Adding Condensate Heater to Improve Boiler Plant Efficiency

Adding Condensate Heater to Improve Boiler Plant Efficiency

As in HRSGs, one may consider adding a condensate heater in case the makeup water flow is high enough as shown in Figure 3.20. A heat exchanger may also be used to lower the feed water temperature and preheat the makeup in a separate heat exchanger or a con- densate heater may be used as shown. One should use appropriate materials when water inlet temperature to condensate heater drops below the water dew point as discussed in Chapters 1 and 6. Another option is shown in Figure 3.20a, where a heat exchanger is used to lower the feed water temperature to the economizer thus preheating the makeup water, which enables reduction in deaeration steam.

110 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

% MCR

To stack

Firing rate

220–230°F

Min. Recirc.

TG0

To DA 45,000 LB/HR at ~150°F Scavenger

Flow control station TG1 300°F From DA

Feed pump 202,000 LB/HR at ~250°F Economizer

Fresh air FD fan

200,000 LB/HR, 900 PSIG, 865°F

FIGURE 3.19

Scheme to improve boiler efficiency. (Patented by Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

Heat exchanger

Condensate heater

FIGURE 3.20

Scheme to increase boiler plant efficiency using a heat exchanger and lower feed water temperature to econo- mizer. Note: (1) turbine, (2) deaerator, (3) HRSG, (4) mixing tank, (5) pump, (6) deaerator coil, (7) condenser, (8) heat exchanger, (9) condensate heater.

Steam Generators 111

TABLE 3.4

Effect of Lower Feed Water Temperature

Steam flow, kg/h

Steam pressure, barg

Feed water temp., °C

Blowdown,%

Exit gas temp., ±°C

Efficiency, % LHV

Efficiency, % HHV

Energy to steam, MM kcal/h

Fuel LHV, MM kcal/h

Example 3.2

A 90,740 kg/h (200,000 lb/h) natural gas–fired D-type package boiler is generating satu- rated steam at 27.6 barg (400 psig) with feed water at 110°C (230°F) in a cogeneration plant. Customer is using a lot of steam in the deaerator and wants to know if the feed water can be used to preheat the makeup water–condensate mixture entering the deaer- ator and thus lower the deaeration steam requirements and whether this scheme has any benefit to him. Customer thinks he can cool the feed water to 60°C and the makeup water mixture can enter the deaerator much hotter and thus save his energy require- ments. With 60°C feed water, there is no concern about water dew point condensation as natural gas is fired in the boiler with partial pressure of water vapor about 0.18. Water dew point corresponds to 57°C from steam tables.

The boiler performance was simulated with feed water at 110°C and at 60°C. Results are shown in Table 3.4. Note that the boiler size has not changed in these options—only the feed water temperature.

The scheme may be evaluated in a few different ways. If deaeration steam is taken from the boiler itself, then one can increase the boiler steam capacity based on deaera- tion steam requirement for each case and see the difference. But it is obvious that even though the fuel-fired is 3.8 MM kcal/h more, the additional energy recovered in the entire system due to the higher condensate temperature entering the deaerator will be 90,740 × 1.01 ×(110−60) = 4.58 MM kcal/h. Hence, the scheme is more efficient; one has to consider the additional cost of the heat exchanger and associated piping valves for cool- ing the feed water, but it is worth considering if an appropriate heat sink is available.