Local regulations may restrict the use of this product to below the conditions quoted.
In the interests of development and improvement of the product, we reserve the right to change the specification without notice. Copyright 2013
TI-P475-06
CH Issue 2
Desuperheater
Online Program Sizing Guidance
Desuperheater overview
In typical process plants, process steam is usually superheated,
or heated to a temperature above saturation. The difference
between the saturation temperature and the actual temperature
of the steam is called 'superheat'.
Desuperheated steam is more efficient in the transfer of thermal
energy, consequently desuperheaters are used to bring the outlet
degree of superheat closer to that of saturation.
Desuperheaters reduce the temperature of superheated process
steam by introducing finely atomized cooling water droplets into
the steam flow. As the droplets evaporate, sensible heat from the
superheated steam is converted into latent heat of vaporization.
A typical desuperheater installation is shown below:
Combined pressure reducing / desuperheating station for venturi and spray type desuperheaters
Pressure
controller
Steam
supply
Desuperheater
Pressure
regulating
control valve
Non return valve
Cooling
water
Temperature
controller
Temperature
regulating
control valve
Page 1 of 12
Desuperheater selection
There are various types of desuperheater available so evaluation of the process duty is crucial to ensure selection of the right equipment.
Turndown capability, pressure drop and outlet superheat play lead roles in desuperheater design and selection:
Turndown: (Maximum steam flowrate Minimum steam flowrate)
Turndown represents the variability of the steam flowrate. For many processes, turndown is very small or fixed. Generally, the higher
the turndown, the more complicated the Desuperheater design.
Outlet superheat:
Although desuperheaters are capable of desuperheating to the saturation temperature of the steam, typically, desuperheaters are
designed to produce steam temperatures at 3C to 5C above saturation. This is because it becomes increasingly difficult to control the
process (and there is very little advantage) at lower temperatures.
Steam pressure drop (for venturi type desuperheaters):
For most pressure systems, a 0.4 to 0.7 bar g drop is considered reasonable. It should be noted that as the required turndown increases,
so does the pressure drop. This is because there is a minimum acceptable pressure drop at the minimum flowrate case that ensures
sufficient velocity to atomise the water droplets. Therefore, as the maximum steam flowrate increases, so does the velocity and hence
the maximum pressure drop.
Water pressure drop (for spray type desuperheaters):
It should be noted that as the required turndown increases, the required cooling water pressure also increases.
General 'Rule-of-thumb':
Over-specifying the thermal load or process requirements is detrimental to efficient operation and will increase the cost
of the desuperheater (and its controls). Under specifying the operating range can result in a unit that cannot handle all
operating cases.
Each type of Spirax Sarco desuperheater, employs a different method to create water droplets. The process by which the water droplets
are created is usually referred to as 'atomisation'.
It must be remembered that the evaporation of the water droplets (and hence cooling of the steam) is a time dependent process and does
not occur instantaneously. Consequently, most of the desuperheating does not occur in the desuperheater itself, but in the pipework
immediately downstream. Thus, the design of the downstream pipework is a crucial factor in a successful desuperheater installation.
It is important that the water droplets remain suspended in the downstream pipework for as long as possible. To ensure this, it is necessary
to maintain sufficient turbulence in the downstream piping by keeping the velocity relatively high higher than is usually encountered in
steam distribution systems (up to 60 m/s). This is the reason why desuperheaters and their associated pipework are often (not always)
smaller than the distribution system in which they are being installed.
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 2 of 12
Types of desuperheater
From the foregoing paragraphs, it is easy to understand why there has to be a period of good contact between the droplets of cooling
water the superheated steam. If good contact is lost, the water can no longer absorb heat effectively from the steam, evaporation stops
and the desuperheating process comes to a halt.
