ENERGY SAVING EQUIPMENT
Chiller efficiency and system design
Chiller product manager for Daikin Applied UK, James Henley, looks at the practicalities of getting the most out of chillers.
O
ver the years there have been significant developments by manufacturers to improve the efficiency of chillers. With advances in component technology and, significantly, the move from fixed speed to inverter solutions, a chiller can now be delivering energy savings of between 25-35%.
HVAC manufacturers will continue to drive and develop new and innovative solutions to improve the efficiency delivered by chillers. However, this must also be closely coupled with a review of the system design to eliminate further possible inefficiencies. Just one such example is being driven by the US market, where 60-70% of all chilled water systems being installed are now Variable Primary Flow systems (VPF). This type of system design is in limited use in the UK market place, where primary/secondary systems are still common practice, even though VPF systems have some significant advantages in cost savings in pumps, piping and hardware, and electrical, as well as energy savings. These benefits are just not being utilised to their full potential. Conceptually, the variable-water-flow (VPF) system resembles the constant primary-variable secondary design more commonly referred to as decoupled system. For example, both systems require a bypass line. For a VPF system, the bypass line is required to ensure that a minimum flow through the chiller evaporators can be maintained. In a VPF system water flow varies through the whole system; the evaporator of each operating chiller as well as the cooling coils. There are two-way modulating control valves on coils, check (or isolation) valves on the chillers and a bypass line with a control valve which are required to correctly implement a VPF system. The by-pass in the case of a VPF system is a smaller pipe size for the minimum flow of the largest chiller. It contains a standard closed control valve that modulates open only when the low flow limit is reached. With chilled water pumps equipped with variable frequency drives operating to maintain a minimum differential pressure at the critical load.
Whilst historically chiller manufacturers may have advised that variable flow through chillers is not recommended and flow rates must be within +/-20% of design flow, this is not the case. With modern chiller controls the operating stability of chiller has dramatically improved allowing for
52 January 2018
a much wider flow rate change. Almost any type of chiller can be used in a VPF system including air cooled and water cooled. The chiller must have a unit controller, using a PID loop control, capable of dealing with both a change in flow and a change in return temperature. Usually, the minimum flow rate of a chiller is 50% of the design flow rate but with a tailored unit selection this could be a much as 30% of design flow, offering even greater savings and is particularly advantageous where there is only one chiller in the system with few operating hours at design flow.
Whilst chillers can be readily installed in VPF systems there are still some basic rules that need to be adhered to to ensure correct system operation. These include: ■ The rate of change in the water flow needs to be slow, not greater than 10% of the change per minute. The chiller needs time to sense a load change and respond; this may require the use of slow-acting, modulating control valves at each chiller to reduce flow variation as chillers are sequenced, especially if pumps are manifolded. This will ensure that the chiller does not trip.
■ The water velocity in the vessels must be between 0.91 and 3.0 m/sec. Below (0.91 m/sec), laminar flow occurs which reduces heat transfer. Above (3.0 m/sec), excessively high pressure drops and tube erosion occur. The chiller has on-board control protection, but these parameters also need to be considered in the plant design and may require a method of monitoring the flow rate through each chiller and a control system to ensure that the flow rate through the evaporator stays within the limits.
■ A bypass is required. The bypass should be sized for the minimum flow rate of the largest chiller being used. It is only used to provide minimum flow to a chiller when the flow requirement through the chiller plant is less than the chiller minimum flow. While this approach ensures minimum chiller flow, it also reduces the pump operating cost savings by increasing the system flow.
■ VPF systems are more complex both to design and to operate when compared to the primary/secondary system. The sequencing of chillers and pumps requires a thorough understanding of the system dynamics, because flow rates will vary through every operating chiller. The control system needs to avoid cycling (restarting the chiller too
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