Deaerator Steam Demand and Vent Rate

A deaerator heats feedwater to saturation to drive out oxygen, and the steam it takes to do that is a real load on the boiler that plants routinely leave out of their steam balance. The heating steam is a mixing calculation: enough steam condenses into the feedwater to bring it from whatever temperature the condensate and makeup arrive at up to the saturation temperature for the operating pressure -- about 227 degF at 5 psig, 212 degF atmospheric. Because it is a mixing calculation, the colder the incoming water the more steam it takes, which is why condensate return moves this number directly. Returning 60% of a 25,000 lb/hr feedwater flow at 190 degF against 40% makeup at 60 degF puts the mixture at 138 degF and calls for about 2,318 lb/hr of steam, 9.3% of throughput; raise the return to 80% and the mixture is 164 degF, the demand falls to 1,641 lb/hr, and 677 lb/hr of steam stops being raised at all. Condensate return pays twice -- once for the water and the treatment it does not need, and once here. One caution on valuing that saving: a pound of steam costs the boiler the enthalpy rise from its FEEDWATER, and the feedwater is already at the deaerator's saturation temperature, so charging deaerator steam all the way from the makeup temperature double-counts heat the deaerator is what supplies. The vent is the part that gets mis-set, and it fails in both directions. Vented too little and the non-condensables the deaerator has just liberated have nowhere to go, so they stay in the water and an expensive vessel is doing nothing while corrosion continues downstream. Vented too much and usable steam goes to atmosphere continuously. The correct setting is a small steady plume, typically a few tenths of a percent of throughput, and the point of putting a dollar figure on it is to make it a deliberate choice rather than a valve someone cracked. The consequence of getting deaeration wrong is not energy, it is boiler tube and condensate line corrosion, which is why the vent is never closed to save steam. This does not size the deaerator, its storage section, or the pegging steam control, and it does not establish whether the unit achieves its rated oxygen removal, which is a dissolved oxygen measurement rather than a calculation. It does not address the net positive suction head available to the boiler feed pumps, which the deaerator's elevation and operating pressure govern and which is the usual reason a feed pump cavitates. It does not cover chemical oxygen scavenging, which is required regardless because mechanical deaeration alone does not reach the required residual. Steam plant operation is a licensed activity in many jurisdictions: the deaerator manufacturer, the water treatment program, ASME, and the jurisdiction's boiler inspector govern.

mixed incoming temperature = condensate fraction x condensate temperature + (1 - fraction) x makeup temperature; heat = feedwater flow x (deaerator saturation temperature - mixed temperature); heating steam = heat / latent heat at the deaerator pressure; vent steam = throughput x vent fraction.

The deaerating feedwater heater mixing heat balance by name. Saturation temperature, latent heat, and steam enthalpy are entered from the steam tables for the operating pressure. ASME, the deaerator manufacturer, the water treatment program, and the jurisdiction's boiler inspector govern.

A mixing heat balance on figures the user enters; no steam table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: feedwater_lb_hr, condensate_fraction, alt_condensate_fraction, condensate_temp_f, makeup_temp_f, da_saturation_temp_f, latent_heat_btu_lb, steam_enthalpy_btu_lb, vent_fraction, boiler_efficiency, fuel_cost_per_mmbtu, hours_per_year, mixed_temp_f, heat_required_btuh, heating_steam_lb_hr, heating_steam_pct, steam_saved_lb_hr, vent_steam_lb_hr

• Steam is valued from feedwater at saturation not from the makeup temperature, which double-countsboiler heat balance
• Oxygen removal is measured, not calculated dissolved oxygen measurement establishes performancewater treatment practice
• Chemical scavenging is required regardless mechanical deaeration alone does not reach the residualthe water treatment program