Improving energy efficiency is a key issue in the design of chillers
for an air-conditioning system. Efficient chillers not only reduce the
operating cost, but also reduce green house gas emissions by reducing
power consumption. According to Michael Cui, senior engineer at the Trane
Company in LaCrosse, WI, U.S., a 1-percent increase in efficiency brings
U.S. $1 million in savings over the life of a medium-sized chiller. The
social and environmental benefits can be even greater. Trane engineers
are using multi-objective optimization to improve chiller efficiency.
Different from the traditional trial-and-error technique, Trane engineers
analyze multiple variables and effects simultaneously and optimize the
design to achieve the maximum benefit.
Motor
temperature and velocity distribution on a cross-section area.
CLICK for full-size graphic.
|
|
A refrigerant-cooled hermetic motor is an integral part of Trane water-
and air-cooled chillers. According to Mr. Cui, this technology provides
high efficiency and
extends the life of high-power induction motors. In these hermetic refrigeration
machines, coolant flow is needed to carry away the heat generated by high power
motors to improve energy efficiency and motor durability. However, the coolant
flow also induces the windage loss to the systems.
Two competing mechanisms
need to be quantified simultaneously to achieve the optimized design of
these hermetic systems. Trane engineers use the latest computational
fluid dynamics
(CFD) technology to simulate heat transfer phenomena in high-power induction
motors and refrigerant flow through these motors concurrently. These simulations,
says Mr. Cui, bring many benefits compared to traditional build-and-test
techniques: temperature distribution in the motor is evaluated to eliminate
hot spots and
obtain uniform cooling; through-flow induced torque and loss are determined;
the impact of rotor surface geometry and speed are assessed; and the required
mass flow rate for effective cooling is predicted. As a result, hermetic
refrigerant machines are designed with optimized configuration for the
best energy efficiency.
Pressure
distribution inside a Trane centrifugal compressor stage. CLICK for
full-size graphic. |
|
In order to improve energy efficiency, design and analysis move from the individual component to the entire system. Compressor air dynamic performance is no longer limited to single parts, but extended to include the interaction of multiple components. This integrated analysis ensures the maximum performance of each component as a part of the compressor system at various working conditions. Trane engineers have developed a Virtual Laboratory for the design of its refrigerant compressors. It is based on CFX-TASCflow, a CFD software package from CFX, AEA Technology in Waterloo, Ontario, Canada, that Trane engineers have found to be ideally suited to modeling indoor comfort systems and which includes a real-gas equation of state for the refrigerant. The result, says Mr. Cui, is that overall performance and local flow field details for complete compressor stages without building a prototype can easily be obtained.
Different from single-component analysis, the Trane Virtual Laboratory simulates
the entire compressor, readily providing information on the interactions between
the components. Since it quantifies the impact of design changes of a single
part on the performance of the whole compressor, this feature has special value
for the overall improvement of machines. This capacity proved very useful when
Trane engineers investigated the options to use different diffusers in compressors.
The impact on the overall compressor performance was analyzed in detail, with
the simulations indicating that the change would lead to different flow fields
inside the upstream and downstream components. The individual performance of
these components was altered when the diffuser was changed. The information
obtained was of great value to designers, guiding product improvements and
helping to avoid unnecessary design iterations.
CFX
software calculates force and torque from pressure and viscous
shear stress on the surface of inlet guide vanes and impeller
of a Trane centrifugal compressor stage. CLICK for
full-size graphic. |
|
Transient simulations in the
Trane Virtual Laboratory provide further physical insights into compressor
performance and flow field unsteadiness, reduction
of which is critical to improve efficiency and reduce vibration and noise
levels. Pressure fluctuation distributions inside the impellers and diffusers
can be
obtained for different compressor designs and loss mechanisms inside the
flow field can be studied thoroughly. CFX software, says Mr. Cui, allows engineers
to explore large-scale unsteady flow phenomena. Removing these instabilities
is said to have the benefit of obtaining the highest energy efficiency in
a
wide range of applications.
Using advanced technology in design and development
of the new product makes Trane a successful indoor comfort solution provider.
The company has earned
numerous industrial and EPA awards and enjoys a large share in the commercial
HVAC system market.
Reducing Environmental Impact
To minimize the impact on the environment, Trane
also develops technology to improve the reliability of its products, increase
service intervals, and reduce
the need for disassembly of the units. Possible emission of refrigerant associated
with these processes reportedly is minimized.
Cross-section
of a Trane scroll compressor. |
|
For the compressors designed
by Trane, the force and torque on components are quantified to ensure safety
and reliability. The force and torque are calculated
from the pressure and viscous shear stress obtained from flow field simulations
on the surface of the components. These force and torque calculations are
also used to predict the vibration and noise of the system. The possibility
of failure
associated with fatigue of the components is therefore reduced. All this
analysis is conducted using a whole system simulation under strict dynamic
loading conditions.
In recent years, the sound produced by HVAC equipment has attracted more attention.
Since the equipment is typically located near building occupants, noise radiation
needs to be controlled. Reducing the acoustic impact on the environment is
particularly important for hospitals, schools, and music halls. The machinery
sound from HVAC equipment is caused by temporal variation in the flow field.
The internal flow field vibrates the machines, setting the air around them
in motion. This unsteadiness in the air reaches the human ear in the form of
noise. According to Mr. Cui, by using CFX to analyze the unsteady flow field
inside the chiller, track down the response of the unit structure, and predict
air motion around the machines, Trane engineers deliver units with the lowest
noise radiation.
Since Trane developed the industrial first scroll compressor,
the Trane 3-D(R) scroll compressor, in 1987, these devices have dominated
the market for small
tonnage air-conditioning equipment. Compared to the reciprocating compressors,
where intake, compression, and discharge occur in discrete steps, scroll
compressors conduct intake, compression, and discharge phases of operation
simultaneously
in an on-going sequence. Their smooth operating characteristics reduce force
and torque variation inside the compressor, making scroll compressors quiet
and reliable.
Oil
droplet path-lines inside a Trane scroll compressor. CLICK for
full-size graphic. |
|
Lubrication is very critical to improve the durability of scroll
compressors since the oil pump is an integral part of the compressor. Trane
engineers developed
the technology to use the latest particle tracking techniques to predict
the oil circulation rate inside the scroll compressors. The oil coming from
different
regions inside the compressors is tracked through the operating process.
Oil circulation features of different designs and oil droplet sizes are obtained
at different operating conditions.
This technology helps designers develop
compressor designs with adequate lubrication and an ample supply of oil.
The fully lubricated moving components extend the
durability of the system and reduce the need to replace parts and service
the units.
This information is provided by Michael Cui, senior engineer at the
Trane Company in LaCrosse, WI, U.S. |
