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Special Application Steam Desuperheaters

Type 6905 Mechanical Atomizing Dump Desuperheater

The Type 6905 units employ one of more special spray nozzles to atomize the water droplets and produce the heat transfer contact area necessary for cooling the steam and evaporating the droplets. Because droplets are relatively coarse and the steam temperature entering the unit is normally above 800° F. We recommend and can supply a thermal sleeve welded to the unit which extends approximately 1 foot downstream in the customer's piping. Water pressure to this type unit differs from all other types in that it is required at a minimum of 25 psi above line pressure.

Application: The mechanical atomizing type desuperheater was developed for economically desuperheating high steam flows in large steam lines i.e. 20", 24", 30", 36", etc. Initial applications were for periodic usage on emergency dump to condenser systems in power stations where precise desuperheating was not necessary and excess water flows were common. These units are also used on controlled over-pressure dump and bypass systems.

Construction: The Type 6905 consists of only three basic components; body, spray shield, and nozzle assembly. Bodies are normally cast carbon, alloy or stainless steel with weld ends. Stainless steel nozzles are removable from body nozzle bosses. Spray shield is stainless steel pipe or rolled plate.

Type 6910 Surface Absorption Desuperheaters

The Type 6910 units desuperheat steam by forcing it to come in contact with wetted metal reaction rings. These desuperheaters can be used where steam flow varies with consequent changes in water flow being required for close control. Water pressure need be not more than 10 pounds above steam pressure. Automatic controls can be used and in general the temperature control bulb can be located nearer (as close as 5' or less) the discharge of the desuperheater than with other types. Saturation temperature or % wet steam is possible with this type unit.

Construction: Body and cover are normally cast carbon or alloy steel with internal basket, reaction rings and deflector plate of 304 stainless steel. Available in ratings to 900 psi.

Operations: Water, entering as indicated, flows on a splash plate and is distributed over a perforated plate in the top of the basket containing the reaction rings. The water flows over these metal rings, wetting them thoroughly and providing ample surface for contacting the steam. The high temperature steam flows through the reaction ring section and is de­superheated by contacting the wetted rings. It flows out through the plate at the bottom of the basket and passes through a water deflector and separator into the desuperheater outlet. Excess water drains to the bottom and should be removed through a trap.

Application: The surface absorption type unit is generally used where space limitations and requirements of minimum water carryover are stipulated. Normally used in the marine, food processing and drying industries. Units have been in operation for over 40 years with minimum service required. See reverse side for control schematic.

Sizes and Approximate Dimensions of Type 6910 Desuperheaters
Size No. 2 3 4 5 6 8 10 12
Over-all width in inches 17 17-1/2 20 22-7/8 26-7/8 31-3/8 39-3/4 44
Over-all height in inches 16-1/2 16-5/16 17-3/4 22-1/4 24-3/4 29-1/4 35-1/2 41-3/4
Desuperheater Control Systems
A desuperheater is not a single piece of equipment that succeeds or fails on its own, but is only one of several distinct system components. For a successful application, system engineering is a must. Neglect of any one component may result in system failure, no matter how excellent the design and engineering application of the other components. Therefore, all of the control components must be carefully selected for the specific application to be handled.

Steam Pressure Reducing Valve-This valve must have a turn­down somewhat greater than that of the system; it must respond to plus and minus control signals even at maximum and minimum flow rates. These valves are selected for a useful control range of 20 to 80% of maximum flow. They are normally an equal­percentage type or have equal-percentage characteristics. This type of valve has the best inherent flow characteristic and range needed for proportional control.

Water Control Valve - This valve must have sufficient rangeability to meet the application. Two valves in parallel may be needed to get this range-one large and one small. Where a large water pressure differential is encountered, be careful of possible cavitation. Consider using a pressure control valve upstream of the main flow valve.

Atomizing Steam Shutoff Valve - This valve is used only with Type 6970-72 desuperheaters in the on-off, not throttling mode, as in above valves. The valve is sized on a constant flow, dependent on unit size and a nominal pressure drop.

Temperature Controller - Must have an adjustable proportional control band wide enough to match response characteristics of the entire desuperheater system. Automatic reset prevents drift in control point. Rate action is seldom needed, but if it is provided, complete cutoff should be possible.

Pressure Controller - Must prevent large pressure variations which might interfere with temperature control; therefore, it needs an adjustable throttling range and automatic reset.

Control Valve Actuators - Pneumatically operated control valve actuators are the most popular type in use, but electric, hydraulic and manual actuators are also used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide an integral positioner capability and high stem force output for demanding service conditions.

Temperature Switches - They are used in connection with alarm systems for high and low temperatures. The temperature sensor uses the expansion principle in which the fluid or element in the sensing bulb reacts to the line temperature.

Pressure Switches - These switches, as above, are generally used in connection with alarm systems for high and low pressures. The alarm can be either audio or visual. Pressure applied to the sensor actuates a mechanism and its movement is then used to control the operation of an electrical snap acting switch, or other actuating medium.

WARNING: A control system cannot successfully hold a temperature that is no higher above saturation than the controller's degree of sensitivity and deadband.
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