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Boiler water is blown down to control the amount of total dissolved solids (TDS) in the boiler. This water is pressurised, hot and dirty, creating large volumes of flash steam and possible disposal problems. A heat recovery system can reclaim large amounts of energy during this essential process.

Heat Recovery from Boiler Blowdown (TDS control only)

The previous module discussed the water to be blown down from a boiler in order to maintain an acceptable TDS level. This water has a number of characteristics:

• It is dirty - This means that:

- The water is generally unsuitable for other applications.

- The dirty water may present a disposal problem.

• It is hot - This means that:

- A proportion of the water will flash to steam at atmospheric pressure.

- The hot water may present a disposal problem. For example, there may be a substantial quantity to dispose of.

A heat recovery system can solve many of these problems.

Energy flowrate in blowdown

Using the data from the blowdown calculation, Example 3.12.5, the amount of energy sent to blowdown can be calculated using the steam tables.

Note: 1 kJ/s = 1 kW

Flash steam

The blowdown water released from the boiler is water at the saturation temperature appropriate to the boiler pressure. In the case of the boiler in Example 3.13.1 - 10 bar g, this temperature is 184°C. Clearly, water cannot exist at 184°C under atmospheric conditions, because there is an excess of enthalpy or energy in the blowdown water.

Assuming the blowdown water is released to a flash steam system operating at 0.5 bar g, steam tables may be used to quantify this energy excess:

This excess energy evaporates a proportion of the water to steam, and the steam is referred to as flash steam.

The quantity of flash steam is readily determined by calculation or can be read from tables or charts.

The specific enthalpy of evaporation at 0.5 bar g (hfg) from steam tables is 2 226 kJ/kg.

Therefore 14.1% of the water blown down from the boiler will change to steam as its pressure drops from 10 to 0.5 bar g across the blowdown valve.

There are two options:

1. Vent this flash steam to atmosphere via the blowdown vessel with the associated waste of energy and potentially good quality water from the condensed steam.

2. Utilise the energy in the flash steam, and recover water by condensing the flash steam. It is useful to quantify the energy flowrate in the flash steam. This can be done using steam tables.

Example 3.13.3 ​

Compare this to the 241 kW rate of energy blown down from the boiler.

It may be possible to use this flash steam: in this example it represents approximately 49% of the energy flowrate in the blowdown, and 14.1% of the water blown down.

Using values from steam tables for the above calculations assumes that feedwater will be supplied at a temperature of 0°C. For greater accuracy, the actual change in feedwater temperature should be used.

Recovering and using flash steam

The flash steam becomes available for recovery at the flash vessel. In essence, a flash vessel provides a space where the velocity is low enough to allow the hot water and flash steam to separate, and from there to be piped to different parts of the plant.
The design of the flash vessel is important not only from a steam/water separation point of view, but structurally it should be designed and built to a recognised pressure vessel standard, such as PD 5500.
This is not only good engineering practice, the boiler inspector will also insist upon this if the plant is to be insured.
The most obvious place for the flash steam to be used is in the boiler feedtank, which is usually nearby.
The water temperature in the feedtank is important. If it is too low, chemicals will be required to de-oxygenate the water; if it is too high the feedpump may cavitate. Clearly, if heat recovery is likely to result in an excessive high feedtank temperature, it is not practical to discharge flash steam into the tank. Other solutions are possible, such as feedwater heating on the pressure side of the feedpump, or heating the combustion air.
Figure 3.13.2 shows a simple installation, which makes recovery of the 97 kW of energy flow, and 157 kg/h of boiler quality water, extremely cost effective.

Example 3.13.4

(continuing from Example 3.13.3)

A typical arrangement for recovering this energy is shown in Figure 3.13.3.

A problem with the arrangement shown in Figure 3.13.3 is that the simultaneous flow of incoming cold make-up water and residual blowdown from the flash vessel may not be guaranteed.

One preferred arrangement is shown in Figure 3.13.4, where a cold water break tank is used as a heat sink. A thermostat is used to control a small circulating pump so that when the residual blowdown is at a high enough temperature, water is pumped through the heat exchanger, raising the average tank temperature and saving energy.

If the temperature of the blowdown effluent exiting the heat exchanger can be above 43°C, then it should be directed to the blowdown vessel rather than straight to the effluent drain (See Module 3.14).

Preferred type of heat exchanger

Plate heat exchangers are preferred for this application, as they are very compact and easily maintained.

Experience shows that the higher velocities and turbulence in plate heat exchangers help to keep them clean, and hence dismantling is rarely required. However, should cleaning be required, it is relatively straightforward to open the heat exchanger and clean the plates.

The cleaning of a shell and tube heat exchanger is more complex, and will involve a complete strip down and often the tubes themselves cannot be removed for cleaning.

When energy is recovered from the flash steam and the condensate, 82% of the total energy contained in the original blowdown has been recovered.

In addition, 14% (by mass) of the water has been recovered, making a further contribution to savings.