Where and how to specify pressure-independent control valves in hydronics
While commercial hydronic systems are generally reliable and efficient, they can experience a range of issues that can impact performance and lead to increased energy costs and equipment failure
- Review how low delta T syndrome can affect performance and energy usage in chilled water systems.
- Learn how pressure-independent control valves (PICV) operate to maintain constant flow with system pressure changes.
- Understand how pressure-independent control valves can solve low delta T syndrome.
- Hydronic systems are typically balanced to ensure the proper flow in an HVAC system.
- Pressure-independent control valves (PICV) can help optimize heat transfer and ensure that the temperature difference or delta T, between the supply and return water is consistent.
- Low delta T syndrome can be a common occurrence that can be solved using pressure-independent control valves.
Hydronic systems operate to maintain flow through a system and typically at the end of a project, flow to different devices and units are balanced to maintain the flow required.
However, as demand from the different coils within the system changes, the pressures throughout the hydronic system also change. As filters become clogged, increasing pressure drop, different pressures are seen within the system, which also affect flow at each coil. If additional coils are added or removed from the system, the entire hydronic system should be rebalanced but is rarely done.
Low delta T syndrome
Low delta T syndrome is a condition that can occur in commercial heating, ventilation and air conditioning (HVAC) hydronic systems when the temperature difference between the supply and return water is lower than expected or designed. This condition results in poor performance and increased energy consumption as additional equipment is required to operate.
When water temperature difference decreases, more flow is required and since pump energy is a cubic function of flow, pump energy costs increase substantially. In addition, more chillers may be required to operate to maintain the flow requirements through the system.
This is expressed by the typical heat capacity equation we are familiar with when accommodating for water:
Eq 1: q = m x C x ΔT
Or, converting for water:
Eq 2: Btuh = 500 x gpm x ΔT
One of the causes of low delta T syndrome is poor water control, which can be caused by several factors:
Poorly balanced system: If the system is not properly balanced, some areas may receive more water flow than others. With improper balancing, typically the coils hydraulically closest to the pumps overflow to meet the flow requirements for coils that are hydraulically furthest away.
Oversized pumps: If the pumps in the system are too large and unable to turn down to appropriate requirements, they may push water through the system too quickly, which can lead to poor heat transfer and low delta T.
Air in the system: If there is air in the system, it can also prevent proper water flow with improper heat transfer, which can cause low delta T syndrome.
Although low delta T syndrome can occur throughout the day, it is more prevalent during unoccupied hours when cooling load is typically lower. When the building is unoccupied, the heat generated by occupants and the equipment is reduced and the outside air temperature may also be lower. As a result, the chilled water supply temperatures remain the same, but the return water temperature is reduced.
Low delta T syndrome can lead to several negative impacts on the system, including reduced heating or cooling capacity, reduced efficiency and increased energy consumption.
Pressure-independent control valves (PICV) are a type of control valve used in hydronic systems that can more accurately maintain the necessary flow rate, regardless of changes in the differential pressure across the valve. One type of PICV operates using a combination of a control valve and a differential pressure regulator. The differential pressure regulator is located downstream of the control valve and maintains a constant differential pressure across the valve. This ensures that the flow rate remains constant, even if the system pressure changes due to varying demand or other factors.
In a variable volume hydronic system, the pump increases and decreases in speed to maintain a system differential pressure depending on the flow requirements of the individual coils. At an individual control valve, as the pump increases in speed and flow, the control valves adjust the flow by closing or opening the orifice to allow flow. As the system pressure changes, the valve must modulate to maintain a constant flow.
If the valve does not modulate, an increase in pressure translates to an increase in flow through the valve. A decrease in pressure translates to a decrease in flow. To maintain a constant flow as the pressure changes, the actuator on the control valve moves the plug up and down to close or open the orifice. By decreasing or increasing the opening, flow is controlled.
PICVs are typically controlled by a building automation system that monitors the temperature in different zones of the building or monitors a coil discharge temperature and adjusts the flow rate to maintain the desired temperature — like a standard modulating two-way control valve. The difference is with the differential pressure regulator, which is the key component and typically consists of a spring-loaded diaphragm in combination with the valve assembly. The spring in the cartridge provides a constant force on the diaphragm, the diaphragm senses the differential pressure across the control valve and adjusts the regulator to maintain a constant pressure drop, allowing it to maintain a set flow over a range of system differential pressures.
