During steam turbine start-up, empty load and low load, the steam flow is small, which is not enough to take away the heat generating by up-draft friction in the low-pressure cylinder, thus the exhaust steam temperature rises and the exhaust cylinder will increase too. Super high temperature of the exhaust cylinder will causes bigger deformation of the cylinder and will damage the consistency of dynamic and static parts of the steam turbine, in  serious condition, it will cause the unit vibration or other accidents. Therefore in the low pressure cylinder, the water spray cooling device is provided. （js）