De-superheating is the process of reducing the temperature of superheated steam. This is typically achieved by injecting a cooling medium, such as water, into the steam flow. The nozzles used in the desuperheating process can be classified as either variable nozzle or fixed nozzle de-superheaters. Here's

The differences between these two types are:

Sl No.	Variable nozzle de-super heater	Fixed nozzle de-super heater
1	Variable nozzle desuperheating systems have adjustable nozzles that allow for controlling the flow rate of the cooling medium injected into the steam flow.	Fixed nozzle desuperheating systems have non-adjustable nozzles, meaning the cooling water flow rate and the degree of desuperheating are fixed
2	The nozzle opening can be adjusted to vary the amount of cooling water injected, thereby controlling the degree of desuperheating and achieving the desired steam temperature.	Separate control valve is required to adjust the water flow
3	More flexibility in adjusting the cooling water flow rate and achieving precise temperature control.	Not much accuracy in temperature control
4	Variable nozzle desuperheating systems are often used in applications that require tight temperature control,	USed where there is much tolerance in temperature control, Ex: In process industries They are commonly used in applications where a constant degree of desuperheating is sufficient, such as in industrial processes with steady steam loads or in small-scale power plants.
5	Complex design	Simple design
6	More costlier than fixed nozzle de-super heaters	Less costlier
7	Little bit complicated operation	Simple operation
8	Can be used for variable inlet flow & temperature	Used only for fixed flow & temperature
9	Size of nozzle is variable	Size of nozzle is fixed
10	Maintenance is difficult & costlier	Maintenance is simple & cheaper

The choice between a variable nozzle and fixed nozzle desuperheater depends on factors such as the required temperature control accuracy, steam flow variability, plant operating conditions, and budget considerations. Variable nozzle desuperheaters are often preferred in applications where precise temperature control and flexibility are crucial, while fixed nozzle desuperheaters can be suitable for applications with relatively stable operating conditions and lower cost requirements.