Catalogue

RLC-PRC042D-EN 9
Application Considerations
Variable Flow in the Evaporator
An attractive chilled water system option may be a variable primary flow (VPF) system. VPF
systems present building owners with several cost saving benefits that are directly related to the
pumps. The most obvious cost savings result from eliminating the secondary distribution pump,
which in turn avoids the expense incurred with the associated piping connections (material, labor),
electrical service, and variable frequency drive. Building owners often cite pump related energy
savings as the reason that prompted them to install a VPF system.
The evaporator on the Stealth can withstand up to 50 percent water flow reduction as long as this
flow is equal to or above the minimum flow rate requirements. The microprocessor and capacity
control algorithms are designed to handle a maximum of 10% change in water flow rate per minute
in order to maintain ± 0.5°F (0.28°C) leaving evaporator temperature control. For applications in
which system energy savings is most important and tight temperature control is classified as +/-
2°F (1.1°C), up to 30 percent changes in flow per minute are possible.
With the help of a software analysis tool such as System Analyzer™, DOE-2 or TRACE™, you can
determine whether the anticipated energy savings justify the use of variable primary flow in a
particular application. It may also be easier to apply variable primary flow in an existing chilled
water plant. Unlike the "decoupled" system design, the bypass can be positioned at various points
in the chilled water loop and an additional pump is unnecessary.
Series Chiller Arrangements
Another energy saving strategy is to design the system around chillers arranged in series. The
actual savings possible with such strategies depends on the application dynamics and should be
researched by consulting your Trane Systems Solutions Representative and applying an analysis
tool from the Trace software family. It is possible to operate a pair of chillers more efficiently in a
series chiller arrangement than in a parallel arrangement. It is also possible to achieve higher
entering to leaving chiller differentials, which may, in turn, provide the opportunity for lower chilled
water design temperature, lower design flow, and resulting installation and operational cost
savings. The Trane screw compressor also has excellent capabilities for “lift,” which affords an
opportunity for “lift,” which affords an opportunity for savings on the evaporator water loop.
Series chiller arrangements can be controlled in several ways. Figure 3, p. 10 shows a strategy
where each chiller is trying to achieve the system design set point. If the cooling load is less than
50 percent of the systems capabilities, either chiller can fulfill the demand. As system loads
increase, the Chiller 2 becomes preferentially loaded as it attempts to meet the leaving chilled water
Figure 2. Temperature out of range system solution
95°F (35°C)
238 gpm (15 l/s)
LOAD
PUMP
PUMP
80°F
(30°C)
238 gpm
(15 l/s)
59°F
(15°C)
60 gpm
(3.8 l/s)
95°F
(35°C)
178 gpm
(11.2 l/s)
59°F
(15°C)
178 gpm
(11.2 l/s)
68°F (20°C)
238 gpm (15 l/s)
59°F(15°C)
238 gpm (15 l/s)
95°F
(35°C)
60 gpm
(3.8 l/s)