Energy Savings: Variable‐Speed Drives in Chilled Water Systems The following article was contributed by Carrier Corporation. Articles highlighting novel HVAC technologies should be submitted to Chapter Technology Transfer Committee Chair Mark Maguire ([email protected]
) for consideration in future newsletters. Variable frequency drive use has rapidly grown in the past decade in HVAC systems as advancements in VFD technology and reductions in first cost have increased return on investment. VFD’s have increased in performance and reliability, leading to VFD’s being standard in VAV and in secondary and primary pumping applications. Why do VFD’s save energy? The energy savings on centrifugal devices (fans, pumps, compressors) coupled with a variable‐ speed drive are dictated by the ideal fan laws (or pump affinity laws).
Power is proportional to speed cubed; this is why VFD’s save energy and have become prevalent in centrifugal devices. As can be seen from the graphs above, lowering the speed reduces the power requirement greatly. The relationship between lift and speed is also very important in applying VFD’s to centrifugal chillers. The centrifugal compressor lift is not a mathematical function of speed as it is for duct and pipe systems; this relationship is examined further later. How do VFD’s work? VFD’s convert AC power to DC, then convert back to AC at a desired frequency. Incoming 3 phase AC power passes through a rectifier, separating the alternating current. Capacitors convert the rectified current to DC. An inverter made up of IGBT’s switch to create the desired electrical current to match the desired motor speed.
Variable Speed Pump Basics VFD’s are applied to pumps to reduce energy costs and increase control. Designing the optimal system for energy efficiency and reliability requires a fundamental understanding of the Pump Affinity Laws. Speed control allows the system to supply the required flow at the lowest possible system pressure and power, generating operational costs savings.
Variable Condenser Water Flow If the flow rate is varied through a chiller condenser, it is critical to respect the onboard chiller safeties such as high condenser pressure. Variable condenser flow is not the norm, especially for centrifugal chillers that depend on head pressure reductions at part load. In centrifugal chillers, reduced condenser water flows may elevate head pressures, resulting in a loss of efficiency and possible chiller instability. Examine the effect on the entire system (chillers, pumps, cooling towers) when considering variable condenser water flow. Variable Speed Drives in Chiller Applications A centrifugal chiller is a turbomachine that follows the same laws of operation as pumps and fans. Similarly, centrifugal chillers can unload with either inlet guide vane control or by reducing compressor speed with vfd’s.
A centrifugal compressor must move enough compressed refrigerant flow to produce tons of capacity, and produce enough lift to overcome system head pressure (difference between condensing and suction temperature). The key differences between pumps and chillers: Pumps: the system head is a function of flow Chillers: the system head is a function of condenser water temperature and not directly related to flow. When can a centrifugal compressor reduce speed? When less flow is required: vfd’s can slow down to move less mass flow or refrigerant when less than design capacity is required. When less lift is required: for a fixed duct or piping system, the lift or system pressure is a function of the flow. However, for chillers, whenever the condensing temperature is less than design (e.g., 85°F tower water), the vfd can reduced speed, saving energy. Centrifugal chillers with vfd’s save energy even at full tonnage when entering condenser water temperature is not at design (e.g., 85°F). Reduced head pressure allows a vfd/compressor to move more capacity at lower speeds while consuming less energy. How much energy does a vfd save on water‐cooled chillers? Is full‐load kW/ton a good indicator of kWh consumption? No – chiller plants rarely operate at 100% capacity and design condenser water temperatures. Philadelphia weather data shows that the majority of chiller run hours occur at average 65‐75°F entering condenser water temperatures. Variable‐speed operation is ideal because of the large amount of operating
hours that occur at less than 85°F entering condenser water temperature. Optimize the chiller for maximum efficiency at the maximum number of run hours by using a vfd.