Chilled Water System Efficiency
Chilled water systems in HVAC are common in large commercial applications utilizing chillers to chill the water, a pumping system to pump the water (water and glycol systems are common), and chilled water coils inside air handlers or duct work to exchange heat from the air that is distributed throughout the building for conditioning. Engineers who design chilled water systems generally consider the air side and the water in design parameters of components in the chilled system to increase efficiency and comfort. Component factors and considerations in designing a chilled water system include:
- Load factors including sensible and latent heat loads
- Pipe size including friction head of the loop (for pressure and flow)
- Pump size including type of pump (constant volume or variable capacity – inverter duty motor)
- Coil size including fin spacing and coil rows for the air side
- The control sequence from chiller operation to pump operation including valves and actuators in the system
- Temperature control of the water or water glycol mix
Chilled Water System Efficiency - Sequence of Operation
Typically in most situations the chiller manufacture will control the sequence of the chiller to provide the water temperature setpoint as defined by the design engineer. The chiller manufacturer typically provides all the solid state controls to control the chillers while the building automation system will usually provide an enable command for the chiller and monitor the chilled water sequence including alarm points inside the chiller program so the end user can have remote notification of any problems with the chiller through the building automation system. With modern controls the chilled water setpoint can be changed with a reset schedule based on load demand of the air side of the chilled water system. This requires a set-up between the building automation system and the chillers local controls. The BAS can poll the load demand throughout the building and when demand is heavy decrease the chilled water temperature setpoint and when the demand is low increase the chilled water temperature setpoint. Setting up a chilled water system this way along with economizer modes can save energy dollars by reducing chiller runtime and loading factors. Each system is unique and has different requirements for load factors but when a given system is evaluated and tweaked for energy efficiency factors significant energy saving sequences can be used to save dollars and reduce runtime of equipment while still maintaining comfort levels. Another big factor in chilled water system efficiency is the balance of the system including the water and the air side. The water flow is generally controlled by pressure and maintaining the proper pressure for the flow through the coil. Again this is generally based on demand and controlled by the BAS using various control algorithms including a PID loop that controls the VFD for the pump and the actuator to the chilled water coil. It is essential for this system to function at optimum efficiency levels that the systems is balanced and PID loops tuned to prevent large swings in performance of the system. By using a control strategy with proper system selection and set-up including controls and balancing the end user can realize a large savings in energy usage. It is important to note that proper training of operators including helping them understand efficiency factors contributes to the long term success of maintain an energy efficient system. This includes a proper maintenance program to maintain the equipment properly.
Chilled Water System Efficiency - Water Side Economizers
There are some chilled water systems that utilize a water side economizer so that when the outside temperatures are ideal the chiller can be shut down and the cooling tower used to replace the chiller by utilizing cold outside temperatures to chill the water. This water side economizer can add a lot of efficiency to a chilled water system. Shutting down the chiller while continuing to provide chilled water to cool the load is a huge savings in energy costs and reduces the run time of the chiller. That translate to a savings in energy costs and reducing maintenance for the chiller with the energy savings resulting in the biggest advantage to having a water side economizer. Water side economizers require that the chilled water system be designed (or redesigned in some cases) with a heat exchanger in the system, a good control system to automate the process, and the piping system to be able to redirect the chilled water system to align for water side economizing. The same as an air side economizer works where the air is redirected in a water side economizer the water is redirected so it does require a change to the original design but this type of system can pay for itself easily in a season or two. There are other methods for saving energy and costs associated with chilled water systems and some it starts with changes in control systems and some it requires a change or redesign of the system. Either way, there are ways to increase efficiency and save money.