A Feedwater Treatment and Condensate Handling System is required with any boiler to properly prepare the inlet feedwater. It should consist of the feedwater (or condensate) receiver, water softener system, chemical injection system and blowdown tank with a cooling water valve. The type of receiver used depends on several factors. The most important is the amount and temperature of the condensate returned from the factory’s steam system.
There are three broad categories of systems:
An Atmospheric or “Open” Deaerating System is normally used when low-temperature condensate is returned from the steam system or a 100% makeup system. The feedwater receiver is a non-pressurized tank (or “hotwell”) that is vented to the atmosphere. Softened makeup water is added as required by means of a level controlling device mounted on the hotwell. Steam and chemicals are mixed in the tank with the makeup water, returned condensate and generator trap returns prior to being fed into the steam generator. A combination of preheat and chemical treatment is used to remove dissolved gases and oxygen from the feedwater.
Among the advantages of this type of feedwater tank are a relatively low cost and the ability to be quickly brought up to operating temperature. It also has a minimal amount of trim and external components.
Pressurized Deaerators are employed in systems to preheat the feedwater and to remove non-condensable gases through a combination of heat and agitation. The primary difference between a “hotwell” and a “deaerator” tank is that the deaerator is a pressurized vessel and includes internal devices to create agitation that aid in the oxygen removal process. The heat is supplied by return condensate, generator trap returns, low pressure steam or a combination of all three. This occurs while agitation is accomplished by means of sprays or cascading tray arrangements. Deaerators typically operate at pressures of 5 psig or higher and can offer a substantial reduction in oxygen scavenger chemical usage compared to hotwells. This depends on the amount of makeup water used in the system.
The advantages of a pressurized deaerator include its increased ability to utilize heat returned from the steam system and reduce chemical usage. Its disadvantages are its higher capital cost, more extensive trim components and longer heat-up time requirements.
In systems where there are large amounts of high temperature condensate return, a “semi-closed” receiver (SCR System) is used. Two receivers are used with this type of system. High pressure and high temperature condensate and the generator trap returns are routed back to the pressurized SCR tank to minimize the loss of valuable heat. The Steam Generator gets its feedwater directly from this tank. Raw makeup water and low temperature condensate are fed to a second atmospheric “makeup” tank (a “hotwell”). Then it is transferred via a high pressure transfer pump to the SCR tank.
Clayton can offer all three types of feedwater systems as required by the application. The components can be provided as individually or skid mounted, factory piped and wired packages.