Specifying chillers for a variable-primary plant conversion
- Examine challenges faced when converting chiller plant from primary-secondary to variable-primary.
- Explore considerations when specifying a chiller to use in an existing system.
- Learn the effects of variable-primary pumping on chiller specification.
- Study considerations made on a large hospital and renovation project.
All chillers in a heating, ventilation and air conditioning system serve the same basic function: to remove heat from the chilled water system. However, the system that a chiller is being installed in can present challenges when specifying a chiller. A number of important factors must be considered when specifying a chiller for an existing system, including capacity needs, existing system layout, system temperatures, existing chiller plant controls, downtime to the system and physical limitations. These factors must be considered on a per-project basis due to how unique chilled water systems can be.
Mechanical, electrical and plumbing engineering firm WSP recently worked with architecture firm Kahler Slater, construction manager The Whiting–Turner Contracting Co. and mechanical contractor Bay Mechanical on an expansion and renovation project at Sentara Virginia Beach General Hospital. The facility is a 425,000–square–foot hospital serving the health care needs of the greater Virginia Beach, Virginia, area.
The project consisted of 55,000 square feet of renovation to existing operating rooms, medical surgical stepdown, intensive care unit and central sterile spaces, and a 10,000–square–foot operating room expansion. The facility has future plans for a 100,000–square–foot patient tower. The existing chilled water system was at capacity for tonnage and flow, so the renovation and expansion necessitated adding capacity to the chilled water system.
The existing chilled water system consisted of three 750–ton water–cooled centrifugal chillers and one 425–ton water-cooled centrifugal chiller in parallel. One of the 750–ton chillers was not operational and the 425–ton chiller was at the end of its service life; the total operational capacity of the plant was 1,925 tons. This capacity barely satisfied the cooling load of the existing facility and provided no redundancy. The system operated on a 10 F maximum temperature differential for both chilled water and condenser water.
Chilled water pumping was done with a primary/secondary pumping system, which included one dedicated, constant–volume pump per chiller and three variable–speed pumps on the secondary side. Condenser water was pumped via a constant–volume primary system to four existing induced–draft cooling towers. After years of additions and renovations to the facility and subsequent additions in cooling load, the existing condenser water system was struggling to meet the required flows that the system demanded.
Load analysis showed that that the expansion, renovation and future patient tower would require 2,450 tons of cooling capacity. This meant that the existing chilled water system needed to be upgraded and presented the opportunity for the facility owner to look at improving the system as a whole to save on energy and operational costs in the future.
Consideration for system redundancy needed to be made as well. Because many spaces in a hospital are critical, a chilled water system failure often is unacceptable. Adding redundancy into the chiller and pumping systems helps to not only protect the building from system failures, but allows individual chillers or pumps to be brought offline for planned maintenance without affecting the supply of chilled water to the building.
WSP focused on converting the constant-volume primary, variable-volume secondary chilled water pumping system to a full variable–primary pumping system. This existing system pumped primary chilled water through the chillers and the secondary set of pumps drew water from the primary loop and supplied chilled water to the building air handling units. While this allowed the secondary pumps to modulate speed to match the flow of the coils in the building, the primary pumps ran 24/7 at their maximum flow.
Converting to variable-primary meant installing a new set of pumps to flow the chillers and building coils in one loop. This eliminated a set of pumps, which reduced maintenance needs for the facility and presented a tremendous energy savings for the facility by reducing water flow through the chillers during part load operation and subsequently reducing pump horsepower. This also simplified the chilled water pumping controls.
WSP’s design for the chilled water system focused on replacing the nonoperational 750-ton chiller, adding a new 750-ton centrifugal chiller to replace the existing 425-ton chiller. This increased the operational capacity to the system from 1,925 tons to 3,000 tons to meet the needs for the renovation, expansion and future patient tower, while also adding 550 tons of redundancy to the system.
