates with response and precise modulation in order scale and sediment may be entrained in it to be discharged through the drain trap. to accurately maintain the required relative the flow—a Y-type strainer is required to humidity. Faulty control can make it remove larger solid particles. Similarly, the Steam from the separating chamber can difficult to provide the desired humidity condensation that occurs in the supply still carry liquid mist which must be level, and can lead to overloading the lines permits water droplets or even slugs removed. Humidifiers equipped with an ducts with moisture and the creation of of condensate to be carried into the inner drying chamber that is jacketed by wet spots. humidifier. the steam in the separating chamber can effectively re-evaporate any remaining Two design factors affect the accuracy of Several steps within the humidifier water droplets before steam is discharged. humidifier control that can be achieved— are required to positively prevent the Similarly, the control valve should be the metering valve and the actuator that discharge of liquid moisture and finer integral with the humidifier. Both the positions the valve. particulate matter along with the humidifier and the distribution pipe should humidifying steam. be jacketed by steam at supply pressure Precise flow control can be achieved and temperature to prevent condensation with a valve designed expressly for the The separating chamber in the humidifier as steam is discharged. purpose of adding steam to air. Parabolic body should provide the volume required plug type valves have been established as for optimum velocity reduction and Only proper design of the humidifier for best for this service. They permit a longer maximum separation of steam from conditioning of steam can assure the stroke than comparable industrial valves, condensate. Properly separated, the essential levels of sanitation and a clean and the plug normally extends into the atmosphere. These guidelines contribute orifice even with the valve in “full open” to better comfort conditions and ensure position. This facilitates full and accurate that the humidifier meets the vital modulation of flow over the complete physical requirements of the system. stroke of the valve. Control of Output The Control Valve As discussed, the duct high-limit humidistat The parabolic plug design also provides can become the controlling humidistat due exceptionally high rangeability. Range- to psychrometric conditions in the duct. In ability is the ratio between the maximum typical duct air flow rates, the high limit controllable flow and the minimum Table 19-1. humidistat senses humidifier output in controllable flow of steam through the Steam Humidifier Valve Rangeabilities one second or less. In most applica- valve. The higher the rangeability of a Valve Size Rangeability tions, humidifiers consistently operate valve, the more accurately it can control at a fraction of maximum output. steam flow. Rangeabilities of the parabolic Ratio of Minimum Flow Equivalent plug valves used in Armstrong Series Diameter Flow as % of Max: Mii Maximuu 9000 Humidifiers shown in Table 19-1 are typical of the ratios that can be achieved 11/2" 63:1 1.6 with this type of valve. 11/4" 69:1 1.4
1 1/8" 61:1 1.6 1" 53:1 1.9
7/8" 44:1 2.3
3/4" 33:1 3.0 Chart 19-1. Desirable modified linear 5/8" 123:1 0.8 characteristic curve for valves used under
9/16" 105:1 0.9 modulating control. The modification of true
1/2" 97:1 1.0 linear characteristics provides more precise control when capacity requirements are very 15/32" 85:1 1.2 low and the valve is just cracked off the seat. Figure 19-1. Parabolic Plug Metering Valve 7/16" 75:1 1.3
13/32" 64:1 1.6 100 3/8" 70:1 1.4 90 3 9 7 11/32" 59:1 1.7 80 /8" /32" /32" 5/16" 49:1 2.0 70 9/32" 40:1 2.5 60 1/4" 31:1 3.2 50 7/32" 24:1 4.2
3/16" 18:1 5.6 40 3/4" 5/32" 59:1 1.7 30 Stroke 1/8" 37:1 2.7 20 7 /64" 28:1 3.5 PERCENT OF FULL CAPACITY 10 3/32" 21:1 4.8 0 3/8" 3/8" 3/8" 5/64" 15:1 6.9 0 102030405060708090100 1/16" 10:1 10.0 PERCENT OF FULL STROKE 19 Considerations in Selecting Steam Humidifiers, Continued... The actuator is another important compo- 1. Large diaphragm area—22 sq in or Distribution of Steam nent in humidity control. Several types are more—to provide ample lifting force. The third essential factor in proper humidifier available to provide compatibility with This permits the use of a spring heavy design is distribution. Steam must be various system types. The actuator must enough to stabilize both the hysteresis discharged as uniformly as possible into be able to position the valve in very nearly effect and the flow velocity effect on the the air to permit the fastest possible identical relationship to the seat on both positioning of the valve stem versus air absorption without creating damp spots opening and closing strokes. This is pressure to the actuator. or saturated zones. essential to provide consistent, accurate