Details on how ASHRAE 90.1 affects the plumbing engineer

Additional topics were covered by the webcast speaker on how plumbing engineers should interpret ASHRAE 90.1 when specifying pumps and drives.

12/29/2014


qAdditional topics were covered by the webcast speaker on how plumbing engineers should interpret ASHRAE 90.1 when specifying pumps and drives.Attendees of the webcast on ASHRAE Standard 90.1—What this means to the plumbing engineer had several questions that the presenter answered after the live event. Watch the webcast on-demand for additional information.

Question: Which is better: Using constant speed pumps set for high-rise in pressure build-up sequence at high demand or using variable speed pumps set to vary the water per the demand?    

Reece Robinson: Constant speed pumps can be considered a "better" choice over variable speed pumps if the flow remains relatively constant and the pump is operating at or near its best efficiency. A high-rise example would be water transfer to a storage tank. Tall buildings such as the John Hancock building in Chicago have a water storage tank near the middle floors (50th floor, for example). The storage tank supplies the lower half of the building (gravity feed down) and also supplies water to a second set of pumps supplying the upper floors. The feed pumps at the bottom of the building switch on to refill the storage tank mid-way up the building and run at a fairly constant flow rate.

Question: Is trimming impellers necessary with variable frequency drives (VFDs)?    

Reece Robinson: In most cases it is more energy efficient not to trim impellers when VFDs are used. When in impellers is trimmed the pump efficiency is decreased. However, leaving an impeller at its full diameter can result in a larger motor and subsequently larger VFD, which results in a higher installed cost. A full diameter impeller may be one or two motors/VFD sizes up from what the design flow/head call for. A lifecycle cost analysis will show whether the added investment will have an attractive rate of return.

Question: If ASHRAE Standard 90.1 dictates that there is no pressure change, how will tanks be effective?    

Reece Robinson: The standard does not state that there shall be no pressure change. The standard states that no device(s) shall be installed for the purpose of reducing the pressure of all of the water supplied by any booster system. Many interpret this as requiring systems to be "constant pressure" variable speed systems. The energy code is simply discouraging the use of pressure regulator valves as a means to control the output pressure of pumps as this method has been proven to be inefficient.

Question: In a high-rise building, how is the pressure controlled by zone?    

Reece Robinson: High-rise buildings are a bit tricky when it comes to delivering ample water pressure. It is generally more energy efficient to divide tall buildings into two or three pressure zones. This means that there will be 2 or 3 booster systems: one for the low zone, one for the middle zone, and one for the high zone. For example, a 100-story building the low zone would be floors 1 to 32, the middle zone would be floors 33 to 67, and the high zone would be floors 68 to 100. If one pump system were to be used, the output pressure from the system would be in excess of 450 psig to get water to the top of the building. This would require a large amount of pressure regulator valves (PRVs) on the lower floors, many of them rated for very high working pressures, which will drive up costs, both initial and maintenance. Increasing the number of pump systems decreases the number of pressure regulator valves required. You cannot avoid using PRVs in high-rise buildings altogether, but you can reduce that number by increasing the number of pumps. The lower floors of each zone will have PRVs that serve each floor because the delivery pressure of each pump system will be well over what is required by the consumers.

Question: Is a flow meter needed for either linear or square control curve?    

Reece Robinson: That depends on the pump system control logic. Some manufacturers have pump curve data loaded into the controller and can calculate an approximate flow rate by measuring the differential pressure across the pump. Using a flow sensor will more accurately depict a linear or squared control curve.

Question: What is the distance from a fitting to a pressure transducer recommended?    

Reece Robinson: For most pressure transmitters used in the industry today, there's no minimum spacing from fittings etc. as long as the area around the sensor is completely water. Things like air or air bubbles caused by cavitation should be taken into consideration for sensor placement.

Question: Why a diaphragm tank, and not a bladder type?    

Reece Robinson: For the presentation, I might have used the terms bladder and diaphragm interchangeably. For the purpose of storing water to allow a pump to stop, both can be used. The bladder type tanks have a higher acceptance volume whereas the diaphragm type tanks have a limited acceptance volume; however, both will have a pressurized air pre-charge allowing for water storage.

Question: How many starts and stops allowed per hour if water demand is on/off quite often? Will I burn the motor?    

Reece Robinson: The allowed starts and stops will depend on the inertia of the pump.


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