Blowdown Losses. Some of the hot water from within a boiler—typically
between 2 and 10% of the feed rate—is drawn off (blown down) to remove contaminants
that would otherwise build up and form scale deposits. Fuel is required to heat the feed
water to the drum operating temperature and pressure. The blowdown flow must be made
up with colder feed water, and heating this makeup water requires additional fuel. This
can lead to reductions of 1% or more in boiler efficiency. Ways to reduce blowdown
losses are discussed in Chapter 17.
As noted earlier, olefins furnaces also generate significant amounts of steam.
Consequently, they too have blowdown streams—typically 2–3% of boiler feed water
flow. However, as the duty is divided between process heating and steam generation, the
impact of blowdown losses on overall energy efficiency in olefins furnaces is generally
less significant than it is for boilers. Many furnaces in other services (e.g., hydrotreater
furnaces in refineries) also generate limited amounts of byproduct steam, and they also
require blowdowns.