metering of steam discharged by the 2. Diaphragm material highly resistant In normal ducts, a single distribution humidifier. to wear or weakening from continuous manifold installed across the long dimen- cycling. sion will provide good distribution of steam. By their design, electric motor modulating In large ducts or plenum chambers, it may actuators provide true linear positioning 3. Actuator stroke long enough (in be necessary to broaden the pattern of characteristics on both opening and conjunction with valve plug and seat vapor discharge to achieve the required closing cycles. Pneumatic actuators may design) to provide high rangeability distribution, thus requiring multiple mani- or may not be able to provide the precise ratios. folds from single or multiple humidifiers. positioning and holding characteristics essential to accurate control. Rolling All modulating actuators, whether electric Humidification for industrial areas without diaphragm type pneumatic actuators are or pneumatic, should incorporate a spring central air handling systems is customarily recommended, providing they meet the return. This is necessary to ensure closing achieved with unit humidifiers discharging following criteria: the valve if there is an interruption of power steam directly into the atmosphere. Proper or control air to the unit. mixing of steam and air can be accom- plished in two ways. A dispersing fan may For industrial in-plant operation and for be mounted on the humidifier or a unit very limited duct applications, a solenoid heater can be positioned to absorb and Chart 20-1. Desirable Operating actuator may be used to provide simple distribute the water vapor. Characteristic for Pneumatic Actuators on-off operation. This type of actuator Position of valve is very nearly identical on should not be specified for duct applica- both opening and closing strokes at any given tions without a detailed analysis of the air pressure to the actuator. system.
11 10 Opening Stroke Closing Stroke 9 8 Figure 20-2. 7 Unit Humidifier for Direct Discharge into Area Humidified 6 5 4 0 102030405060708090100
CONTROL AIR PRESSURE TO ACTUATOR-PSIG PERCENT OF FULL STROKE
Solenoid Valve Figure 20-1.
Steam Out
Distribution Strainer Manifold E
Distribution Manifold Inlet C
Duct Cross Section Duct Cross Section Temp. Single Multiple Distribution Switch Distribution Manifolds in a Large Drain Manifold in a Duct or Housing Normal Duct Trap 20 Operating Noise Suggested Specification: D. The stainless steel metering valve shall 3 In addition to these crucial performance Steam humidifiers for pneumatic or be a parabolic plug, with a full /4" stroke. characteristics, operating noise is a electric modulating control: humidifier shall This valve shall provide the high range- consideration in selecting steam humidifi- be the steam separator type providing full abilities required to achieve full and ers for areas where quiet operation is separation ahead of an integral steam accurate modulation of steam flow over essential or desirable, i.e., hospitals, office jacketed control valve. Steam shall be the entire stroke of the valve. buildings, schools, etc. discharged through an internal steam jacketed drying chamber, a silencing E. The internal drying chamber shall chamber and a steam jacketed distribution receive total steam flow at essentially manifold. atmospheric pressure and be jacketed by steam at supply pressure. A. Humidifier shall receive steam at supply pressure and discharge at F. The silencing chamber shall be steam atmospheric pressure. It shall be jacketed and utilize a stainless steel furnished with inlet strainer and silencing medium. external inverted bucket steam trap. G. The distribution manifold shall provide B. Separating chamber shall be of a uniform distribution over its entire volume and design that will disengage length and be jacketed by steam at and remove all water droplets and all supply pressure to assure that vapor particulate matter larger than 3 discharged is free of water droplets. microns when humidifier is operating A full length stainless steel internal at maximum capacity. silencing screen shall be provided.
C. The stainless steel metering valve H. Humidifier shall be equipped with an shall be integral within the body of the interlocked temperature switch to humidifier, and shall be jacketed by prevent the humidifier from operating steam at supply pressure and tempera- before start-up condensate is drained. ture to prevent condensation.
Figure 21-1. Figure 21-2. The noise of escaping steam is generated at Steam moving at high velocity through the the control valve. Muffling materials around the distribution manifold can create loud, high- valve are necessary to minimize this noise. pitched whistling as it passes the discharge holes. An internal silencing screen or similar material is required to interrupt the air flow and prevent whistling.
Stainless Steel Mesh
Silencing Material
21