The Pressure Independent Control Valve (PICV) technology was brought to market 18 years ago. This technology has now matured and is accepted as a proven application for optimizing hydronic system delta T, enhancing system performance and reducing energy cost. A high quality, industrial grade PICV offers precise hydronic flow control for heating and cooling applications. This article describes how PICVs operate resulting in improved system efficiency and control. Image courtesy of Flow Control Industries, Inc.
In the early 1990’s Flow Control Industries of Woodinville WA, (www.flowcontrol.com) patented their Delta P Valve, the first two-way PICV for use in the HVAC industry. Their unique design consists of precisely machined valve components coupled with a self-contained differential pressure regulator installed in the valve body, upstream of the actual control valve, which is also installed in the same valve body.
The machined valve components result in high rangeability performance while the self contained pressure regulator maintains a constant differential pressure across the control valve irrespective of the differential pressure across the valve body. Maintaining a constant differential pressure across the control valve independent of system differential pressure ensures elicited modulating flow response only to system controller signal input to the control valve actuator. Changes in flow through the PICV will not occur due to system changes in differential pressure across the PICV valve body. Non PICV installations will experience changes in flow through the control valve due to changes in differential pressures across the control valve body irrespective of actuator induced flow changes.
The major operational features of the PICV are:
- Accurate flow control, High Rangeability over a wide range of flow rates.
- Eliminates the need for system balancing due to pressure independent operation.
- Enhances system delta T performance which results in optimizing overall system efficiency.
- Enhanced system zone control.
The primary purpose of the PICV in an HVAC system is to minimize flow through the coil in an air handler while maintaining process variable set point control (coil discharge air temperature). Hydronic system performance has varied from Constant Volume to Variable Volume to Variable Volume with Pressure Independent Flow Control.
Historically, constant volume hydronic systems were widely used to provide flow through the system coils. Control valves used for this application were three-way control valves which are constant flow devices. This type of control resulted in modulating flow through the coil in response to the controller signal output to the actuator but total system flow to the coil assembly was constant with excess flow bypassing the coil and returning to the distribution system via the three-way control valve bypass port. The bypass leg of the three-way control valve required a balancing valve to limit excessive bypass flow. Balancing these systems was usually performed with each control valve at maximum flow conditions. Flow limiting devices were later introduced and used to limit flow to a maximum value with graduated maximum flow control up to scheduled maximum flow. These installations still require a balancing contractor to set and verify flows for both the coil and bypass leg of the three-way control valve.
Constant Volume systems were the industry standard prior to the advent of the two-way control valve and variable flow pumping technology. Constant Volume systems require constant flow and associated pumping horsepower to support the system flow requirements. Process variable control is not exact as three-way control valves are not pressure independent and characteristically have low rangeability. These performance characteristics coupled with flow bypass for other than maximum flow conditions result in a system delta T that is always less than design delta T (Low Delta T) for the installed coils. Low delta T results in higher required system flow rates. High system flow rates result in a flow limited plant condition which requires chillers to stage in response to flow and not load. These combined system factors result in inefficient operation with resulting high operating costs.
Variable flow pumping technology provides an energy efficient pumping method to accommodate system variable flow resulting when two-way control valves are used to control coil flow in a hydronic system. A variable flow, two-way control valve system will, by its nature, experience variations in differential pressure due to varying cooling coil loads and resultant flow demands. Standard, commercial-duty, pressure dependent two-way control valves have low rangeability performance characteristics and a wider control span. Couple that with the fact that as the system differential pressure changes across the valve body, flow will change through the valve without any movement of the actuator and control valve.
This pressure dependent change in flow is in response to the change in differential pressure across the control valve. This non-actuator induced control variation will result in a change of the process variable. This in turn, initiates a controller signal to the valve actuator, changing the valve position as required. All this activity is in response to a process variable non-event. These changes in flow rates also result in additional pumping energy requirements that are excessive since there was no change in the process variable due to cooling zone requirements.
Using a Variable Speed Drive (VSD) for pump control, enables the pump to change speed based on control differential pressure set point at the hydraulically most remote point(s) in the system. A variable speed pumping system will provide modulated flow to meet the changing flow requirements in the system and also reduce system differential pressure across the control valves. Differential pressure (DP) control for a variable speed pumping system enables pumps to develop only the head required for supporting the most hydraulically remote component in the system. However, both flow limiting devices and system balancing are required for this system configuration to ensure excessive flow is not experienced by system coils, especially when calling for maximum coil flow rates.
Variable Flow with Pressure Independent Flow Control
Installing pressure independent two-way control valves, instead of pressure dependent two-way control valves typically used in variable flow systems is the best of both worlds. The PICV integrated differential pressure regulator located in the valve body eliminates any differential pressure variation across the control valve located within the same valve body. This results in control valve modulation that is only in response to a controller signal to the actuator and not in response to a change in flow due to system differential pressure change across the valve body. The high rangeability PICV provides enhanced control resulting in elevated delta T performance and reduced flow to serve system loads.