What is a desuperheater and how does it work?
Desuperheating is the process of reducing the superheat temperature or returning superheated steam to its saturated condition. There have been many nozzle designs over the years but sometimes the simplest solution is the best. Read on to find out more about the general principles of desuperheating and why the MASCOT Mechanical Spray is the best choice for your next project.
What is a desuperheater?
Desuperheating is the process of reducing the superheat temperature or returning superheated steam to its saturated condition. The majority of desuperheaters that are employed to return the system to a saturated condition produce discharge temperatures that are close to saturation, usually within 3°C of the saturation temperature.
An attemperator controls the steam temperature while a desuperheater removes whatever superheat there is in steam and reduces the temperature to a point at or nearly at saturation temperature.
Desuperheaters are generally found in steam lines away from the boiler or boiler outlet piping where there is a downstream use for saturated steam.
A desuperheater is known as a secondary heat exchanger device that transfers heat efficiently. When steam is used for heating or other heat transfer or in industrial processes, it is most efficient when saturated. It is a device that lowers the temperature of superheated steam so that it can be used efficiently for heating or for other industrial use. This steam temperature minimization is also called as attemperation, is performed by the desuperheater cooling water heater (a device that injects a prescribed amount of water into a steam flow stream).
The general purpose, of this device is to provide effective and continuous heat transfer between steam and water. And a highly effective desuperheater application and design ensures that costly problems to be avoided, including general wear of downstream piping and equipment, long startup and shutdown times, and reduced process efficiency.
Desuperheater Working Principle/ Function:
The primary function of a desuperheater water is to minimize the temperature of superheated steam and other vapours. This temperature reduction is achieved by the process vapor being brought into direct contact with a liquid such as water. The injected water is then evaporated.
MASCOT’s Mechanical Spray Desuperheater (MSD) Overview
This is the simplest type of Desuperheater, consisting of a fixed area nozzle arranged to face the steam flow through periphery of Desuperheating pipe. The MSD relies on the pressure differential available across the jets to achieve the conditions for rapid absorption & atomisation of the water into the steam.
A cooling water Control Valve installed in the water line ahead of the MSD will vary the water supplied to the jets. This maintains the downstream steam temperature at the measuring point. While the MSD is working between approximately 5% and 100% of maximum rated capacity, there will be adequate pressure differential across the jets to ensure a spray pattern in the form of a hollow cone of finely divided water particles.
When Desuperheating loads go above the maximum rated capacity, the steam velocity at the exit of the nozzle will ensure turbulent flow, which helps to mix water particles into the steam thoroughly.
With reduction in steam flow, the steam velocity and turbulence will also reduce. The reduced steam flow of the water requirement will also be proportionately reduced to maintain constant temperature. The plug of the Control Valve will automatically move towards the closed position to achieve this.
Given the spray jets have a fixed orifice; the pressure differential across them will reduce proportionally to square off the reduction flow in accordance with Bernoulli’s law. This causes a reduction in differential velocity between the steam and the water particles, a reduction in the cone angle so that the water is projected more directly downstream, and an increase in water droplet size. All of these phenomena adversely affect the ability of the Desuperheater to work efficiently.
MASCOT’s Mechanical Spray Desuperheater (MSD) Details
MSD relies on the pressure drop across the spray jets to achieve the necessary atomization. The unit consists of an orifice plate jet through which the water is discharged, and the pressure drop across this creates a spray pattern to achieve the necessary atomization.
The Mechanical Spray Desuperheater can only be designed for maximum flow conditions and since the pressure drop across the jet hole follows square laws, as the flow is reduced, the pressure drop across the jet hole is substantially reduced and the atomizing characteristic is rapidly lost. For example, if the flow is reduced by 50% of pressure drop across the orifice is reduced by 4.
If the load further reduces, a stage would be reached where the spray jet ceases spraying, and the water is projected straight downstream and collects at the bottom of the steam pipe. Since the steam is not being desuprerheated, the temperature senses a rise in steam temperature, the Control Valve plug moves towards the open position, and the jet commences to spray again. The steam is thus overcooled, which causes the Control Valve to close again. The whole cycle then repeats with consequent swings in temperature and intermittent flooding of the steam line with cooling water.
Mechanical Spray Desuperheater Design
Many attempts have been made to produce a nozzle that has better rangeability, such as a swirl nozzle. However, such nozzles still suffer from the fact that as steam and water flow reduces. Desuperheating efficiency falls away proportional to the square of the flow.
More recently, different variable nozzle spray Desuperheater designs have been attempted. In these designs MASCOT has attempted to maintain a constant water pressure differential that ensures effective atomization, a wide-angle cone, and high water velocity at all loads, changing the orifice area proportionally to load varies throughout.
However, the MSD is widely used with successful results in installations where the load is relatively steady. The inevitable variation in the steam temperature downstream of the Desuperheater and the water, which is collected and trapped out, of low order are acceptable.
For MASCOT to design the most efficient Desuperheater we require the following parameters:
- Operating pressure of steam
- Maximum inlet temperature of steam**
- Required outlet temperature of steam
- Steam flow rate**
- Available cooling water pressure
- Available cooling water temperature
** You can give all possible values for these parameters
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