Designing the chilled water system
Part of the renovation and expansion included replacing existing AHUs and adding a new unit for the expansion. Because the chilled water system was being renovated simultaneously, the design team was able to select chilled water coils to work best with the upgraded chilled water system. Because one of the goals for saving energy on the new system was to reduce water flows and pumping energy, the chilled water coils were selected at a higher water temperature differential than the existing coils.
With all existing AHU coils in the building sized based on 42 F entering chilled water and a 10 F delta, the supply water temperature needed to remain at 42 F to satisfy the rest of the facility’s dehumidification needs. To reduce flows, the new coils in replacement and new AHUs were sized for a 14 F temperature differential, for a return water temperature of 56 F. This reduced flow and pumping energy by 28% for these units.
The future patient tower AHUs will be sized at a 14 F chilled water temperature differential, as well as all future AHU replacements in the existing facility. This means that as units are added or replaced in the future, the chilled water return temperature from these coils and to the plant will be increased, subsequently reducing the gallons per minute per ton of the chillers, reducing pump energy and reducing overall building energy usage.
Until all units are replaced with coils sized for a 14 F delta, the plant will be getting slightly lower return water temperature than 56 F. This is because return water temperature from each coil depends on how that coil was initially sized and on airside conditions and can’t be controlled by the chiller plant. New coils must be sized for the desired chilled water temperature differential to see increased return water temperatures at the plant and reduce flows.
This approach meant that the chillers would see varying flows through their evaporators, as well as different return water temperatures. The combination of these two factors meant that the chillers needed to be selected to ensure stable conditions throughout the full range of operating conditions. During the design phase, the chiller manufacturer used chiller selection software to simulate chiller operation at both 10 F and 14 F chilled water delta Ts, at minimum and maximum flows for the evaporators and for a range of condenser water temperatures. This ensured that the chillers would operate efficiently and prevent surge issues at all operating conditions.
Saving energy, creating efficiency
The specification of the lead chiller was critical for ensuring maximum savings from pump energy reduction. As air handler chilled water coil control valves close down during part load operation, the chilled water plant has a lower demand to produce chilled water. If the chilled water flow is reduced below the minimum evaporator flow on the lead chiller, the chiller will shut off. This minimum flow is specified by the chiller manufacturer and can vary greatly between different chillers.
An accurate control system is important to ensure that this minimum flow is met. To prevent short–cycling and extend the life of the chiller, water is bypassed from the supply to the return to maintain the minimum flow through the chiller as specified by the chiller manufacturer. The energy required to flow water through the bypass is essentially wasted energy as it is not directly used for cooling the building, so a chiller with a minimum flow of less than 50% of maximum flow was selected as the lead chiller. This allowed for the chilled water pumps to slow down to a lower speed than if a chiller with a higher minimum flow was selected, resulting in additional energy savings. This savings is especially significant because a proportional reduction in flow through a pump equates to a cubed reduction in power.
The new chillers were specified with low–pressure R123 for maximum efficiency and to match the refrigerant of the other chillers in the plant. Due to the phase-out of R123, this refrigerant will not be manufactured or imported after 2030. Although the refrigerant will be available for purchase after this date, the chiller was specified with the ability to be retrofitted to operate using R514A, which is a drop-in replacement for low–pressure R123.
R514A has a global warming potential of 2, which is significantly lower than the already-low GWP of 77 for R123. This consideration ensured that the chillers are future-proof against future environmental regulations, phased-out refrigerant shortages and gives the owner flexibility to convert the chiller to R514 if desired to meet sustainability goals or to maintain a consistent refrigerant type for all chillers at the facility.
Central utility plant
The existing central utility plant was extremely congested and allowed little room for new equipment to be installed without first removing existing equipment. Because the hospital is a 24-hour facility and operates year-round, downtime to the chilled water system needed to be minimized. Phasing was critical and the WSP team worked closely with the contractors to minimize the downtime to the facility. The replacement 750-ton chiller was installed at the beginning of construction to allow the facility to meet the cooling load through construction. New variable-primary pumps were installed once the existing 425-ton chiller was removed, before the removal of the existing primary-secondary pumping system.
An expansion of the central utility plant was constructed adjacent to the existing plant. The new 750-ton centrifugal chiller was installed in this new space along with the new condenser water pumps before demolishing the existing condenser water pumps. This also allowed for the chiller to be installed outside of the boiler room, where the existing chillers were installed. Space was allocated in the central plant expansion for future chillers to be installed when the existing 750-ton chillers need to be replaced.
The existing condenser water system was struggling to meet capacity before the project began, due to the condenser water supply piping being undersized. The condenser water pumps were having cavitation issues due to a 14-foot deficit in net positive suction head. In other words, the head pressure on the inlet side of the condenser water pumps was 14 feet less than the minimum required inlet pressure as specified by the pump manufacturer.
At the beginning of construction, WSP recommended to reduce the condenser water flow through the chillers was reduced from 3 gpm per ton to 2 gpm per ton. This reduced the flow through the cooling towers and condenser water pumps by 2,250 gpm, allowing the pumps to function and the undersized piping to be used throughout construction, until the new condenser water system could be put in place.
New condenser water pumps and a new 1,000-ton cooling tower were installed as part of the project. The condenser water pumps were installed in the central plant expansion to allow the existing pumps to operate until the new pumps were brought online. New condenser water piping was installed and sized for the future patient tower expansion load as estimated by WSP.
To minimize impact to the facility, the new chillers and pumping system was brought online in October during low cooling load conditions. All AHUs in the building were put into economizer operation to eliminate the need for chilled water. Final piping and electrical connections took place over a short two-day window. During this time, leaving air temperatures on critical AHUs were monitored to ensure supply air temperatures were maintained. This low–impact changeover was critical for a facility containing numerous critical spaces that must meet temperature and humidity requirements all day, every day.
Another key part to the project was the implementation of a heat recovery chiller installed in sidecar position, upstream of the centrifugal chillers. This heat recovery chiller implementation pulls chilled water off the return main upstream of the centrifugal chillers, cools it and pumps the chilled water back into the return main, essentially pre-cooling the return water.
On the condenser size, heating hot water is pulled out of the return main and sent through the condenser, which heats the water. Water is then pumped back into the return main, essentially pre-heating the heating water return. This moves energy from the chilled water loop to the heating water loop, instead of rejecting it to the atmosphere via cooling towers. Taps in the chilled water return main were installed to allow for this system to be installed. The existing air handlers in the facility operate at 180 F entering hot water with a 20 F delta T. The new AHUs for the renovation and expansion were installed with heating coils sized for more heat transfer and a 40 F delta T.
Once the future patient tower is built with air handler coils returning lower temperature water and future air handler replacements are completed in a similar fashion, the main hot water return temperature to the plant will be lower, yielding very favorable conditions to the application of a heat recovery chiller. Currently, existing hot water temperatures are being lowered to reduce lift and increase efficiency on the heat recovery chiller while maintaining comfortable space temperatures.
At the end of construction, the system will be commissioned by an independent commissioning agent. This will ensure that the equipment and controls are installed and functioning properly and ensure maximum operating efficiency is being achieved.
A chiller is only one part of the chilled water system and as shown by this project, a number of factors must be considered to achieve a successful chiller renovation. When looking at an existing chilled water system, it is important to understand how the system will operate during construction and once construction is complete, and to understand future needs of the facility to ensure that the chiller is specified to work at all system operating conditions.
The minimum chiller flow, chiller sequencing, refrigerant and operating temperatures must all be considered and specified. Physical constraints and phasing also must be considered and incorporated into design to minimize system downtime. Due to the careful consideration of chiller specification and all aspects of the chilled water system, this facility will now have an efficient and stable supply of chilled water for decades to come.