Most hospital facilities we work with have chilled water system issues. These issues tend to manifest as hot offices, unstable OR temperatures, or concerns of critical equipment failure. Many hospitals think the solution is to “add another chiller.” In contrast, we have found it less expensive to identify and resolve system issues than to simply add chiller capacity. But, this requires a shift in perspective. Facility managers who can make the leap to a different mindset can expect to see their operating costs go down.
Problems with central chilled water systems tend to fall in one of two categories. They may suffer from the inability to maximize chiller capacity. Or they may have difficulty managing chilled water flow.
Difficulty Using Installed Chiller Capacity
Many systems were originally installed before the use of variable speed equipment and systems. Older systems with primary, secondary, and even tertiary pumping loops were designed to supply a constant volume of water. Three-way valves were common. These techniques create continuous chilled water flow regardless of load. The net effect: wasted pump energy and chillers that only fully load (and function at peak efficiency) during the hottest days of the year.
Difficulty Managing Chilled Water Flow
Over time, hospitals reduced pumping costs by converting three-way valves to pseudo two-way valves by plugging the third port. They also eliminated tertiary pumps. New variable frequency drives (VFDs) controlled the speed of pumps to vary the flow of chilled water. Theoretically this can reduce pumping costs and improve the loading (and efficiency) of chiller. But it has proven difficult to apply to older piping systems not originally designed with variable pumping in mind.
Moreover, hospitals have overridden chilled water valve controls. This attempts to exceed design cooling from air handling equipment. Yet, changes to the flow and control of the original design result in a system that does not function well. Such modifications make issues difficult to diagnose when they occur.
Decreased capacity usually stems from the accumulated effect of minor modifications over time: usually, a single precipitating event isn’t the cause. Investigating the causes and effects of chilled water system maladies goes beyond chillers and pumps. Root cause analysis means getting acquainted with the functionality of cooling coils, controls, control valves, pumping pressure controls, and interconnected piping systems.
When facility directors begin to consider causes and effects from this standpoint, most have the “aha” moment that the apparent cause was not the true culprit.
How to Address Chiller Issues
By “tuning up” an existing chilled water system, upgrading weak links, and determining optimum flows, one can eliminate the need (and cost) of adding a chiller. The resulting system can then provide the maximum comfort for the least cost. The improved efficiency is also realized year after year in reduced energy costs.
Start small by trying one thing to prove that the concept works. Even minor tweaks can increase the Delta T by 25 to 30 percent and put more load on chillers. Doing so allows one to see the possibility of a more efficient plant. Seeing the effects of change, and showing the boss that you didn’t have to spend money to achieve it, is often the inspiration to do more.
One long-time hospital client called FreemanWhite about an inability to load chillers. Upon visiting the site, we observed chilled water temperature entering cooling coils around 44 degrees and leaving around 47-48 degrees. In contrast, the chiller plant was designed for water to enter the chiller at 54 degrees and exit at 44 degrees. ( a 10 degree Delta T.)
A control valve controlled the chilled water flow, as was commonplace at the time the system was installed. A BMS system recorded supply air discharge temperature set points and sensor readings. A set point of 50 degrees in conjunction with 52 degree discharge temperatures caused the chilled water valve to be wide open.
Upon reviewing the plans, we discovered that the air handling units were designed for a 55 degree set point, well above the current state. Staff recalled resetting the temperature in years past to address staff complaints. Further, building renovations added heat load when more air could not be forced through the duct system.
We manually throttled back the chilled water system flow with service valves. This increased the difference in temperature between the chilled water entering versus leaving the coils. While it did not raise the cooling coil discharge air temperature, it did increase the efficiency of chilled water use. By restricting chilled water flow through the upstream coils, cooling coils further down the piping system received more chilled water.
While not a complete solution, this was an inexpensive way to address the problem. These modifications provided better loading of existing chillers and forced chilled water to units connected farther away from the chiller pumps.
When money for facility upgrades and capital improvements is increasingly scarce, alternatives to purchasing new chillers become more important. Moreover, we’ve seen that new chillers are no guarantee that a hospital won’t still have the same problems.
Get to know FreemanWhite Engineers and see how they can help you address chilled water system issues: