Optimization Efficiency on De-Coupler
1.
Background
The all-variable speed technologies offer substantial energy use reductions and also extend the life of
the plant's chillers while lowering their maintenance requirements.
However, in many large cooling systems, the chilled water distribution system poses a much more
immediate problem to overall cooling system performance and efficiency. This is due to many chilled
water systems fail to attain their design differential temperature (∆T), loads at the end of the distribution
system may be starved at peak periods, while at the same time the chiller plant is not able to utilize its
full design capacity.
Many "fixes" worsen the problem by raising the chilled water supply temperature to loads which reduce
their latent cooling capacity and result in an endless stream of complaints. At some facilities, the
inability to solve nagging distribution problems has undermined the integrity of the entire central plant
and new approaches for cooling are being considered by unhappy end-users.
Truly effective solutions to such problems are relatively straightforward, and extending all-variable
speed principles to the chilled water distribution system facilitates such solutions. I would like to outline
how all-variable speed technologies can be most effectively extended to upgrade chilled water
distribution systems for better performance.
2.
Distribution System Problems
Though chilled water distribution systems vary enormously in sizes and configurations, the problems
associated with these systems are quite universal:
•
Low deltaT,
•
Inability to fully load chillers,
•
Inadequate flow in sections of the distribution system, and
•
Excessive pumping pressure requirements at peak cooling demand conditions.
Some of these problems plague nearly all chilled water distribution systems. Figure-1 shows a typical
primary-secondary variable flow distribution system.
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In smaller systems, the primary loop and the secondary distribution pumps may all be located in the
plant. In a large building complex, the primary (or a secondary) loop may extend throughout the
campus and secondary (or tertiary) distribution pumps are typically located in the individual buildings
served by the distribution system. Actual configurations may have more or fewer distribution circuits
and usually will have multiple pumps at each pumping station. However, the lessons discussed here are
generally scalable and are easy to apply to a wide variety of distribution systems.
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Figure 1: Typical Primary-Secondary (De-coupler) Chilled Water Distribution System Configuration.
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In Figure-1, the primary chilled water pumps (PCHWP1 – 3) are mainly always constant speed pumps
and the secondary chilled water pumps (SCHWP1 – 3) are variable speed pumps.
The primary pumps are cycled on and off with the chiller each serves, and the speed of the secondary
pumps is modulated to meet a differential pressure set-point as measured at the end of the distribution
circuit each serves. A decoupling line shown at the lower right end of the figure permits flows in either
direction at the end of the primary circuit since the "stepped" primary flow will likely always be different
from the continuously variable secondary flow.
This system is widely employed, but has two inherent problems that lead to low ∆T and poor
performance:
i.
When primary flow is greater than secondary flow, low ∆T in the primary circuit results
from the re-circulating primary chilled water through the decoupling line and directly
back to the chillers. The lower than expected return chilled water temperature makes it
impossible to fully load the online chillers because the primary pumps are fixed flow. This
wastes energy and if it occurs at peak conditions, it affects the plant performances.
ii.
Whenever secondary flow exceeds the primary flow, the flow reverses in the decoupling
line and is mixed to degrade the supply water temperature. This reduces the cooling
capacity of the loads in distribution circuits closest to the decoupling line. The result is
greatly increased flow in those circuits and reduced ∆T, which also affects the system of
its full capacity capabilities.
Due to primary and secondary flow is rarely exactly balanced and actual ∆T always varies somewhat
from design, one of the two problems is almost always at play in such systems, both of which can
reduce the design ∆T of the system and both of which make it difficult to operate the system effectively
at full capacity.
Numerous solutions have been proposed to correct this problem, but such "cures" often destroy the
system's ability to meet the cooling load requirements. One popular method of correcting low
secondary circuit ∆T problems is shown in Figure 2.
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Figure-2: Diagram of a Typical Delta T "Enhancement"
While the Figure-2 diagram, or some variation of it, is often touted as a cure for low ∆T, it much more
often has disastrous effects on system operation.
The idea is that the diverting valve on each secondary (or tertiary in some cases) circuit return
(sometimes a mixing valve is used on the chilled water supply) will modulate some return water back
to the pump anytime the return temperature is below design. It is reasoned that the elevated supply
temperature will raise the return temperature and ensure that the design ∆T from the circuit is
maintained at all times.
However, this fix rarely has the desired results. When air is the medium being cooled, return chilled
water temperature is much more affected by entering air temperature than chilled water supply
temperature. Raising the chilled water supply temperature thus has little effect on return chilled water
temperature, but it does profoundly reduce cooling coil capacity, especially latent cooling capacity.
As the supply chilled water temperature rises, load valves open further and flow in the circuit increases
dramatically, often without a significant increase in the return water temperature and usually with a
reduction in cooling effect. Thus, when the scheme shown in Figure 2 is installed on a distribution
circuit, one poorly operating load in the circuit can severely compromise the capacity of all loads in
the circuit.
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In large systems, it is also possible at times to have the flow reversal such that return chilled water from
the mains travels to the supply header through the diverting or mixing valve. Thus the Figure 2 "fix",
and the many schemes that are similar to it, do not fix system operation at all. Instead, it is a "poison
pill" to the chilled water distribution systems.
3.
Getting Real About Low ∆T
So what is the solution to low ∆T problems? To configure a successful solution we must recognize what
helps and hinders ∆T. DeltaT problems are sometimes caused by the designs themselves which may
include added bypasses and three-way valves scattered through the system to keep water moving at
low load conditions. Solutions that involve mixing return water with supply water undermine the
thermodynamic efficiency of the system, destroy the capacity of the coils to meet their loads, and add
further to low ∆T problems. To solve the types of distribution problems that lead to low ∆T, the design
or retrofit needs to follow these rules:
a)
Eliminate all possibility of direct mixing between chilled water supply and return – This
means eliminating all decoupling lines and three-way valves for chilled water system if
feasible. In this era of networked DDC and variable speed control, pumping systems may
not require decoupled. Furthermore, modern chillers accommodate varying flows over
substantial ranges without any loss of efficiency or operational stability. By selecting
equipment wisely, it is not difficult to design "all-variable speed" chilled water generating
and distribution systems without any mixing so that every bit of supply water must pass
through a load before returning to the plant and supply chilled water at design
temperature is available to all loads at all times.
b)
Focus ZERO Surplus and/or Deficit flow at De-coupler Line between primary and
secondary circuits – This means to maintain the primary chilled water loop and secondary
loops to matches its flow demands by varying or modulate the PCHWPs to keep the
decoupling line at zero surplus and/or deficit flow at all time. This is possible by making
use of the flow meter installed along the de-coupler line to control the VFD of the
PCHWPs as shown in Figure 3. Thus, by implementing this, the mixing issues of supply
and return chilled water will be mitigated and low ∆T will be controlled.
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4.
Configuring The Solution
Figure-3 shows a system incorporating the above points that ensure every load will be satisfied and
guarantees that the design ∆T is maintained at all times.
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Figure-3: "All-Variable Speed" Primary-Secondary De-coupling Chilled Water Distribution System Configuration with Network Controls
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Figure-3 is similar to Figure-1. Because there are decoupling lines in Figure-3, it is called an "all-variable
speed series Primary-Secondary system." Here are how the rules listed above have been implemented
to convert the conventional Primary-Secondary system to an all-variable Primary-Secondary and solve
the problems typically associated with distribution systems:
•
5.
Eliminate all possibility of direct mixing between chilled water supply and return – Notice
in Figure-3 that the decoupling line in the primary headers have been installed with an
in-line magnetic flow meter and the PCHWPs have been converted to variable speed
control. With a DDC network coordinating the primary and secondary pumps, the
pumping systems are maintaining zero surplus and/or deficit flow within the decoupling
line. Modern chillers easily accommodate the varying flows over wide ranges (depending
on chiller manufacturer), so varying the flow throughout the entire system as conditions
change works very well. The primary pumps operate with their respective chillers to
maintain matching demand flow in the primary distribution header as measured by an
in-line magnetic flow meter shown along the decoupling line by using varying and/or
modulating the primary pumps. The entire control concepts are similar to the all-variable
primary chilled water system.
Evaluation Costs and Savings for an All-Variable Speed Upgrade
The savings that can be expected from upgrading an existing decoupled primary-secondary distribution
system to an all-variable speed primary-secondary system could be quite substantial, the costs for such
an upgrade are also often justifiable from the savings. Adding variable speed drives to the constant
speed primary pumps and implementing new network control can be accomplished quite economically
in many systems. The energy economics may not be enormously but justifiable at first blush, the
potential benefits to the overall cooling system are. The main driving forces for retrofitting to an allvariable speed distribution system are:
i.
The desire to recapture chiller plant capacity that is being lost to low ∆T problems, or
ii.
The desire to correct distribution flow problems resulting from the low ∆T make it
difficult to properly cool some areas of the facility under certain conditions.
These are big problems for some facilities, and converting to an all-variable speed primary/secondary
chilled water distribution system can usually lead to enormous improvements at relatively low costs.
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However, such a conversion needs to be very carefully analyzed and designed to be sure the retrofit
does not introduce new hydraulic problems into the system and that the chillers and other plant
equipment maintain adequate minimum chilled water flow at all times. Cost is not generally a major
consideration for a primary/secondary upgrade, but a careful design process should be.
6.
Summary & Conclusion
Low ∆T problems are very widely experienced by chilled water distribution systems in operation today.
Many of the “fixes” that have been suggested to mitigate low ∆T offer no solution at all, only more
problems. However, reconfiguring such chilled water distribution systems as primary/secondary" allvariable speed" systems with decoupling lines will mitigate the low ∆T problems. Furthermore, such a
system can alert operators to potential problems at loads that are underperforming so that these
problems can be corrected before they adversely affect the comfort of the spaces served. With a
carefully developed design, an economical upgrade is often achievable that will greatly improve overall
cooling system performance.
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