Attemperator – Working Principle, vs. Desuperheater

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An attemperator is a coil of pipe that usually lies downstream of a steam turbine or boiler. Its basic function is to regulate the temperature of steam by either running cold or hot water. In this article, you will learn an attemperator’s working principle, its use in boilers, compare an attemperator vs a desuperheater, and learn how an attemperator valve works.

Working Principle

Mostly, attemperators serve to reduce the temperature of superheated steam so that downstream equipment does not fail. This is common in industrial processes that deliver both power generation and heating, or when deploying various stages of heating. An attemperator comes in-between stages to ensure the steam is at the right temperature before subsequent steps in the cycle. The way it achieves this depends on its type, which can be either spray or indirect contact.

Spray Type Attemperator

The basic working principle for all attemperators is to take the heat out of a system by the introduction of liquid water. For the spray type, multiple nozzles spray liquid water directly on the steam, intermittently. As a result, the liquid takes up some energy from the steam and vaporizes. Thereby it reduces the temperature of the system. The temperature and the flow rate of the incoming steam determine the influx rate from the nozzles. Because of the direct contact, this method is efficient in heat exchange. However, exposing the nozzle assembly to thermal cycles results in deterioration over time, leading to the sporadic release of water into the steam. Mitigating this requires the use of durable materials and mechanisms for the nozzle assembly.

Indirect Contact Attemperator

In the indirect contact attemperator, the heat exchange between the liquid water and superheated steam can occur via a solid boundary. Typically, this involves a shell and tube setup, where the steam runs through the tubes and the liquid water occupies the larger shell area.

Controlling the temperature of the desuperheated steam occurs by regulating the pressure or flow rate of the liquid water. Controlling the flow rate of the water is preferable on most occasions.

Although the indirect contact attemperator eliminates nozzle assembly challenges, it is less efficient. Also, the setup is bulkier, more expensive, and prone to scale buildup on its walls, which hinders heat exchange.

Indirect contact attemperator with steam in tubes and liquid water in shell
Courtesy: enggcyclopedia

Operation Parameters

No matter the application of an attemperator, there are two key parameters that serve to qualify its performance. First is its turndown, which describes the range of flow rates over which it can operate. Moreover, this turndown is a ratio of the maximum flow to the minimum flow and can be determined for the steam or the cooling water flow.

    \[ Turndown=\frac{Maximum flow}{Minimum flow} \]

Another important parameter is the mass flow rate of the cooling water (mcw). Its value varies according to the mass flow rate of the superheated steam (ms) per time. Both parameters are a function of the desired enthalpy of the desuperheated steam (hd), as well as those of the superheated steam (hs) and cooling water (hcw).

    \[ m_{cw}=\frac{m_{s}\left ( h_{s}-h_{d} \right )}{h_{d}-h_{cw}} \]

Attemperator in a Boiler

Generally, boilers have several stages of heating to take the feedwater temperature to a desirable level to achieve a purpose. After each stage, an attemperator reduces the steam temperature to allow intermediate pipes and ancillary equipment to handle the steam safely.

Typically, the goal is to take the steam to within 5 °F to 10 °F above its saturation temperature, depending on the system demands. Thus, a thermometer obtains the temperature of the steam after each stage of superheating, then injects liquid water accordingly. Modern boiler systems utilize either the spray type or indirect contact attemperator.

Attemperators in multistage boiler system
Courtesy: mdpi

Attemperator vs Desuperheater

Attemperators and desuperheaters are often regarded as the same because they largely perform the same function. Both serve to reduce the temperature of steam in a system either by direct or indirect contact with liquid water. However, there are a few aspects that distinguish between both as the table below highlights.

Generally, attemperators operate over a wider range of pressure and temperature. So they have a larger turndown ratio.Temperatures and pressure ranges associated with desuperheaters are limited.
Only the introduction of liquid water is used in reducing steam temperature.Some versions make use of a venturi tail to reduce the temperature of steam.
The target of its temperature control could be either to increase or reduce. Although, it serves to reduce temperature in most cases.Desuperheaters target only reduction in steam temperature.
Attemperator vs Desuperheater

Attemperator Valve

Attemperators use control valves because to deliver a precise amount of water to achieve specific temperature changes. In indirect contact attemperators, the valve function is simply to alter the cooling water flow rate to suit steam requirements per time. However, the demands are more in the direct contact system, as the valve’s parts interface with the steam. This exposure to thermal cycles makes the valve prone to failure, especially its moving parts. So, the use of superior materials that is wear and erosion resistant is necessary, as well as having a predictive maintenance strategy. Despite this downside, the direct contact system is preferable in most systems as it delivers superior efficiency. Typically, this valve system uses multiple nozzles to atomize the introduction of cooling water. Also, these nozzles could be placed in a variety of angles to alter the cooling effect.

Valve nozzle and diffuser of a spray type attemperator
Courtesy: cncontrolvalve

When in operation, the control valve varies the inflow of cooling water according to the sensor measurements of steam flowrate and temperature. Then the diffuser sprays this water into the shell to lower the steam temperature. If the system is operating a low load condition, the steam influx reduces as well as the cooling water. In addition, some of the nozzles are shut off to avoid excessive reduction of the steam temperature.