ENERGY SAVING EQUIPMENT
soon and too often) and maintain the rate-of- flow variation through the chiller evaporators within the allowable limits. This becomes more and more complicated with the increasing number of chillers in the system.
■ VPF systems take more time and planning to design and commission properly compared to other systems. The design engineer must thoroughly define the control sequence early in the design process, and clearly communicate it to the controls provider.
Additionally, in larger water cooled applications it is recommended that variable flow is considered only in the evaporator because there is virtually no change in chiller efficiency compared to constant flow. In other words, there is no chiller energy penalty. Although variable speed pumping can be done in the condenser loop, it is usually unwise. The intent of variable flow is to reduce pump power input. However, reducing condenser water flow increases the chiller’s condensing pressure, increasing the lift that the compressor must overcome which, in turn, increases the compressor’s energy use. Consequently, pump energy savings can be lost because the chiller operating power is significantly increased. Low condenser flow can also cause premature tube fouling and, with an increase of cleaning or chemical use, subsequent increased compressor power consumption can result. It is best to have variable flow in the evaporator only, not in the condenser.
VPF systems present building owners with several cost saving benefits that are directly related to the pumps. The most obvious cost saving comes from eliminating the secondary distribution pumps, which in turn, avoids the material and labour expenses incurred with the associated piping connections, electrical service and variable frequency drive. Although the number of pumps is reduced, the sizes of both the pumps and the variable frequency drives increase since the pumps must be sized to
overcome the entire system’s pressure drop. This offsets some of the installed cost savings due to fewer pumps. Overall, VPF systems generally cost less to install and once the construction team is familiar with the requirements, they are easier to design and build because there are fewer pumps and pipe fittings. Benefits are as follows: ■ There is no wasted pump work as is the case in a primary/secondary system. Unless the flow drops below the minimum required by the chiller, any flow that is pumped in a variable primary flow system is sent to the building and used.
■ In many applications there are fewer pipe fittings associated with pumps (e.g. for check valves, isolating valves, and elbows) because there is only one set of pumps. As a result, the overall design pressure drop for a variable primary flow system will be about 10% less
Wa Watkins Hir
than a primary/secondary system in the same application.
■ Variable primary flow creates the opportunity to operate chillers in their ‘efficiency sweet spot’ without paying for extra, unused pump energy. In other words, variable primary flow allows two chillers to be used when it is still possible to meet the load with only one chiller. This strategy will save energy and operating costs over the life of the installation.
Given the real benefits of potential cost savings and system energy savings that are available, deciding to use a VPF system should be a ‘no brainer’ and should become more common on chiller installations in the UK. It just takes a closer working between design and delivery to provide better system solutions to the market.
re has unrivalled knowledge and capabilities in the field Watkins Hire
Process Chillers designed fo with your industry in
www.acr-news.com
or hire n mind
FOOD MANUFACTURING IT & DATA CENTRES AT
of temperature control and process cooling. W e offer a complete range of high performance, high efficiency chillers and heat pump units suitable for a wide range of applications.
Our combinati on of in-depth technical experience and comprehensive equipment range means we can cater for all scales and types of temperature contr ol and process cooling applications including:
PUBLIC SECTOR SOLUTIONS
WAREHOUSE & STORAGE
MANUFACTURING & PROCESS Call: 0800 999 6 365 January 2018 53 E:
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Chillers available from 30kWto 750kW
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