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Using
Demand
Expanders
in Compressed
Air
Systems
by: R. Scot Foss, Plant Air Technology
Increasing the amount of
air in a system doesn't have to mean adding compressors.
Artificial demand is the excess volume of compressed air created for
unregulated users as a result of supplying higher line pressure than
necessary. It includes all unregulated consumption, including
appropriate and inappropriate production usage, open blowing, leaks, and
points of use with regulators adjusted to their maximum setting. These
applications track the supply pressure as though there were no
regulators being used.
As the supply pressure fluctuates, artificial demand changes from a
minimum to a maximum waste level. When real production demand decreases
and pressure rises, artificial demand increases. Eliminating leaks
causes the pressure to rise and all unregulated demand increases in
proportion to the pressure rise, including remaining leads.
Many systems have as much as 80% of their total volume uncontrolled.
This condition results from regulator use based on the recommendations
of equipment manufacturers. Most plants have regulators on 50% of their
use points, which normally represent only a small portion of the total
volume. Since little care is used in the selection of regulators and
filters, they frequently have high pressure drops and require high
settings.
Operators increase pressure to improve equipment performance. When an
operator can no longer elevate the pressure, the supply pressure limit
of the system has been reached. At this point, the application follows
supply pressure. The volume required is artificial demand and can
represent 10% to 25% of total compressed air used.
A demand expander can correct these problems, when adjusted to the
minimum required system pressure. It is a main line valve that controls
the maximum pressure at which demand air can be removed from the system
(Fig. 1).
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Fig 1
A Demand Expander with two expansion paths has a
backup valve to prevent a compressed air system shutdown.
(Courtesy APT, Inc.) |
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Expanders maximize air compressor and system efficiency by separating
the supply side (compressors) from the demand side (users). They are
designed to provide for the expansion of compressed air from storage to
the system with a minimum loss of energy; results are a consistently
lower plant air pressure. As pressure is lowered in the piping system,
all unregulated flows and leads are reduced. The pressure dew point is
also lowered as a result of expansion.
Expanders require little supply energy to function properly - as
compared to a regulator that can require 5% to 10% of the system's input
energy to overcome resistance to flow. It is a precise control device
that has a control and response sensitivity within tenths of a psig.
Using an expander allows storage to be maintained in the upstream supply
system for handling variations in demand, rather than utilizing
compressor power (fig. 2)
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Fig 2
Compressed air storage is
maintained in the upstream supply system by a demand expander and
subsequently released as required by various plant processes. |
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The expander allows compressed air to be stored on the upstream side and
be instantly available to demand at the lower, downstream control
pressure. Rather than operating the system at an elevated pressure all
the time, preventing an event from dropping pressure below an acceptable
level, storage is maintained for this purpose while not activating
another compressor. Amount of available storage is a function of vessel
size and useful pressure differential.
On the supply side, compressors that once served to dynamically meet
every change in demand, now can serve storage. Compressor controls can
be adjusted to a design point maximizing efficiency. Changes in demand
do not result in immediate compressor activity. With proper compressor
management, motor starts can be minimized and one or more compressors
could be shut down.
Without an expander, demand is the same as supply pressure. Ideally,
there are no leaks, everything is regulated, and all regulators are set
at the minimum required pressure. However, this is not realistic.
Central or sector control of maximum demand pressures is far more
reliable than hoping that all production and maintenance personnel are
diligent in the installation and use of air operated equipment. If there
are a number of use sectors that need different pressures, an expander
can be used for each area to minimize the system's energy requirements.
When a new use of air comes on line (demand event), it can elevate
operating pressures and generate a great deal of artificial demand. The
excess of demand over supply energy is expressed as negative cfm. Until
the supply system responds to the event, the air required is taken from
the demand piping system. This action causes pressure to drop.
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Fig 3
Pressure decay from a demand event
is greater at the point of use and causes a delay in system
response, something a demand expander minimizes. |
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The effect of pressure decay is greater at the point of use and
diminishes moving toward the supply (Fig. 3). The decay continues system
wide until the supply adjusts. Next, the system assumes a positive rate
of change until the event and the air removed is replaced, pressure
comes back to the original control point, and a neutral point is again
reached.
The supply never sees the pressure drop that occurs at the point of
application, but users in the vicinity of the event do. The magnitude of
the differential is a function of the size of the event, distance from
the supply or transmission speed, amount of storage in the system, and
response time of the supply equipment controls.
The usual way of dealing with this problem is to raise production
pressure at the point of use so it never drops below an acceptable
level. This approach involves adding compressors to achieve this
elevated pressure, and significantly increases the artificial demand
volume in the system. 'The compressor room only knows it has to run the
system at a higher pressure to avoid phone calls from production.
This problem is usually rationalized as pressure losses in the piping.
If the piping system is reworked, it does not improve the situation
other than provide a minimal increase in storage. If the compressor
controls must see a 1 psig drop to respond, pressure at the point of use
can drop more than 5 psig. If the supply must drop 2 psig for a supply
control response, the demand event may see between 5 and 10 psig drop.
If an expander has a 0.1 psig response range, the event would only drop
between 0.1 and 0.5 psig. An expander would see the initiation of the
event an allow control storage to stop system decay. Control storage
should be large enough to limit upstream pressure drop while a
compressor is started up and to prevent it from loading, if the event
duration is short. Responsiveness of an expander determines production
side pressure fluctuations.
Why are expanders designed to maintain small pressure range
differentials, typically 0.1 to 0.2 psig? Many systems, prior to using
an expander, fluctuated more than 15 psig at the point of use. The small
pressure range controls the response of the expander to events that
occur in the system. An expander should open to allow control storage to
stop decay at the other end of a system regardless of the distance.
Because of this response, demand can always be operated at the lowest
required pressure. This figure requires less power and provides support
for the largest system events with minimum artificial demand and the
tightest possible control of pressure to production. Supply corrects to
a neutral rate of change to events that occur and production sees no
change in pressure.
Aside from the accuracy maintained in the production piping system,
operating costs should drop. In a supply-controlled system, support of
an event diminishes volume in the system until a compressor responds.
The compressor must not only match the added demand event, it must also
replace the lost storage in the system. The time between the event
beginning and the response occurring determines how much volume must be
replaced.
Typical pressure-only controls on compressors respond as fast as they
can. The next available compressor or compressors bring the system back
to the unload pressure. If the supply capacity is twice that of the
event, response is fast.
An electrical peak is created and rapid cycling of the compressors
begins. A typical solution is to turn too many compressors on and put
them in modulation. This approach stabilizes system pressure but
increases operating costs. Automated operation in conjunction with an
expander can use a pressure, rate of change, and time protocol to limit
the response to an event volume and not require any added compressor
capacity.
Reprinted with permission from R. Scot Foss, president of
Plant Air Technology, Charlotte, N.C., a company specializing in system
auditing and design. This article is based on his book, "Compressed Air
System Solution." A portion of the proceeds from sales of the book is
donated to children's charities. To order a copy of the book, please
contact Southern Corporation. |
