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10-10
Page 28
2008-10-01
2. STEAM ABSORPTION AND DISTRIBUTION
A. VARIABLES THAT AFFECT ABSORPTION DISTANCES
(1) This manual will help in the design of a steam distribution system to provide the
optimum performance from the humidifier and absorption of steam within the duct
system.
(2) Experience has shown that absorption of steam is affected primarily by the
psychrometric conditions of the air based on how much residual capacity at a given air
volume and temperature exist to hold the moisture being introduced.
(3) To properly introduce steam into an air stream, five variables must be known:
(a) Temperature (°F or °C). If the air temperature varies, pick the lowest temperature.
(b) Highest humidity level before the distributor (%rh).
(c) The amount of moisture being added resulting in a change of rh in the duct or
plenum (
Δ %rh).
(d) Minimum velocity of the air (FPM).
(e) The size of the duct or plenum (cross section area).
(4) As a guide, Table 8 shows how each variable affects the absorption distance.
(5) Table 9 shows examples of typical absorption distances in air handling systems.
(6) Referring to Figure 3, ‘D’ can vary between 1 to 20 feet depending on the airflow’s:
(a) Velocity (V)
(b) Temperature (T)
(c) Humidity (H)
(7) When (V) is 500 fpm, (T) is high (90 to 130°F) and (H) is low (10% to 30% rh) the ‘D’ will
be 1 to 2 feet. As the (H) goes up ‘D’ becomes longer. Similarly, as (T) goes down, ‘D’
goes up. High velocity also increases ‘D’ by moving the steam further down stream
before it can mix. Multiple distributors will shorten the steam absorption distance.
Table 8. Variables That Affect Absorption Distances
Variables
Effect On Absorption Distance
(NOTE: ↑ Increase ↓ Decrease
Air Temperature ↑
↓
Air Temperature ↓
↑
Air Humidity ↑
↑
Air Humidity ↓
↓
Large Increase of RH
↑
Small Increase of RH
↓
High Air Velocity (1000 fpm or above)
↑
Low air Velocity (between 100 and 500 fpm)
↓
Small Duct or Plenum (short distributor)
↑
Large Duct or Plenum (long distributor)
↓