There are three stages to desuperheating: primary, secondary, and tertiary.
Spray water is admitted into the steam via the desuperheating nozzle. The nozzle may be mechanical or pneumatic.
Momentum of steam accelerates water droplets. This action breaks up the water droplets. The higher the velocity of the steam, the better the secondary atomisation.
Water droplets evaporate in steam while being transported. If the velocity is too low or the size of the water droplets too large, there will be water fallout. The process takes a certain critical time to complete.
To achieve the best primary desuperheating you need to:
- use variable area nozzles to maintain excellent spray pattern and fine constant droplet size regardless of water flow
- use a swirl chamber to improve the coverage of the spray pattern
- ensure even distribution over the total cross-section
- avoid multiple spray patterns recombining to form larger droplets
- accurately control spray water using a carefully chosen control valve.