When the steam velocity is too low, 'water droplet fall-out' occurs and a pool of water is formed which runs along the bottom of pipe. At
this point good contact between cooling water and the steam is lost and effective desuperheating will not occur. By following the guidelines
presented in this document or using the Spirax Sarco online sizing software, problems due to droplet fall-out can be avoided.
There are three basic types of Spirax Sarco desuperheater (shown below) which all use a different method to atomise the cooling water
droplets. Each one has its own merits and the desuperheater selection chart shown on the following page determines which type should
be selected.
Cooling water
Spray nozzle assembly
Thermal sleeve
Superheated
steam
Desuperheated
steam
Complete spray type desuperheater
Cooling water
Internal
nozzle
Internal
diffuser
Main
diffuser
Steam
flow
Seal
Seal
Venturi type desuperheater
Atomising
steam
Cooling
water
Internal
nozzle
Internal
diffuser
Steam
flow
Steam
flow
Seal
Seal
Steam atomising desuperheater
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Finer droplets, therefore smaller
absorbtion length and / or better
turndown and approach to
saturation
Page 3 of 12
Desuperheater selection chart
Turndown 1:1 2:1
Spray type desuperheater (STD)
Spray nozzle type desuperheater (SND)
'Cost effective option for turndown duties'
Minimum water pressure: Steam +0.5 bar
3:1
Approach to
saturation
Tsat +5C
Max. W.P:
Steam +4.5 bar
4:1
Tsat +5C
Max. W.P:
Steam +8 bar
5:1
Tsat +3C
Steam +0.9 bar
10:1 20:1 50:1
Tsat +7C
Max. W.P:
St +12.5 bar
Vertical
installations
only
Tsat +5C
Tsat +7C
Venturi type desuperheater (VND)
'Our most popular desuperheater, ideal for most duties'
Max. W.P:
Minimum water pressure: Steam +0.1 bar
7:1
Tsat +3C
Max. W.P:
Steam +2.5 bar
Max. W.P:
St +4.9 bar
Max. W.P:
St +10 bar
Steam atomising type desuperheater (SAD)
'Special desuperheater for high turndown duties'
Required water pressure: Same as steam pressure
Atomising steam required at 1.5 x
Desuperheater inlet steam pressure or 3 bar g minimum
Tsat +3C
SAD
(with recycle)
Tsat +3C
Other considerations
Desuperheater orientation
Desuperheaters may be installed either horizontally or vertically (with the steam flowing upwards). In a vertical installation, increased
turndown can be achieved; as the steam and water are countered by gravity, the water is less likely to fall out of suspension. Spirax Sarco
strongly advises against installations in which the steam flow is vertically downwards, as the opposite would occur.
In the case of a horizontal installation the cooling water connection (and the atomising steam connection on a SAD (steam atomising
desuperheater) should ideally point downwards, as this gives the best orientation for drainage of fluids in a shutdown situation. Other
orientations are acceptable for satisfactory operation, but drainage is not as effective.
In a vertical installation we recommend that, the cooling water pipework (and atomising steam pipework, if applicable) should be brought
to the desuperheater from below the corresponding connections on the desuperheater. This will provide the best layout for drainage of
fluids on shutdown.
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 4 of 12
Distances
The diagram below indicates the recommended straight length distances between the desuperheater and upstream / downstream
equipment. The distances are represented by length 'A' and 'B'.
Combined pressure reducing / desuperheating station
Pressure
controller
Pressure
regulating
control valve
Steam
supply
B
Desuperheater
Non return valve
Cooling
water
Temperature
controller
Temperature
regulating
control valve
Recommended distances for location of pressure and temperature sensors and equipment:
Residual superheat
100C
Downstream distance required (straight)
Upstream distance required (straight)
2.5 m
50C
3.7 m
5.0 m
30C
15C
6.25 m
6.8 m
10C
5C
7.0 m
5 pipe diameters or 1.5 m minimum
'The greater the residual superheat, the faster the water droplets are absorbed'
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 5 of 12
Other considerations (continued)
Cooling water supply
Typical cooling water supply options are as follows:
- Boiler feedwater (BFW) (taken from the pressure side of the boiler feedpump).
