Motorized Valve Capacity and Diameter Selection Based on m³/h Value
To calculate valve capacity and select the correct valve diameter, please enter the required data into the calculation fields below.
If the temperature difference (°C) and pressure drop (kPa) values in the system are not known, the following standard values may be used:
- For heating systems: Temperature difference: 20 °C, Pressure drop: 10 kPa
- For cooling systems: Temperature difference: 5 °C, Pressure drop: 20 kPa
A valve with a value lower than the calculated Kvs value should not be used. It is normal for the selected valve to be one or maximum two sizes smaller than the installation pipe diameter.
Valve Capacity Calculation
Correct Motorized Valve Selection in HVAC Applications
In HVAC installations, motorized valves are among the most important components for controlling fluid flow and conditioning the ambient temperature.
In addition to their main function, motorized valves today may also include additional functions depending on market requirements and competition. These functions include the management of differential pressure and flow control, which are important for maintaining temperature comfort.
This article focuses only on the selection and control parameters of motorized valves operating based on temperature control. Motorized control valves are classified into several categories according to actuator operating principles, number of ports, and functional differences.
Basic Operating Principle of Motorized Valves
Let us consider a motorized control valve system diagram with a heating system example commonly encountered in field applications. In a typical residential heating installation, the system consists of two circuits: the primary circuit and the secondary circuit.
The heat exchanger separating these two systems provides the desired temperature set value on the secondary side through the motorized control valve located on the primary side. The motorized valve is controlled by an electronic controller. The system compares temperature values and sends the required control signal to the motorized valve.
Valve Control Characteristics
Valve characteristic refers to the relationship between the valve stroke position, or opening, and the flow passing through the valve under constant pressure. Different characteristics such as linear, logarithmic, and split characteristics are selected according to the application requirements.
The main purpose of logarithmic characteristic motorized valves, which are commonly used in the sector, is to provide more linear and balanced heat transfer control for heat terminal units with logarithmic characteristics.
Valve Control Theory
The most important definition encountered in motorized control valve selection is the Kvs value. In motorized valve sizing, the Kvs value is related to the flow rate and the differential pressure across the valve.
Therefore, valve selection should not be based only on pipe diameter. Capacity is a function of the flow passing through the valve and the differential pressure across the valve.
To ensure that the valve actuator can control the differential pressure across the valve, the maximum operating differential pressure value of the actuator for the selected diameter must be checked.
Valve Authority
Valve authority is a dimensionless term that expresses the ratio of the valve pressure drop to the total pressure loss in the installation where the valve is located. Higher valve authority allows the valve to operate more stably and provides more precise temperature control.
- For 3-way valves: Valve authority should preferably be greater than 30%.
- For 2-way valves: Valve authority should preferably be greater than 50%.
On/Off Control
On/off motorized valves are used to fully allow or completely stop the fluid flow. They are generally preferred for shutting off or isolating a line in the installation.
3-Point Control
In 3-point control valves, the valve stem is moved to the desired position using open, close, or stop signals from the system. This method can be used in installations where less precise regulation is required.
Proportional Control
Proportional control is one of the most common control methods used in precise closed-loop installations. The control signal is transmitted as 0(4)-20 mA or 0(2)-10 V. Since it can provide precise control even at low flow rates, it is preferred under variable fluid conditions.
In summary, the most important parameter in motorized control valve selection is correctly determining the valve body capacity required for the application. Then, valve authority must be within the appropriate range in the installation, the differential pressure closing value specified by the manufacturer must be checked, and the most suitable actuator control type should be selected according to the application sensitivity.