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(19) Adaptive Steam
Control
Horizon steam-fired chillers have added adaptive steam
flow control. The flow control algorithm directly controls
the rate at which steam enters the unit. Direct control of
steam flow guarantees that steam consumption will not
exceed the maximum desired flow rate under any
condition. This function replaces the start up demand limit
used on old absorption chillers. It provides superior
protection for marginally sized boilers because it is active
all the time, but at the same time does not interfere with
normal operation of the chiller. Steam flow control will
compensate for variations in steam supply pressure or
generator pressure variations at start up or low cooling
water temperatures. This gives the unit better carry-over
protection and improved crystallization temperature
margin control.
% Energy input Auto Command verses Energy Valve
Position
•
Steam flow control converts the non-dimension %
Energy input Auto Command (cycle report; second from
last item) to a required steam flow rate. Two factory
mounted pressure transducers measure steam supply
pressure and pressure drop across the steam valve.
Steam flow characteristics for all of the valve types and
sizes available for Horizon chillers have been
programmed into the software. Given steam flow rate
and pressure drop across the valve, the characteristic
equation for steam flow is solved for valve position. If
steam pressure fluctuates, the energy valve is
repositioned to maintain the desired energy input;
therefore, the energy valve mechanical position likely
will not always be the same for a given energy input
auto command. Energy valve position is indicated as
the last item in cycle report.
•
For a given valve opening position there is greater
steam flow when a greater differential pressure exists
than when less differential exists. Because the steam
flow rate will change with given pressures the % Energy
Input Auto Command must be used to determine the
firing rate. The valve position simply states physically
where the valve position is not the steam flow rate.
Field set-up of the unit is simplified because energy valve
end stops are automatically set by the inputs in the
machine configuration menu: unit size in tons, valve size in
inches, and valve type (V-ball or butterfly). Steam flow
control is possible because of the addition of two factory
mounted pressure transducers on the steam line. In
addition to flow control, steam pressure measurements are
valuable pieces of information to a service technician or
plant operator and are available on the clear language
display.
In service test, manual control uses steam flow control to
convert the energy input to a steam valve position. Steam
valve position in while in manual control will have demand
limit and carry-over protection active. Because steam flow
control is active, the valve position will adjust to changes in
steam pressure.