- Demineralised water.
- De-ionised water.
- Condensate.
Towns water or process water may also be used, but depending on hardness, salts may be deposited on the inside of downstream
pipework and the face of valve seats and plugs.
Cooling water quality
The quality of the injected water is important. The TDS (Total Dissolved Solids) of the injection water should be as low as possible
since all these solids will come out of solution and be deposited on the faces of valves and could block up the small orifices in the
desuperheater nozzles.
Cooling water temperature
Generally, the hotter the better. This is because hot droplets need to absorb less heat to reach their flash temperature than cold ones.
Hence, hot droplets will evaporate more quickly, producing a more efficient desuperheating process. Using hot water also has the
additional advantage that smaller amounts of water will fall onto the inside walls of the pipework.
Because of the benefits of using hot water, it is logical to insulate the water supply pipes to minimise heat loss.
Cooling water pressure and flowrate
In order to inject the cooling water, its pressure at the desuperheater nozzle must be equal to or greater than the operating steam
pressure in the pipe. The requirement varies from one type of desuperheater to another, but typical minimum values are:
- Spray type desuperheater
steam pressure + 0.5 bar
- Venturi type desuperheater
steam pressure + 0.1 bar
- Steam atomising type desuperheater equal to steam pressure
For the spray and Venturi type desuperheaters, the highest water inlet pressure required will be at the highest cooling water flowrate.
It should be noted that the water flowrate is a function of the square of the pressure difference between cooling water and the steam.
So if the water flowrate is to be increased by a factor of 4 for example, then the pressure difference must increase by a factor of 42 = 16.
This is the reason why it is important not to over-specify the turndown as high cooling water pressures are quickly reached (especially
with spray type desuperheaters).
If an independent or booster pump is used, a spill-back will be required to ensure that there is always flow through the pump.
Cooling water control valve
A pressure drop will be required over the water control valve. We have already said that ideally the water should be as hot as possible
so care is needed to ensure that flashing conditions do not exist across the control valve.
Superheated steam pressure control
It is desirable that a constant steam supply pressure be maintained.
The temperature of the steam after the desuperheater controls the amount of water added. The higher the temperature, the more the
control valve will open and the greater the amount of water that is added. Usually the target is to reduce the steam temperature to within
a small margin of saturation temperature. In virtually all applications the upstream pressure will be controlled and constant, however, if
the superheated steam supply pressure is increased, the saturation temperature will also increase. The set value on the Controller will
not change, and an excessive amount of water will be added as the control system tries to achieve the set temperature. This would result
in very wet saturated steam with its attendant problems.
Control
In this document we have frequently used the term 'turndown' to describe the performance of the different types of desuperheater. However,
as far as an installation is concerned, it should remembered that the desuperheater is only one element of a desuperheating station.
Obviously, if the controls that are fitted have lower turndown than the desuperheater, then the turndown of the desuperheater station will
be reduced.
For example, in a particular pressure reducing / desuperheating station, the rangability of the cooling water valve may not be as high as
the desuperheater. In this case it will be the rangability of the water control valve that limits the turndown of the desuperheating station.
Separator station
In applications where there must be no moisture present in the resulting steam (such as prior to a turbine for example) it is recommended
that a separator is installed downstream of the desuperheater. This will protect downstream pipework and equipment from the effects of
moisture in the event of a control system failure or abnormal operating conditions, for example at start-up.
The separator must be located after the temperature sensor thereby giving the water droplets as much time as possible to evaporate.
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 6 of 12
Strainer
Spirax Sarco recommend that a strainer is incorporated in the cooling water supply line to protect both the cooling water control valve
and the small bores within the desuperheater from becoming blocked.
Isolation valves
To allow maintenance to be safely carried out, isolation valves are recommended upstream of:
- The superheated steam pressure control valve.
