Keeping New York water running may take a VFD.

In the current boom of high-rise and luxury residential construction in New York there are several challenges and opportunities for providing a suitable and cost effective domestic water system. In designing a large number of these projects in New York City, ADS Consulting Engineers finds that most are between 15 and 20 stories tall. Since street pressure is usually available to supply only the lowest four or five floors, an auxiliary domestic water system is required.

In the past, wooden or steel roof tanks provided this water. The city skyline is known for these tanks, which have held water for both domestic and fire purposes for over a century. However, as real estate becomes more valuable, developers want to capitalize on the amount of saleable floor area by making buildings taller. In many instances it is profitable to extend the height of the building, leaving no room on the roof for the tanks.

In response to this, a fire pump is provided for the fire protection system and a constant pressure booster system is used for domestic water purposes. Depending on the project, the constant pressure booster can consist of many parts: two or three pumps, controls, interconnecting piping, a tank, shutoff valves and pressure regulating valves (PRVs). In response to the request for more energy efficient and durable systems, ADS has recently been specifying variable frequency drives (VFDs) in lieu of constant speed for the booster systems. There are both pros and cons to this scenario.

One major advantage of VFDs over constant pressure systems is the elimination of the PRVs. Being complex mechanical devices, PRVS are prone to wear and tear and require periodic rebuilding and adjustment.

Also, when a constant pressure booster system is running, the motor runs at constant speed, generally 1750 or 3500 rpm. Regardless of the system demand the PRVs must adjust to maintain this constant pressure, thus wasting energy. Even when system demand is zero, the motors may continue to run due to the controller's minimum run timer, which keeps the motors from starting and stopping too many times per hour.

VFDs are more efficient because they regulate motor speed by varying the frequency of the electricity to the pump so system demand is satisfied without running the motors at full speed. Some VFD systems still incorporate PRVs in case of VFD failure, which would lead to full-speed running of the pumps, but alternatives exist--such that the pumps shut off if a VFD or sensor fails.

Another benefit of VFDs is that they do not start the motors at full speed, but rather ramp up to the required RPM. This "soft-start" feature greatly increases motor life and reduces energy consumption.

A final benefit is that a two-pump system can be used to replace a three-pump system. A typical constant pressure design usually calls for a three pump booster system with a small lead pump sized for 10-20% demand and two lag pumps sized for 40-50% demand. This provides redundancy for pump failure and expandability for future demand. VFD systems are usually specified with two pumps sized for 65-70% demand. This provides equal redundancy and expandability in comparison to three-pump system without the extra pump, which saves both space and cost.

Disadvantages of the VFD systems have traditionally been the cost and size of the VFDs themselves. These issues have been minimized over time due to production improvements and technology enhancements and VFD prices and sizes have come down dramatically. Typically, a two-pump VFD system does not cost more than a three-pump constant pressure system and the VFD system size is smaller.

Another disadvantage has been the complexity of the drives themselves. PRVs, although complex, can be repaired or replaced quickly by field technicians. In the past, VFDs, being controlled by computers, required specialized test equipment and trained personnel to service them. This added up to costly service charges. With recent innovations in computer technology and training, such service professionals have become commonplace and knowledgeable in VFD equipment.

A final disadvantage, and one that has kept VFDs from being popular for plumbing systems, was the belief that VFDs are only appropriate in large-scale HVAC systems, where friction loss is high compared to static pressure loss. This may have been true when VFD installation costs were high, but now that VFD costs have dropped, the benefits from energy savings are available to a number of different project sizes and system types.

In New York City, water tanks were the original method of providing water for high-rise buildings. These were later replaced with constant pressure booster systems using PRVs. The most recent booster systems incorporate VFDs, which can be smaller in size, more energy efficient and require less maintenance than the systems they replace, while installation costs remain the same. For these reasons, we expect to see many more high-rise buildings use VFD controls on their domestic booster systems.