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Control valve: to explore the balance of the circulating system and pressure stability
Features of Control Valves The primary goal in designing any HVAC system is to achieve a comfortable indoor climate while minimizing costs and reducing operational issues. In theory, the new control technology seems to be sufficient to meet the most demanding requirements, enabling greater comfort and energy savings. In practice, however, even the most sophisticated controllers do not fully achieve their theoretical performance. The reason is simple: the proper operating conditions that must be met are often overlooked, and its importance for comfort and cost can not be ignored. If we systematically analyze the operation of a HVAC system, we often notice the following problems: Not all rooms can achieve the required room temperature, especially after a high load change occurs. When the desired room temperature can be achieved, it will continue to fluctuate despite the use of sophisticated controls on the end devices. This phenomenon usually occurs in the case of low load and medium load. Although the production plant has sufficient installed capacity, it can not be transported under high loads, especially during the start-up phase. Even sophisticated controllers do not correct these faults. The reason is often related to the design defects of the circulatory system itself, mainly because the three basic conditions have not been met. The traffic at the system interface must be compatible with each other. Design flows must be achieved at all ends. The pressure difference across the control valve must not change too much. Control Valve Features The characteristics of a control valve are determined by the relationship between water flow and valve opening. When the pressure is constant, these two quantities are expressed as a percentage of the maximum value. For valves with linear characteristics, the flow rate is proportional to the valve opening. At low and medium loads, a small opening of the control valve significantly increases the flow through the valve due to the non-linear nature of the end fittings (Figure 1a). Therefore, the control loop under low load may not be stable. This article is mostly about some of the information about the second condition, and some implications for the use of two-way control valves on the end fittings. This problem can be solved by choosing the characteristics of the control valve to compensate for the non-linearity so that the output of the end unit is proportional to the valve opening. At 20% of its designed flow rate, the valve can be set to only allow 20% of the design flow at 50% opening if the end unit output is 50% of its design value. In this way, a 50% calorific value is achieved when the valve reaches 50% opening (Figure 1c). With all these flows, the valve characteristics we receive can compensate for the non-linearity of a typical end heat exchanger. This feature (Figure 1b) is called equal percentage correction "EQM". However, there are two conditions that must be met to achieve this compensation: If the pressure differential across the control valve is not constant, or if the valve size is too large, the control valve characteristics may shift, which may affect the regulatory control performance. The pressure differential across the control valve must be constant. When the control valve is fully open, design flow must be available. Control valve valve authority When the control valve is closed, the end, pipes and accessories reduce the flow and pressure drop. As a result, the differential pressure on the control valve increases. Increasing the pressure differential will shift the characteristics of the control valve. This offset can be demonstrated by controlling valve weights. The numerator is constant and depends only on the choice of control valve and the design flow value. The denominator corresponds to the ΔH achieved on the circuit. The balancing valve installed in series with the selected control valve does not change these two factors and therefore has no effect on the control valve valve power. The selected control valve should be able to achieve the best possible valve authority. According to the range of products supplied by the market, the size is generally large. The balancing valve allows the design flow to be achieved when the control valve is fully open. Its characteristics are closer to the theoretical ones, thus improving the control function (Figure 3b). In an out-of-range allocation (Figure 2a), the remote loop can sustain a higher delta H variation. In the case of low flow, the valve control valve obtained the worst. That is, when the control valve almost withstand all pump head. When using variable speed pumps, the pressure differential near the last circuit is typically kept constant (Figure 2b). In this case, the change in ΔH will be transferred to the first loop. The Δp sensor is used to control the variable speed pump. If it is located close to the last circuit, it is theoretically possible to minimize pumping costs, but problems close to the pump circuit can also be a problem. When the system is operating under a condition of average small load, these circuits may be under-flowed or, if the control valve is designed with the minimum Δp, the valve authority under design conditions will be poor. For this reason, a better compromise is to set the Δp sensor in the middle of the system to reduce Δp by more than 50% compared to a constant speed pump. Figure 2c shows the relationship between power output and valve opening (EQM control valve selected for proper fully open flow with a valve rating of 0.25). When the ΔH obtained on the circuit increases, the control valve characteristics may deteriorate, causing the control loop to oscillate irregularly. In this case, a local differential pressure controller stabilizes the Δp at both ends of the valve so that its valve strength approaches one (Figure 4a). Adjusting Control Valve Selection The size of the two-way control valve is correct when: 1- The control valve, which is fully open at design conditions, achieves the design flow. 2- Maintain sufficient control valve authority, generally higher than 0.25. When the control valve remains fully open for a longer period of time, the first condition needs to be met in order to avoid over-flow and insufficient flow in the other coils. This occurs when the coil is undersized during the start-up night after a nighttime shutdown at night, when the thermostat is set to the lowest cooling condition (which is a common practice), and the control loop is unstable Happening. To obtain design flow at design conditions, the pressure drop in the fully open control valve at design flow must be equal to the locally available pressure difference, ΔH, minus the design pressure drop in the coils and accessories (Figure 3a). Can I use this information when selecting a control valve? We assume that we can. For a 1.6 l / s flow, the control valve available on the market produces a design pressure differential of 13, 30 or 70 kPa without intermediate values. Calculated value generally can not find the corresponding product in the market. Therefore, the control valve is generally large size. Therefore, a counterbalance valve should be installed in order to obtain design flow under design conditions and to improve the characteristics of the control valve without increasing the pressure drop (Figure 3b). After selecting the control valve, we must verify that its valve power ΔpVc / ΔHmax is sufficient. If not enough, the design of the system must be reconsidered in order to be able to create a higher Δp across the smaller control valve. Some Special Designs for Local Problems For some special cases, it is better to work alone than to have the rest of the system react to abnormal conditions. When the selection of the control valve is at a critical state, or when there is a large ΔH variation in the circuit, a local differential pressure controller can be used to stabilize the pressure differential across the valve, as shown in Figure 4a. This is the case of a typical control valve with a minimum valve weight of less than 0.25. The principle is very simple. The diaphragm of the self-operated differential pressure control valve STAP communicates with the inlet and outlet of the temperature control valve. As this pressure differential increases, the force on the diaphragm increases and the STAP is proportionally closed. In this way, the pressure difference on the control valve can actually be kept constant. This pressure differential is selected so that design flow is available at the STAM when the control valve is fully open. Control valve will not be too large size, the valve authority also remained close to one. All extra pressure drops are applied to the STAP. Compared with the temperature control, the pressure control is easier, with a suitable ratio to avoid irregular oscillation. The combination of a local differential pressure controller and a variable speed pump guarantees optimal control conditions, increased comfort, and saves pump energy and reduces noise in the system. For economic reasons, this solution is usually suitable for small systems. For larger systems, ΔH varies widely and a flow rate can be limited by a differential pressure sensor connected to the balance valve (Figure 4b). When the measured pressure differential is consistent with the design flow, the control valve is not allowed to open further. This solution is well suited when the BMS system simultaneously requires that the measured flow rate be around the design value. When the end device is controlled by a switch control valve or a time proportional control valve, the pressure drop limit reduces noise and simplifies the balancing process. In this case, the differential pressure controller can apply a steady pressure differential across a set of end devices, as shown in Figure 5. This solution can also be applied to a small group of devices controlled by a regulating control valve while also increasing its valve authority. These examples are not limiting, but merely show that some special problems can be solved by a specific solution. Stable distribution of variable pressure in heating system Variable flow distribution In a heating system with a radiator, the presetting of the thermostatic valve is usually designed with the pressure difference ΔHo = 10 kPa. In the balancing process, the balancing valve STAD in the branch is set to obtain the correct total flow on the branch. This will confirm the default value, and the center of the branch can reach the expected 10kPa. If the differential pressure on the thermostatic valve can exceed 30 kPa, noise may build up in the unit, especially if some air remains in the water. In this case, it is best to use an STAP as shown in Figure 7 to stabilize the pressure drop. On each branch or small riser, a STAP can stabilize the pressure drop. The flow qs can be measured by the measuring valve STAM. Constant flow distribution in residential buildings, water temperature is adjusted by the central controller based on outdoor conditions. Stimulating the pump head may be