APPLICATION GUIDE

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VARIABLE SPEED DRIVES (VSD)

In Hong Kong, most of the 3-phase ac motors in buildings are fitted to fans and pumps. The flow from most fans and pumps is either constant or controlled by restricting the flow by mechanical means, e.g. dampers are used on fans and valves are used on pumps. This mechanical constriction will control the flow and may reduce the load on the fan or pump motor, but the constriction itself adds an energy loss that is obviously inefficient. Hence if the flow can be controlled by reducing the speed of the fan or pump motor, more efficient means of achieving flow control could be offered.

As the speed of the fan or pump is reduced, the flow will reduce proportionally, while the power required by the fan or the pump will reduce with the cube of the speed. This level of potential energy saving makes the use of Variable Speed Drive (VSD) a cost-effective investments in energy efficiency which can be considered for motors.

In recent years, development in power semiconductors and microprocessors have allowed the introduction of electronic VSDs which have improved performance and reliability over earlier systems while reducing the equipment cost. Hence a range of motors in building services can now be considered for retrofitting with VSD based on the economics of energy saving.

A VSD can be regarded as a frequency converter rectifying ac voltages from the mains supply into dc, and then modifies this into a ac voltage with variable amplitude and frequency. The motor is thus supplied with variable voltage and frequency, which enables infinitely variable speed regulation of three-phase, asynchronous standard induction motors.

Application of VSD in Primary Air-Handling Units (PAU)

Conventional fresh air supply introduced into a high-rise commercial building is normally fed via a Primary Air-handling Unit (PAU) at constant air volume. The conventional PAU is part of a central air conditioning system usually used to supply fresh conditioned air via riser ductwork to the floors it served at constant rate, regardless of the actual needs of the zones served. The PAU brings outside air, at a designed temperature of 33.5?X C, into filters prior to pre-cooling it to 20?X C and delivers it via fans and ductwork to serve individual floors. This system is designed for "worst case" condition and end up wasting energy relative to the needs of the building for most of the operational life. No modulation method had normally been allowed in the design other than the original balancing of the system. The system is normally operated continuously from 8:00 am to 6:00 pm for general offices building.

Demand control on PAUs using carbon dioxide offers a unique opportunity for building services engineers and building owners to resolve the problem of how to reduce energy costs while optimising indoor air quality.

In a typical building, the amount of CO₂ exhaled by people is diluted by outside air introduced by mechanical ventilation, air leakage, and open windows. The lowest concentration of CO₂ measured in outside air in Hong Kong ranging from 400 to 500 ppm. The CO₂ concentration measured in Causeway Bay was found to be on the high side of 500 ppm. CO₂ is generally not considered a health-threatening contaminant at the 500 to 3,000 ppm levels typically found in most buildings. Many people have observed symptoms of stuffiness, sleepy, inattention, unpleasant odours, and a general feeling of discomfort as CO₂ levels rise about 1,400 ppm. It is important to note that these symptoms are not directly related to CO₂ or a corresponding lack of oxygen. Rather these reactions are more related to the build-up of other contaminants and irritants in the space when ventilation levels are low. CO₂ is therefore often considered a good surrogate indicator of indoor air quality.

According to ASHRAE Standard 62-1989 " Ventilation for Acceptable Indoor Air Quality", ventilation maintaining an indoor CO₂ content of 1000 ppm is considered ideal. CO₂ lower than 800 ppm is considered as over-ventilated. Some of the government office buildings investigated under the Pilot EMO Implementation Programme have average measured CO₂ concentration below 700 ppm. Therefore CO₂ based demand control ventilation has good potential to reduce energy consumption while optimising indoor air quality.

The CO₂ based demand control can also be achieved by the direct application of CO₂ sensors for real time speed control of PAU. Recent innovations in gas sensor designs have considerably improved the long-term performance and cost of CO₂ sensors, making it one of the fast growing segments of the HVAC control industry. The CO₂ sensors should be located at some strategic location where "worst case" occurred. The figure below shows a possible arrangement of a variable flow PAU using VSD and CO₂ sensors.

Application of VSD in Variable Air Volume (VAV) Air-Handling Units

Variable Air Volume (VAV) systems typically bring conditioned air from PAU and returned air from the air-conditioned space into Air Handling Units (AHU) where the air temperature and humidity can be adjusted. Fans blow air across filter, cooling coils and volume control dampers or inlet guide vanes into ductwork, which distributes the air throughout the zones served. The air passes into each zone from the ductwork through individual VAV terminal boxes. A temperature sensor located in each zone is connected to its VAV box and opens or closes the VAV box to maintain the defined temperature setpoint. As the zone becomes satisfied, the VAV box modulates to a close position. The pressure in the ductwork would then begin to rise as the openings in the VAV box close.

Traditionally, inlet guide vanes or discharged dampers are installed in the AHUs to prevent this over pressurisation and save energy. These devices work by creating resistance and a pressure drop to the air entering the ductwork or reducing the efficiency of the fan. The more the VAV boxes in the system close, the more the dampers close to maintain static duct pressure. The dampers or inlet guide vanes for the fan are commonly controlled by a controller maintaining a fixed pressure in the supply ductwork downstream of the AHU.

While dampers and inlet guide vanes work to maintain a constant pressure in the ductwork of a VAV system, the utilisation of VSD could save much more energy and reduce the complexity of the installation. Instead of creating an artificial pressure drop or causing a decrease in fan efficiency, the VSD decrease the speed of the fan to provide the flow and pressure required by the system. The figure below shows a modified VAV system with VSD in lieu of the conventional star-delta motor starter and motorised dampers for static pressure control in the ductwork.

For new high-rise building projects involving CO₂-based demand control ventilation via central PAUs, it would be more appropriate to include individual duct-mounted CO₂ sensors at the return air ducts to control the amount of fresh air drawn from the main riser duct at the AHU rooms on each floor. The total demand of fresh air required to be handled by PAUs should then be control either via static pressure sensors in the main air duct or summation of individual fresh air requirement at each floor together with appropriate DDC controllers and VSDs. A typical configuration diagram of the system is shown in the figure below

Application of VSD in Secondary Chilled Water Circuit

Primary pumps in a primary/secondary pumping can be used to maintain a constant flow through chillers that encounter operation or control difficulties when exposed to variable flow.

In chilled water systems, the primary loop consists of primary pumps sized to handle the chillers designed flow rate at a discharge pressure just high enough to circulate the water through the chiller and the rest of the primary piping loop. The secondary chilled water loop is a variable flow system consists of secondary pumps sized to circulate chilled water to handle full capacity of the cooling loads connected on the circuit. During light load condition, most of the two-port control valves on the loads are not fully open resulting in pressure rise in the secondary chilled water loop. In a conventional system, a by-pass valve connected across the cooling loads will be used to by-pass the secondary water flow and regulate the flow to loads and balance the water pressure in the system. A differential pressure sensor normally controls the by-pass valve.

The figure shows a new arrangement of the secondary chilled water circuit with VSD in lieu of by-pass valve for regulation chilled water flow according to the actual loading requirement. Energy saving is achieved in pump motors in most of the time when the cooling loads are not at full capacity and maximum chilled water flow is not required.

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Last revision date: 8 April 2009