FEED TANK and HEATER (HOTWELL)
The next item added to this system is the hotwell or feed water tank.
The hotwell ideally should be heated as hot as
practically possible to drive off any dissolved oxygen and the
dissolved solids should also be as low as possible as these concentrate up in
the boiler.
The Totally Dissolved Solids (TDS) content are calculated from the make-up
characteristics and the condensate
returns (see later for a description of this).
This program allows the engineer to evaluate the economies of fitting make-up water pre-treatment systems.
Many feed tanks have a level switch, when the
water falls below this level then water is added from the make up tank.
this water is generally cold and has the effect of lowering the hotwell
temperature and so increasing the dissolved oxygen
content. The program calculates the temperature drop and turns on hotwell
heating to counter the effect.
The feedwater heater parameters and size of the
hotwell , make-up switch level are configurable from the Configure Feed Tank
and Heater menu.
CONDENSATE RETURNS
The next item to be added are the Condensate returns
The user can set the returns rate and any
residual TDS in the condensate, this is added to the feedwater
parameters in the calculation. The program estimates the condensate chemicals
demand (amines) and
then calculates how much is returned in the condensate lines to the hotwell.
Amines are costly and their demand is related to the alkalinity of the make-up
water.
FEED PUMP SUCTION HEAD
CALCULATOR
With hotwater in the feed tank there is only a certain temperature that the
pumps will operate at before the pumps
cavitate. A useful calculator has been included here, when next is clicked the
following appears
Varying the suction head the results box indicates the maximum feedwater
temperature that can be achieved
before the feed pumps cavitate, it also estimates the amount of dissolved oxygen
that may be present in the
feed water.
Given that it takes approximately 8 mg of sodium sulphite (one common oxygen
scavenger) to neutralise 1mg
of dissolved oxygen and solutions are approx 30% strength. then it does not take
too long to realise that at 86°C
you need 58mg chemical per litre of water being fed to your boiler.
A 3.5 metre head on the other hand allows the feed water to be kept at 95°C with
only 0.9mg/l of dissolved oxygen
so you only need 24mg of oxygen scavenger - the oxygen scavenger chemical demand
has reduced by 60% by just
raising the feed tank by 2 metres.
This little calculation is so often overlooked that the authors have decided to
include it to encourage better design.
Please note the calculator figures do not take part in the models simulation; it
is in place for the training aspect of the software.