Control valves

Desuperheaters

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.

4.8.13

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 3oC to 5oC 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.

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.

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TI-P475-06 CH Issue 3