- The cooling water control valve.
Safety valve
In applications involving simultaneous pressure reduction, a safety relief valve may be needed to protect both the desuperheater and
downstream equipment from the effects of:
- Excess pressure in the event of pressure control system failure.
- Excess temperature in the event of temperature control system failure.
It is essential that the desuperheater and downstream equipment are suitable for the maximum temperature of the superheated steam.
This is to protect these items in the event of a failure of both the pressure and temperature control systems.
Recycle loop
For SAD steam atomising desuperheaters with a very high turndown a 'catchpot and recycle loop' are often installed as shown on the
diagram below. The recycled condensate is hot which leads to faster absorption.
The desuperheater generates a small suction effect to draw the recycle water back to the desuperheater ensuring that the water doesnt
'by-pass' the desuperheater.
Combined pressure reducing / desuperheating station for steam atomising type desuperheaters
Pressure
controller
Pressure
regulating
control valve
Steam
supply
Desuperheater
3m
On / off
control
Non return
valve
Cooling
water
Temperature
controller
Temperature
regulating
control valve
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 7 of 12
Detailed example
Steam is required to heat a vessel jacket at 2 bar g and 133.7C. A low pressure steam supply is available at 10 bar g and 200C
for this purpose.
Condensate at 95C and 5 bar g is available as the cooling medium.
Minimum steam flow required is 1000 kg/h, maximum flow 5000 kg/h.
Pressure
controller
Steam
supply
Desuperheater
Pressure
regulating
control valve
Non return valve
Cooling
water
Temperature
controller
4
Temperature
regulating
control valve
Method
As the turndown required equals 5:1 and the application
involves simultaneous pressure reduction, this would seem
to be a good application for a venturi type desuperheater
(VTD). Please note that a steam atomising desuperheater
(SAD) could also be used for this duty, but the overall
installation cost would be higher due to the greater
complexity of the installation.
As already stated, it is not possible to control at the
saturated temperature of the steam so any associated
temperature controller should have a set point of 3C
above the saturation temperature (which is 136.7C in this
example).
We can use the Spirax Sarco Online Sizing Program to
calculate the unit size (available at www.spiraxsarco.com/
uk with password). The form would be completed as follows:
The steam temperature at the inlet to the desuperheater is
calculated by the program automatically and is shown on
the datasheet.
Once the calculation button is pressed the software will
calculate the required cooling water pressure and flowrate.
It will also calculate the pressure drop across the unit. The
user will see a summary screen, at which point the desired
flange rating can be selected and then the 'Save & Email'
button can be pressed. You will then receive an e-mail with
a drawing and datasheet to your chosen e-mail address.
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 8 of 12
Detailed example (continued)
The datasheet generated by the program for this example is as follows:
Spirax Sarco Venturi Type Desuperheater Data Sheet
1
Client :
Spirax-Sarco Limited
Client's Ref :
EXAMPLE CALCULATION
Plant Location :
Spirax Sarco Ref :
VTD01582
Equip. Item No. :
Description :
Size 6 Venturi Type Desuperheater
No. OFF :
Unit Ref :
VTD150CS6F0
Operation :
Serial No. :
Drawing No. :
DE-VTDSDSO
Unit Size :
v 1.4
Client Project Ref :
INLET CONDITIONS
Case 1
Case 2
Case 3
Flowrate (kg/hr)
5000
3000
1000
CONDITIONS UPSTREAM OF LET-DOWN VALVE
