Every component in a hydronic system has some level of influence on the control and performance of a hydronic system. Valves are key because they are the controlling devices at the critical areas of heat transfer.
- Hydronic systems are highly interdependent systems.
- Each system component will interact with one another and impact performance.
- Valves are the critical devices that control heat transfer in hydronic systems.
- Proper valve sizing and selection involves looking at the complete system.
- Valves not sized and selected correctly lead to undesirable outcomes not only at the heat transfer device, but also the overall mechanical system.
Why do we need to be aware of the valve characteristics?
Simply put, it is because of the nature of the equipment we are using. For control of the system, we desire a linear response and output, but we are dealing with a heat transfer device that is highly non-linear. From image A, we have a typical heating coil characteristic. You can see it is highly non-linear. A small percentage of flow change at the high end does not result in a reduction in output capacity until you start going past 50% flow.
The flow through the coils are regulated by our control valves and the characteristics they inherently hold. In order to get the desired linear response/output for the control system, we need to match up coil and valve characteristics that give us the desired response type. In this case, we’re matching up an equal percentage valve characteristic with the heating coil to product the linear response system. Keep in mind that this is an ideal condition though, and the real world does not always happen in this fashion.
- Equal Percentage Characteristics
- Typically used for water applications
- Used in systems with large changes in pressure drops
- Used where a small percentage of the total pressure drop is permitted by the valve
- Used in temperature and pressure control loops
- Linear Characteristics
- Typically used for steam applications
- Used in liquid level or flow loops
- Used in steady state systems
- Used when pressure drops across the valve is a large portion of the total pressure drop
- Quick Opening Characteristics
- Used for frequent on/off service
- Used for systems where “instant” large flow is required (i.e. safety systems)