The control valve within the PICV operates in the same fashion as a conventional modulating control valve with the valve plug moving up and down to adjust the flow based on the coil requirements. By decreasing or increasing the opening, flow is controlled.
In a PICV, the differential pressure regulator use a small capillary tube to operate the spring-loaded diaphragm, which adjusts an orifice to maintain a constant pressure. As system pressure increases, the diaphragm moves toward a closed position to maintain constant pressure.
As pressure in the system decreases, the diaphragm increases the flow area to maintain the constant pressure again. The spring controlling the movement of the diaphragm is selected and calibrated to maintain the constant pressure. This means that the control valve can be fully modulated to control flow while the differential control valve maintains pressure.
By combining the function of a control valve and a differential pressure regulator, a PICV can provide precise control over the flow rate while maintaining a constant pressure drop across the valve.
An additional benefit of the PICV is to limit the movement of the actuator, prolonging its life. As the system flow and pressure continuously change throughout the system as, the actuator on a conventional two-way modulating control valve must constantly adjust to maintain a constant flow. As the differential pressure regulator maintains the constant differential pressure, the actuator can remain in the same position and movement is limited.
The differential pressure regulator is typically set to a specific differential pressure during installation, based on the requirements of the system. This pressure drop will vary depending on the flow rate and the specific system requirements, but maintaining a constant differential pressure across the control valve is what allows for accurate flow.
There are many types of PICVs, some of the more common types include:
Fixed orifice: A fixed orifice PICV has a fixed flow orifice size that provides a constant flow rate regardless of pressure variations. It is the simplest and most economical types of PICV and is commonly used in small or low-flow applications.
Dynamic balancing: A dynamic balancing PICV incorporates a pressure regulator that adjust the valve opening to maintain a constant flow rate as the system pressure varies. It is an ideal choice for systems with variable pressure drops, such as those with multiple coils.
Self-acting: A self-acting PICV has a control element that responds to changes in the system pressure to maintain a constant flow rate. It does not require external power or a control signal and is an excellent choice for systems with no power supply.
Electronic: An electronic PICV uses an electronic actuator to control the valve opening, providing precise control of flow rate regardless of pressure variations. It is commonly used in large or complex systems where accurate control is critical.
Ball valve: A ball valve PICV has a flow control cartridge that maintains a constant flow rate regardless of pressure changes. It is often used in high-flow applications.
When comparing the total installed cost of pressure-independent control valves to the cost of conventional two-way modulating valves, components such as circuit setters may no longer be required and time on-site calibrating each valve is reduced.
How to use PICVs
Pressure-independent control valves can be a helpful solution to address low delta T syndrome in HVAC hydronic systems. PICVs can help maintain a constant maximum flow rate through coils they serve, regardless of changes in the system pressure or load, which can help prevent low delta T syndrome and ensure efficient operation.
By maintaining a constant maximum flow rate, PICVs can help to optimize heat transfer and ensure that the temperature difference (delta T) between the supply and return water is consistent. This can be particularly helpful in systems that are prone to low delta T syndrome, as it can help to overcome issues related to poor water flow, such as oversized pumps.
PICVs can also help maintain system balance even after changes in the system are made. Any changes to the system, with an addition or a removal of a coil, can change the dynamics of the system. In this way, PICVs are beneficial as the system does not have to be rebalanced after any changes as the differential pressure regulator allows the valve to continue to maintain the proper flow.
When low delta T syndrome is present in a chilled water system, additional chillers may be required to operate based on the system flow requirement but not necessarily the load. Meeting building cooling requirements can be difficult when on a design day if the hydronic system is not properly balanced.
It is important to properly size and commission these hydronic valves. If not done, the following are items to consider:
Valve sticking or binding: Over time, debris or mineral buildup in the system may cause the valve to stick or bind, resulting in poor control and reduced flow rates.
Incorrect valve sizing: PICVs are easy to size based on the flow rates and will always maintain good controllability. However, always ensure that the design flow is within the recommended gallon per minute range for a given size.
Inadequate maintenance: Lack of proper maintenance may lead to valve failure or malfunction. Regular visual inspections and general cleaning can help prevent unnecessary issues from general dirt and grime buildup.
Control system integration: The PICV is a critical component of the HVAC system and must be properly integrated with the control system. Issues with communication, calibration or control algorithms may result in poor performance or instability, like typical control valves.
As with all control valves, proper sizing and commissioning is critical to ensure optimal performance and longevity.