9 Heat Flow
04.09.2016 Mk8 MM Expansion Features Installation and Set-Up Guide Page 133
Example 2: Steam with Deaerator
A common practice in steam generation is the use of a “deaerator” to remove the oxygen from the
feed water and hence reduce the incidence of oxygen corrosion in the boiler, steam and condensate
pipe work.
The principle of a deaerator is to mix the make-up water with the condensate return and live steam
direct from the boiler in a tank. Flash steam may also be directed back to the tank. The effect is to
mechanically “scrub” the oxygen from the feed water and also to preheat it before it is pumped to the
boiler.
In the system as set out above it is no longer valid to measure the temperature of the feed water just
before it enters the boiler as the inlet temperature for the “steam meter” calculation. This water has
already been preheated by steam from the boiler and therefore this additional energy should not be
taken into the software calculation.
The solution is to treat the boiler and deaerator as one system. The energy into the system is supplied
by the burner and the inlet temperature is the “weighted average” of the condensate return
temperature and makeup water temperature. The outlet steam temperature is measured by a
temperature sensor in exactly the same way as in a system without a deaerator.
1: First the percentage “Make up“ in the “Feed water” must be calculated.
Where %Mu = % Cold make up water
V1 = Volume flow rate of condensate return water
V2 = Volume flow rate of cold make up water
%Mu
100xV2
V2V1
Example: 1
Steam boiler with a volume of condensate return at 40 GPM and make up water at 8 GPM
Makeup%
100x8
840
16.7%
To calculate the second part to establish the “Weighted Average Temperature” the following equation
is used.
Where Tave = Weighted Average temperature
T3 = Temperature of condensate
T1 = Temperature of make up water
%Mu = Percentage of make up water
Tave T3
%MuxT1AT1
100
Steam boiler with condensate return temperature of 176°F and a make up water temperature of
41°F. From the above example (1) the make up percentage is 16.7%
WeightedAverage 176
16.7x176–41
100
153.4°F
To implement the above control form the following calculations have been imbedded in the revised
software to obtain the “Weighted Average Temperature” (T ave).
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