Spring is upon us and soon summer. So many people, technician and manager alike, are looking at their chiller(s) in agony, hoping this year will be better than last year. This is an unfortunate circumstance, usually stemming from years of babysitting the machine, repeated resetting, calling tech support, and finally accepting that this is just how they are supposed to operate. I say unfortunate because this status quo is too often accepted as normal; and well, it shouldn’t be.
The function of a chiller, whether a high pressure or low pressure, water cooled or air cooled, is simple, lower system chill water temperature, utilizing the refrigeration cycle, to a preset temperature when the demand calls. A typical chiller system has outside air lockouts to prevent operation below 45 degrees Fahrenheit. Working in ambient temperatures below that is certainly possible but will require some engineering and design. But an operating chiller that turns off due to low ambient temperature, will be a controlled shutdown and restart by itself once within acceptable ambient temperatures.
To get a better idea, let’s look at a simplified sequence of operations. When chill water temperature rises above a predetermined temperature, the chill water pump (and condenser pump if applicable) will start and prove flow, then the chiller will start under its lowest mechanical load possible. Once warm and settled (usually based on timers), the chiller will stage up to required demand. As the system water starts to cool, demand falls, and chiller starts to stage down. The chiller will try to maintain this temperature with staging. Once a predetermined set point is reached under the lowest mechanical load possible, the chiller will shut off, followed by associated pumps after a predetermined time interval. A rise in water temperature will start the sequence over.
While this is a simplified version, it is important to understand that this process is an internal function of the chiller controls and should not require any external reset to restart the sequence. If an automated system is connected, it should act as a switch like the low ambient cutout described above (a BAS is designed for system optimization, not individual equipment control). Any physical reset is a safety built into the system to protect the equipment. Some systems do use low pressure cutout to pump down and stop the system, but this is an automatic reset. Safeties such as high-pressure cutout and low water temperature cutout are considered hard safeties and require an action to reset (this forces a technician to leave the control room and physically look at the system). Head pressure trip is self-explanatory in diagnosing. But low water temp can be a little more difficult. I would recommend starting at a full cycle. If the system is not turning off in its lowest mechanical load, i.e. unloaded or 20-30% at your vfd, then take a close look at your dead bands.
A well operating chiller does require diligent maintenance, logs, and fine tuning. But it should never require constant attention to keep running. If yours does, don’t get to the point of acceptance. This is not how they are supposed to work. If you are having issues getting to the root cause or not sure how or what maintenance is required, feel free to contact us.