Engineers at MIT were well aware that Low Delta-T Syndrome problems were a significant energy cost to the campus. Campus wide, they determined that this problem alone was costing the campus nearly $1.5 million annually when all associated energy consumption from chillers, pumps, and cooling tower fans was taken into account.
The problem was particularly pronounced at the Charles Hayden Library where data showed that coils throughout the building had average Delta-T readings as low as 6ºF, resulting in far more water being pumped through the building than was necessary or even advantageous.
According to Henry, the library’s problems were caused by a combination of several factors. First, many of buildings coils were designed with a lower Delta-T than would be done today and also because originally the building had its own chiller plant and operated under constant flow conditions. Second, many if not all of the coils had lost much of their heat transfer capacity due to fouling. Other factors might also have included oversized control valves, improper or non-dynamic water balancing, and air handling unit (AHU) control logic reliance on air sensor inputs.
According to Peter Cooper, Manager of Sustainable Engineering and Utility Planning at MIT, Hayden Library had fairly homogeneous demand patterns (i.e. no laboratories or other special demand spaces) to factor into the control scheme, making it a more straightforward test site. Plus, the library is a far distance from the chiller plant so by increasing Delta-T and thus reducing flow to and from the building, there was greater potential for energy savings.
The Belimo Energy Valve technology was retrofitted to (5) air-handling units (AHUs) at the library. Operating under this control technology, the valves eliminate the over-pumping through coils that occurs when the coil becomes power saturated – meaning that its heat transfer capability has been exhausted and any additional flow will only result in wasted pump energy.
The Energy Valve provides accurate automatic flow control through its characterizing disc, which has high rangeability and turndown ratio. The equal percentage characteristic of the disc decreases “hunting” and stabilizes system output through small, incremental changes in water flow during the first 10 to 30 degrees of valve opening—where control accuracy is most critical.
Precise pressure independent flow regulation of the valve is the result of continuous monitoring and analysis of flow and Delta-T. The Energy Valve continuously monitors the coil Delta-T and compares this value with the desired Delta-T value setpoint. Ideally these values will be the same. However, if the actual Delta-T deviates from the setpoint, the valve will readjust itself to bring it back in line. Once the appropriate Delta-T is established, the valve logic resumes its normal pressure independent operating mode.
Inherent software also monitors and can trend all sensed or calculated values, which include but aren’t limited to water flow, return and supply water temperature, power, and energy. All of this information is reported back to the BAS via BACnet MSTP or BACnet IP where it can be used for additional trending and analysis.
Before the retrofit, the total average Delta-T for the coils at the Hayden Library was 6.15°F, based on metering data taken from the period of August 9, 2010 thru October 9, 2010. After the retrofit, the building was metered for the exact same calendar period (August 9, 2010 - October 9) in 2011. The average Delta-T had risen from 6.15°F to 12.14°F, reducing chilled water flow to the building by 49%. This percentage is weather normalized, meaning that it has been adjusted for variances in outdoor temperature that occurred during the metered period. Thus, the 49% is an exact and accurate representation of the overall flow reduction.
What does such a reduction in flow amount to, in terms of actual energy savings for a facility like MIT? According to Peter Cooper, the savings from reducing flow and increasing Delta-T is substantial.
“We’ve had studies done at MIT that indicated annual savings would be as high as $1.5 million if we were able to fix all our low Delta-T issues,” said Cooper. “You can save many times that much by avoiding the cost of extra chillers,” he added, pointing out that better control allows you to squeeze more capacity out of existing equipment. The beta testing at the Hayden library convinced Cooper of something else: the value of data.
“One thing that impressed us was having such intelligence right on the valve actuator,” said Cooper. “You can characterize a coil’s performance with just a couple of pieces of data and with that information you can observe the degradation of coils and refocus your maintenance efforts accordingly. That’s very useful when you are a campus dealing with limited maintenance resources.”