The above example is typical of a modern boiler plant using base exchange softening only. Blowdown rates are much lower when de-mineralized feedwater is used. In the example, the heat loss is equivalent to 1.8% of the fuel fired.
Operated continuously over a year the fuel wasted per boiler represents approximately 46,500 m3 of natural gas, 44,500 litres of fuel oil or 70 tonnes of coal. Added to this is also the cost of acquiring and treating the water that is used within the boiler system.
Blowdown control can be broken down into instantaneous or continuous systems and may be manual, semi-automatic or fully automatic.
Instantaneous manual system
The simplest implementation of blowdown control is an instantaneous manual system that is operated once per shift to reduce the boiler total dissolved solids (TDS) to a sufficient level well below the boiler specified maximum limit. The TDS are then allowed to build up during the next shift until they reach the maximum level again.
A TDS test should be carried out prior to blowdown so that the time can be adjusted to reflect changes in average boiler load conditions which may occur on a day-to-day basis.
Automatically timed control
Figure 2 (above) shows a simple semi-automatic system where a timer is used to control blowdown for short periods according to a pre-set schedule. Again, with this system, daily testing of the boiler is necessary so that the timing schedule can be adjusted to take into account changes in boiler and system operation.
The system can be made fully automatic by installing a TDS monitoring facility as pictured in Figure 3. This will override the timer in the event of variation from the desired TDS level.
Continuous blowdown systems are preferable where heat recovery is required. In its simplest form, such a system consists of a valve, adjusted after regular boiler water testing. The valve position is determined from the boiler pressure, TDS levels and the blowdown rate required.
As shown in Figure 4, a control module is used to modulate the blowdown valve using inputs from a TDS detector located in the cooled blowdown sidestream. For this system to operate correctly, cooled blowdown must flow continuously over the detector.
Blowdown can also be achieved in the boiler evaporators where sediments are deposited. This process is carried out intermittently by opening the appropriate valve and allowing the sediments to be flushed out.
Eurotherm Process Automation offers a control module that can be configured for continuous, intermittent or both continuous and intermittent blowdown control.