Case 1
Case 2
Case 3
10
Pressure (bar g)
2.302
2.108
2.012
10
10
10
11
Temperature (C)
179.7
179.2
178.9
200
200
200
12
13
COOLING MEDIUM
Case 1
Case 2
Case 3
14
Flowrate (kg/hr)
193.5
116.1
38.7
15
Pressure (bar g)
5.34
3.2
2.13
16
Temperature (C)
95
95
95
17
Medium -
Water
18
19
DISCHARGE CONDITIONS
Case 1
Case 2
20
Pressure (bar g)
21
Temperature (C)
136.7
136.7
136.7
22
Flowrate (kg/hr)
5193.5
3116.1
1038.7
Case 3
23
24
MATERIALS OF CONSTRUCTION
25
Main Body
Carbon Steel ASTM A 106 Grade B
Flanges
Carbon Steel ASTM A 105
Gaskets
Soft Copper
26
Inner Nozzle
Stainless Steel BS 970 S11/13
Inner Body
Carbon Steel ASTM A350 LF2
27
Inner Venturi
Stainless Steel BS 970 S11/13
Main Venturi
C.Steel BS 1501-151/161-430A/B
CONNECTION DETAILS
28
29
MECHANICAL DESIGN
30
31
Max.Design Pressure (bar g)
Steam
Water
Side
Side
15
15
Inlet Steam
Size
Rating
300 LB
32
Max. Design Temperature (C)
220
220
Discharge Steam
300 LB
33
Internal Corrosion Allowance (mm)
1.5
1.5
Cooling Medium
300 LB
Flange Type
ASME B16.5 Slip-On
34
Mechanical Design Code
ASME B31.3
35
Welding Standard
To Code
36
External Surface Finish
High Temp. Silicone Aluminium
37
Weight (kg)
68
38
39
DIMENSIONS
40
A (" NB)
41
B (mm)
42
C (mm)
127
43
D (mm)
533
44
E (mm)
660
178
Note: Letters refer to Drawing
Issued for sizing only
PDA
08/01/2010
Rev
Description
By
Date
Checked
Date
A Spirax Sarco Size 6 venturi type desuperheater has been selected. It can be seen from the 'Cooling Medium' section that the
maximum cooling water pressure required is 5.34 bar g consequently a water booster pump should also be installed (as the cooling
water is only available at 5 bar g.
The program has calculated a steam pressure drop across the desuperheater of 0.302 bar g (maximum). As there is a pressure
reducing valve installed, the program has automatically added this pressure drop to the desuperheater inlet pressure to 'compensate'
for the pressure drop. This will ensure that the outlet pressure from the desuperheater is 2 bar g. The datasheet therefore shows the
correct pressures and flowrates for sizing both the pressure reducing valve and the water control valve.
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 9 of 12
Detailed example 2
Steam is required at 5 bar g (saturated) for use in a Shell & Tube Heat Exchanger. The Client has steam available at 5 bar g, 350C.
Minimum steam flow is 8 500 kg/h, maximum is 25 000 kg/h. Boiler feedwater at 20C is available as the cooling medium.
B
Steam
supply
Desuperheater
Non return
valve
Cooling
water
Temperature
controller
4
Temperature
regulating
control valve
Method
As the turndown is approximately 3:1 we can select
any type of Desuperheater. There are 3 options:
SAD - Steam atomising desuperheater
This would require atomising steam at 7.5 bar g (min).
This section
does not
need to be
completed
VTD - Venturi type desuperheater
This could be selected, however, this desuperheater
is not part of a pressure reducing station,
consequently if we select a VTD venturi type
desuperheater, the discharge steam pressure will be
reduced by the pressure drop across the unit. The
steam could be desuperheated to Tsat+3C.
STD - Spray type desuperheater
The unit is capable of handling the turndown with no
steam-side pressure drop and can desuperheat to
Tsat+5C.
In this case the client states that Tsat+5C is
acceptable so we will select an STD.
The Spirax Sarco Online Spray Type Desuperheater
Sizing Program would be completed as shown
opposite:
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Page 10 of 12
Detailed example 2 (continued)
A Spirax Sarco Size 10 spray type desuperheater has been selected. The maximum cooling water pressure required is 9.79 bar g and
there is a negligible steam-side pressure drop.
