Chiller system problems solved with Belimo Energy Valve
The use of chillers is becoming increasingly popular in temperature control, however, they do tend to spend much more energy than needed due to design flaws of the system which lead to Low Delta-T Syndrome. Low Delta-T Syndrome is a costly problem in many large facilities, and folks from the Massachusetts Institute of Technology (MIT) partnered with Belimo in order to find a solution for this problem on their campus.
“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 Peter Cooper, Manager of Sustainable Engineering and Utility Planning at MIT. “You can save many times that much by avoiding the cost of extra chillers.”
Low Delta-T Syndrome explained
Since most of you probably aren’t familiar with chillers, I’ll describe a scenario of how Low Delta T Syndrome occurs. Chillers operate by circulating cooled water, which is distributed to heat exchangers (or coils) which cool the air at distributed spaces. When the difference between the temperature of cooled water being sent and water being returned to get cooled again is less than projected, another chiller kicks in thus requiring more energy to serve only a part of the volume.
This can occur due to dirty coils (inside the coil piping or in the air side or both), improper piping design, poor control valve selection/operation, etc. Since the coils aren’t helping the system to achieve the desired temperature, the Building Automation System (BAS) in charge of the control increases the water flow rate in the secondary side of the system. Increased volume of water being circulated at a time causes circulation of increased amount of return water and a little chilled water and the water temperature going out to the secondary is warmer. This can cause the system to activate yet another chiller, while other previous chillers operate at just a part of their capacity.
MIT and Belimo cooperation story
Back in 2010 when Walt Henry, director of the Systems Engineering Group at MIT’s Department of Facilities, mentioned to Marc Thuillard, Head of Research at Belimo, that the University would be interested in partnering with Belimo on some product development testing – particularly if it was a control technology that might solve some of the campus energy problems.
“Marc asked for an example of such a problem and I mentioned our problems with maintaining an acceptable difference between entering and exiting chilled water temperatures at our chilled water plant”, said Henry. “I thought that a modification of some existing Belimo control valves might help.”
At that time, Belimo was already developing Belimo Energy Valve – a “smart” valve with enhanced capability to help combat Low Delta-T Syndrome. MIT had the perfect test case opportunity for this developing technology – the Charles Hayden Library.
According to Henry, the library’s problems were caused by a combination of several factors. Many of buildings coils were designed with a lower Delta-T than would be done today, and the building originally had its own chiller plant and operated under constant flow conditions. Other factors might also have included oversized control valves, lost heat transfer capacity of the coils due to fouling, improper or non-dynamic water balancing, and air handling unit (AHU) control logic reliance on air sensor inputs.
How do Belimo Energy Valves solve the problem?
The valve combines the function of an electronic pressure independent control valve that also included a flow sensor, and a return and supply water temperature sensor, along with the communications and logic capability to use these sensed values for improving coil performance.
The technology is used to optimize the heat transfer performance of individual coils by continuously monitoring the coil Delta-T and comparing this value to the desired Delta-T setpoint. If these values were not the same, or suitably close, the valve would readjust itself. The data gathered at the valve is reported back to the BAS and used for further trending and diagnostics.
The Energy Valve provides accurate automatic flow control through its characterizing disc, which has high turndown ratio (the ratio of the maximum flowrate to the minimum flowrate). The equal percentage characteristic of the disc decreases “hunting” and stabilizes system output through small, incremental changes in water flow during most critical range of control – the first 10 to 30 degrees of valve opening.
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. Ideally, the coil Delta-T and the desired Delta-T value setpoint should 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 (metering data taken from the period of August 9, 2010 – October 9, 2010). The Belimo Energy Valve technology was retrofitted to (5) air-handling units (AHUs) at the library.
After the retrofit, the building was metered for the exact same calendar period (August 9, 2010 – October 9) in 2011. Operating under this control technology developed by Belimo, the valves eliminate the over-pumping through coils that occurs when the coil becomes power saturated.
The average Delta-T had risen from 3.6°C to 6.74°C (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.
“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.”