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.
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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)
FREQUENTLY ASKED QUESTIONS
Valve characteristics, such as flow coefficient, pressure drop, and valve authority, significantly impact the performance of a hydronic system. For example, a valve with a high flow coefficient can handle higher flow rates, while a valve with a low pressure drop can reduce energy losses. Improperly sized or selected valves can lead to poor system performance, including reduced heat transfer, increased energy consumption, and decreased system reliability. Moreover, valve characteristics can affect the stability and controllability of the system, making it essential to consider these factors during system design and operation.
Valve authority is a dimensionless parameter that represents the valve’s ability to control flow in a hydronic system. It is defined as the ratio of the valve’s flow coefficient to the system’s flow coefficient. A valve with high authority can effectively control flow, while a valve with low authority may not be able to maintain the desired flow rate. In hydronic systems, valve authority is critical because it affects the system’s stability, controllability, and overall performance. A valve with adequate authority ensures that the system operates within the desired parameters, while a valve with inadequate authority can lead to system instability and poor performance.
To determine the required valve rangeability for a hydronic system, you need to consider the system’s operating conditions, including the maximum and minimum flow rates, pressure drops, and temperatures. Rangeability is typically expressed as a ratio of the maximum to minimum flow rates and should be sufficient to accommodate the system’s turndown requirements. A valve with inadequate rangeability may not be able to maintain the desired flow rate during part-load conditions, leading to poor system performance and reduced efficiency. In general, a rangeability of 10:1 or higher is recommended for most hydronic systems.
Undersizing or oversizing valves in a hydronic system can have significant consequences on system performance and efficiency. Undersized valves can lead to inadequate flow rates, increased pressure drops, and reduced heat transfer, resulting in poor system performance and decreased efficiency. Oversized valves, on the other hand, can cause unnecessary energy losses, increased wear and tear on the valve, and reduced system controllability. In extreme cases, undersized or oversized valves can lead to system instability, reduced reliability, and even premature failure of system components. Therefore, it’s essential to properly size and select valves for a hydronic system to ensure optimal performance and efficiency.
To ensure proper valve sizing and selection for a hydronic system, it’s essential to consider the system’s operating conditions, including flow rates, pressure drops, temperatures, and control requirements. You should also consult the valve manufacturer’s specifications and performance data to select a valve that meets the system’s requirements. Additionally, it’s recommended to perform hydraulic calculations and simulations to validate the valve selection and ensure that it can operate within the desired parameters. Finally, it’s crucial to consider factors such as valve authority, rangeability, and materials compatibility to ensure that the selected valve can provide reliable and efficient operation over the system’s lifespan.
