Instruction Manual
D103643X012
DFA Desuperheater
August 2013
3
Educational Services
For information on available courses for the Fisher DFA Desuperheater, as well as a variety of other products, contact:
Emerson Process Management
Educational Services, Registration
P.O. Box 190; 301 S. 1
st
Ave.
Marshalltown, IA 50158-2823
Phone: 800-338-8158 or
Phone: 641-754-3771
FAX: 641-754-3431
e-mail: [email protected]
Principle of Operation
The DFA desuperheater reduces steam temperatures through the introduction of cooling water directly into the hot
steam flow stream. By regulating the quantity of water that is injected, accurate downstream steam temperature can
be both controlled and maintained.
The rate of vaporization, and/or cooling, is a function of droplet size, distribution, mass flow, and temperature. Steam
velocity is critical and should be maintained at 6.1 to 9.1 meters per second (20 to 30 feet per second) as the
minimum. Actual minimum steam velocity requirements will vary by application. As steam velocity increases, a longer
distance is required to achieve homogeneous mixing and to complete vaporization.
In DFA desuperheater nozzle styles, the spraywater quantity is controlled by internal control valve which responds to
signals received from the temperature control system. The water enters the main tube of the desuperheater, passes
through the spray nozzle, and discharges into the steam line as a fine, atomized spray (see figure 4).
Each particular nozzle, or set of nozzles, in the sprayhead is tailored to meet a specific set of operating conditions. The
nozzle design optimizes the spraywater droplet size promoting rapid atomization and complete vaporization of water
in the steam flow stream to obtain precise temperature control. The DFA desuperheater uses a variable geometry AF
nozzle. In the AF nozzle design (see figure 4), water enters the swirl chamber via compound angled orifices, thus
creating a rotational flow stream. This flow stream is further accelerated as it is forced up and out through the spray
annulus. The cone-shaped plug varies the geometry of the spray annulus using a force balance principle between
water pressure and the preload exerted by a helical spring. This variable geometry design sprays a thin hollow cone
over a wide range of flow rates, resulting in excellent temperature control over a wide range of operating conditions.
Figure 2. AF Nozzle Cross Section
STEM
SPRING
NUT
PIN
WATER INJECTION HOLES
PLUG
BODY
A7191