high (relative to the thermostatic valve too high), it will cause noise. If the backwater temperature is not limited, you can use constant flow distribution. One solution is to arrange a bypass pipe AB and a counterbalance valve STAD-1 for each house (Figure 8a). This balance valve takes away the resulting ΔH. Each house also has a secondary pump with a suitable pump head (less than 30kPa). When the thermostatic valve is closed, Δp applied to the thermostat valve is maintained at an acceptable level to avoid creating noise in the system. The designed secondary flow must be slightly less than one flow to avoid backflow in bypass AB, creating a mixing point at A and lowering the water supply temperature. This is why the balance valve STAD-2 is installed on the secondary circuit. To avoid the use of a secondary pump and STAD-2, a Proportional Pressure Relief Valve, BPV, can be provided per house as shown in Figure 8b. The BPV is connected to the balance valve STAD-1 to obtain the required primary flow rate. BPV set point to choose to meet the requirements. When the thermostatic valve is closed, the pressure differential between A and B increases and tends to exceed the BPV setpoint. With BPV open, the pressure differential between A and B is kept constant. General Design Recommendations The design of a circulatory system depends on its specific characteristics and working conditions. For example, in the design phase of variable flow distribution system, we should consider whether the outage or the same procedure. Constant speed pump or variable speed pump. End device is the regulation control or switch control. Some general recommendations are valid in all cases: 1. The system must be hydraulically balanced under design conditions so that installed power can be delivered. From this point of view, there is no difference between the end device using the adjustment mode or the switch mode control. 2. The compensation method or "TA balance" must be used to optimize the balance process. This avoids the entire system back and forth repeatedly detected, thus greatly reducing labor costs. With both methods, it is possible to clearly identify if the pump size is large and modify the pump, thus reducing pumping costs. The balance program offers the possibility of detecting most of the irregular cycle points. Manual balancing valves are always capable of measuring flow for diagnostic purposes. 3. Care must be taken to adjust the correct characteristics of the two-way control valve (mostly EQ% or EQM). Correct dimensions: The control valve must be able to withstand most of the available circuit pressure differential under design conditions at full open and design flow rates. Control valve valve authority should not fall below 0.25. 4. If the last condition above can not be met in some circuits, install a local Δp controller in these circuits to increase the valve authority of the control valve and reduce the risk of noise. 5. When using a variable speed pump, the Δp sensor must be placed in the correct position to reduce pumping costs and reduce the Δp variation across the control valve to achieve the best compromise between the two. Using computer simulation means that it's easy to find the best sensor location. Conclusion A HVAC system is designed for a certain maximum load. If the system is not fully loaded due to system imbalances under design conditions, the total investment in the entire plant will not be rewarded. When the maximum load is required, the control valve fully opens and can not handle this condition. It is very difficult to determine the size of the two-way control valve. The calculated valve is generally not available on the market, so the size is usually larger. This balance of the circulating system becomes critical, and in general this part of the investment is less than one percent of the total investment in HVAC. Every morning, after a night of shutdown, you need to mobilize all the power to restore comfort as soon as possible. A well-balanced system can do it quickly. If the start-up time can save 30 minutes, compared to 8 hours of work time, you can save 6% of energy consumption per day. This is more than all dispensing pump costs. In variable flow distribution, pump power consumption generally accounts for less than 5% of the seasonal consumption of refrigeration units. The cost of lowering the room temperature by one degree is 10 to 16%. Therefore, getting the right comfort is the best way to save energy. Therefore, all measures must be taken to reduce pumping energy consumption so that they do not adversely affect the operation of the end-device control circuit. Pumping costs can be reduced by maximizing the design water temperature difference, T, and variable speed pumps using the best-positioned Δp sensor. Under moderate load, stable regulation of PI control (Figure 1a) is less than the flow required for switching control, thus reducing pumping costs as well. But the most important thing is to compensate for the pump size is too large. Counterbalance valves that are set with the compensation method show this oversized condition. It can be seen that all overpressures are located on the balance valve near the pump. After correcting the pump, the balancing valve can be reopened. Circulating balance requires the right tools, the latest programs and effective measuring devices. The manual balancing valve is clearly the most reliable and simplest product to get the correct flow under design conditions and always measures the flow for diagnostic purposes. If necessary, can also be connected with the pressure controller.