Spirax Sarco Spray Type Desuperheater Data Sheet
Client :
Spirax-Sarco Limited
Client Project Ref :
Client's Ref :
DETAILED EXAMPLE 2
Plant Location :
Spirax Sarco Ref :
STD00519
Equip. Item No. :
Description :
Size 10 Spray Type Desuperheater
No. OFF :
Unit Ref :
Size STD250CS6F0
Operation :
Drawing No. :
DESTD00519-1
Serial No. :
Unit Size :
10
INLET CONDITIONS
Case 1
Case 2
Case 3
Flowrate (kg/hr)
25000
18000
8500
10
Pressure (bar g)
11
Temperature (C)
350
350
350
v2.0
CONDITIONS UPSTREAM OF LET-DOWN VALVE
Case 1
Case 2
Case 3
12
13
COOLING MEDIUM
Case 1
Case 2
Case 3
14
Flowrate (kg/hr)
3733
2678.8
1269.2
15
Pressure (bar g)
9.79
7.48
5.55
16
Temperature (C)
25
25
25
17
Medium -
Water
18
19
DISCHARGE CONDITIONS
Case 1
Case 2
20
Pressure (bar g)
21
Temperature (C)
163.9
163.9
163.9
22
Flowrate (kg/hr)
28733
20687.8
9769.2
Case 3
23
24
MATERIALS OF CONSTRUCTION
Nozzle Housing
Carbon Steel ASTM A 350 LF2
25
Main Body
Carbon Steel ASTM A 106 Grade B
Flanges
Carbon Steel ASTM A 105
26
Water Branch
Carbon Steel ASTM A 106 Grade B
Gaskets
Soft Copper
27
Thermal Sleeve
Stainless Steel ASTM A312 TP316L
Spray Nozzle
Stainless Steel
CONNECTION DETAILS
28
29
MECHANICAL DESIGN
30
31
Max.Design Pressure (bar g)
Steam
Water
Side
Side
10
30
Inlet Steam
Size
Rating
10
300 LB
32
Max. Design Temperature (C)
374
374
Discharge Steam
10
300 LB
33
Internal Corrosion Allowance (mm)
1.5
1.5
Cooling Medium
300 LB
Flange Type
ASME B16.5 Slip-On
34
Mechanical Design Code -
ASME B31.3
35
Welding Standard -
To Code
36
External Surface Finish -
High Temp. Silicone Aluminium
37
Weight (kg)
180
38
39
DIMENSIONS
40
A (" NB)
41
B (mm)
430
10
42
C (mm)
250
43
D (mm)
800
44
E (mm)
1050
Note: Letters refer to Drawing
Issued for sizing only
PDA
22/01/2010
Rev
Description
By
Date
TI-P475-06
CH Issue 2
Desuperheater Online Program Sizing Guidance
Checked
Date
Page 11 of 12
Typical applications
The following is a list of applications where desuperheaters have been supplied:
Power generation
To reduce the temperature of steam discharged from turbine bypass
systems to that required for other parts of the plant.
Turbine washing
Process industries
In process industries, desuperheaters are used as part of a system for
reducing the temperature and pressure of steam from boilers to
economic levels of operation.
Paper and board industry
Paper drying machines
Food industry
Steam cooking kettles
Evaporator heat exchanger
Product conditioning
Textile industry
Fabric finishing autoclaves
Tobacco industry
Tobacco leaf drying plants
Chemical and pharmaceutical industry
Reactor heater jackets and coils
Steam supply to process heaters
Oil and petrochemical industry
Vacuum distillation start-up heaters
Steam supply to process heaters
Let-down station and turbine bypass
Thermocompressor discharge
Mechanical vapour re-compression
Brewing and distilling industry
Steam heating system
Boiler and turbine installations
Power generation
Ship building
Coffee
Chemical
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Desuperheater Online Program Sizing Guidance
Page 12 of 12
