Motorized Valve Capacity and Diameter Selection Based on kW Value
To calculate valve capacity and select the correct valve diameter, please enter the required data in the calculation fields below.
If the temperature difference (°C) and pressure drop (kPa) values of 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 Kvs value lower than the calculated 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 maintaining indoor temperature conditions. In addition to their basic function, motorized valves today may also include additional functions in line with 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.
Basic Operating Principle of Motorized Valves
Let us consider a heating system example commonly encountered in the field. 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 circuits allows the desired temperature setpoint on the secondary side to be achieved through the motorized control valve located on the primary side.
The motorized valve is controlled by an electronic controller in the system. 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 rate passing through the valve under constant pressure. The use of linear, logarithmic or split characteristics depends on the requirements of the application.
The main purpose of logarithmic characteristic motorized valves, which are commonly used in the sector, is to provide a more linear and balanced heat transfer control for heat terminal units that have a logarithmic characteristic.
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.
For this reason, valve selection should not be based only on pipe diameter. Capacity is a function of the flow rate 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 valve size 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. A higher valve authority allows the valve to operate more steadily and enables 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 either fully allow or completely stop the flow of the fluid. They are generally preferred for closing or isolating a line in the installation.
3-Point Control
In 3-point control valves, the valve stem is moved to the desired position through open, close or stop signals coming from the system. It may be used in installations where less precise regulation is required.
Proportional Control
Proportional control is one of the most common control methods used in sensitive 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, the valve authority should be ensured to remain within the appropriate range in the installation, the differential pressure closing value specified by the manufacturer should be checked, and the most suitable actuator control type should be selected according to the sensitivity required by the application.