We are in the midst of summer here in Atlanta and our air conditioning systems are working hard to keep us comfortable. Operating a chilled water system in larger facilities can be a challenging thing. Understanding what your largest energy consumer in your system wants can be the key to operating these systems as efficiently as possible. As is the case with any system, there is a balance that must be found to truly optimize performance. There is not one answer…you need to understand what you have and have a strategy.
In many buildings, the chiller consumes more energy than any other piece of machinery. In fact, a chiller typically consumes as much as 20% to 40% of a building’s electricity. Delivering cooling water to the chiller’s condenser at optimum temperatures along with maximizing the temperature of chilled water being delivered to the loads can save an enormous amount of energy across its operational ranges.
It is not uncommon for the chillers in buildings to be running below their design efficiency for a variety of reasons. Many of these reasons can relate to the cooling water entering the condenser and how it interacts with the refrigerant within the chiller. The temperature of the condensed refrigerant is designed to correspond to a specific condenser head pressure for a given load. If this temperature is increased above the design specifications, the compressor lift is increased and the compressor has to work harder to bring the refrigerant gas to a higher head pressure to allow it to be converted to a higher pressure liquid. Higher lift reduces overall efficiency. Generally speaking, each additional degree the chiller’s refrigerant is above the design temperature condition, the compressor will consume 1.5% more energy. The reverse of this is also true and therefore lower water temperature entering the condenser barrel will reduce chiller energy consumption.
The compressor in a chiller is designed for a specific condenser water temperature. As the condenser water approaches this temperature, the chiller runs very near its design or expected efficiency. As the condenser water temperature increases past design or optimum points, the chiller becomes exponentially more inefficient. In newer chillers, the water temperature delivered to the condenser barrel can be much lower than what could be delivered to many older chillers. Lower temperatures in older chillers (and even some newer chillers) will typically cause the compressors to “surge” which creates noise, wear and tear on the machine and ultimately failure. High head pressure also reduces the chiller capacity by minimizing heat transfer on hotter days when the cooling demand is very high. If the loss in heat transfer is great enough, due to condenser water that is not cold enough, the condenser head pressure will reach its upper limit causing the chiller to shut down.
Understanding what your building needs, the limitations of the equipment you have and the range of operation possible during weather extremes can help you operate your building in a safe, reliable and efficient manner.
Let us help you take advantage of the weather extremes to best understand your limitations and optimize your operations.
