HVAC AND BUILDING SERVICES
Optimisation
of pumps for HVAC and building services
Heating, ventilation and air conditioning (HVAC) generally consumes the largest proportion of energy in commercial buildings, typically accounting for around 40 per cent of total building consumption and around 70 per cent of base building electricity consumption. Unlike other energy efficiency strategies such as plant upgrades that can be more expensive, improving the performance of HVAC systems can provide a quick reduction in energy use and costs. Here, we look at some of the ways pumping systems for HVAC services can be optimised.
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here are a number of key end uses in HVAC systems where energy consumption occurs which can be further broken down with the national average being: • 34 per cent of HVAC energy consumption is from fans for air circulation and ventilation • 27 per cent of energy consumption is from cooling – most frequently the energy use of chillers for cooling via chilled water, but also the use of direct expansion cooling systems such as packaged air conditioners • 17 per cent of energy consumption is from heating – most frequently the energy use of boilers for hot water for heating, as well as the use of electric heaters for zonal reheat • 16 per cent of energy consumption is from pumps for the circulation of hot water, chilled water and condenser water • Six per cent of energy consumption is from cooling towers for heat rejection, being primarily the cooling tower fan energy HVAC optimisation strategies can vary, but something as simple as changing control algorithms, altering control schedules and set points, and completing minor mechanical repairs and alterations to existing equipment and systems1. The before and after energy consumption of an individual end use for some buildings following an efficiency upgrade can vary by a factor of five or more, especially for fans as they are highly non-linear, and gas as it is often very inefficiently used. However, to realise the benefit, building owners and facility managers need to see energy efficiency strategies as an investment rather than a cost, and implement several strategies at the same time as true optimisation will not be achieved by deploying strategies alone.
Optimising existing fan/pump distribution systems
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The first approach to optimising existing distribution systems is to assess it for problems before establishing system pressure and flow rate requirements and quantifying the installed fan or pump capabilities. This process requires system performance data to be collected over time as logging data such as flow rate, discharge pressure and energy consumption is essential for diagnosing optimum system performance. pump industry | Autumn 2022 | Issue 39
Many existing fan and pumping systems in HVAC can improve their efficiency and performance by identifying system problems due to incorrect fan or pump selection and operation. Incorrect fan or pump size can be determined by comparing the system pressure/flow requirements with the fan or pump performance curve. In many cases, the fan or pump performance can be better matched to the system requirements, with maximum efficiency and minimum power consumption achieved by ensuring that the flow and pressure at the fan or pump’s best efficiency point (BEP) closely matches the system operating point. Where fan or pump distribution systems are not performing optimally, there are a number of solutions that can be explored2: • Replace old fans/pumps with modern energy-efficient replacements • Replace constant volume systems with variable volume systems • High pressure drop – clean filters/strainers and heat exchangers • High pressure drop – reduce system resistance by upsizing ducts/pipes and fittings, removing unnecessary fittings, removing throttling dampers/valves and installing low pressure-drop plan • Oversized fan/pump – trim/change impeller, smaller impeller, VSD, two-speed drive, lower rpm. Undersized fan/pump – replace fan/pump or reduce system resistance (optimise flows) • Multiple fans/pumps operating continuously – review and update the control system • High maintenance costs – match fan/pump capacity with system requirements; check materials compatibility (corrosive environment etc.) • High flow rates – adjust system operating temperatures to maximise temperature differentials and reduce flow rates • Over throttled system – modify fan/pump performance to reduce the need for throttling
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