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Control valve characteristics
The primary goal in designing any heating and air conditioning system is to achieve a comfortable indoor climate while minimizing costs and reducing operational problems.
In theory, the new control technology seems to be sufficient to meet the most demanding requirements, to improve comfort and to save energy.
But in practice, even the most sophisticated controllers can't fully realize their theoretical performance. The reason is simple: often overlook the correct operating conditions that must be met, and its importance for comfort and cost cannot be ignored.
If we systematically analyze the operation of HVAC systems, we will often notice the following problems:
Not all rooms can reach the required room temperature, especially after a high load change.
When the desired room temperature is reached, despite the use of a sophisticated controller on the end device, it will continue to fluctuate. This phenomenon usually occurs under low load and moderate load conditions.
Although the production unit has sufficient installed capacity, it cannot be transported under high load, especially during the start-up phase.
Even with a sophisticated controller, these faults cannot be corrected. The reason is often related to the design flaws of the circulatory system itself, mainly because the three basic conditions are not met.
The traffic at the system interface must be compatible with each other.
The design flow must be reached at all ends.
The pressure difference across the control valve cannot be changed too much.
Control valve characteristics
The characteristics of the control valve are determined by the relationship between the water flow and the valve opening. When the pressure difference is constant, these two quantities are expressed as a percentage of the maximum value.
For valves with linear characteristics, the water flow is proportional to the valve opening. Under low and medium loads, the slight opening of the control valve significantly increases the flow through the valve due to the non-linear nature of the end device. Therefore, the control loop may be unstable under low load.
This article is primarily about some of the information about the second condition, as well as some implications for using a two-way control valve on the end device.
This problem can be solved by selecting the control valve characteristics to compensate for the non-linearity, so that the output of the end device is proportional to the valve opening.
When the water supply flow is 20% of its design flow rate, if the output of the end device is 50% of its design value, the valve can be set so that it can only allow 20% of the design flow rate when the opening degree reaches 50%. Thus, when the valve reaches 50% of the opening, 50% of the heat output can be obtained. By analogy to all flows, the characteristics of the valves we obtain compensate for the nonlinearity of typical end heat exchangers. This feature is called the equal percentage correction "EQM".
However, to obtain this compensation, two conditions must be met:
If the pressure difference across the control valve is not constant, or if the valve size is too large, the control valve characteristics are offset, which may affect the regulation control performance.
The pressure differential across the control valve must be constant.
When the control valve is fully open, the design flow must be available.
Control valve authority
When the control valve is closed, the flow and pressure drop in the ends, pipes, and fittings are reduced. This causes the pressure difference across the control valve to rise. An increase in the differential pressure shifts the characteristics of the control valve. This offset can be manifested by controlling the valve authority.
The numerator is constant and depends only on the choice of control valve and the design flow value.
The denominator corresponds to the ΔH reached on the loop. The balancing valve installed in series with the selected control valve does not change these two factors and thus has no effect on the control valve authority.
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 too large. The balancing valve allows the design flow to be achieved when the control valve is fully open. Improve the control function by bringing its characteristics closer to the theoretical characteristics.
In an alien assignment, the remote loop will experience a high ΔH change. In the case of low flow rates, the resulting control valve has the worst valve authority. That is, when the control valve is almost full of the pump lift.
When using a variable speed pump, it is generally necessary to keep the pressure difference close to the last circuit constant. In this case, the change in ΔH will shift to the first loop.
The Δp sensor is used to control the variable speed pump. If it is located close to the last loop, it is theoretically possible to minimize pumping costs, but loops close to the pump can also cause problems. These circuits may run out of flow when the system is operating under average small load conditions; or, if the control valve is designed for a minimum Δp, the valve authority under design conditions will be poor. To this end, a better compromise is to set the Δp sensor in the middle of the system to reduce the change in Δp by more than 50% compared to a fixed-speed pump.
When the ΔH obtained on the loop increases, the control valve characteristics may deteriorate, causing the control loop to generate irregular oscillations. In this case, a partial differential pressure controller can be used to stabilize the Δp at both ends of the valve so that its valve weight is close to one.
Adjustment control valve selection
The size of the two-way control valve is correct under the following conditions:
1- Design valves can be reached by fully opening the control valve under design conditions.
2- Maintain adequate control valve authority, typically above 0.25.
When the control valve remains fully open for a longer period of time, the first condition needs to be met to avoid overcurrent and insufficient flow in other coils. This occurs during the morning start-up after the nighttime shutdown, when the coil size is small, the thermostat is set to the lowest value of the cooling condition (this is a common practice), and the control loop is unstable. Happening.
In order to obtain the design flow under design conditions, the pressure drop in the fully open control valve at the design flow must be equal to the locally available differential pressure ΔH minus the design pressure drop in the coil and accessories. Can I use this information when selecting a control valve? We assume that we can.
For flow rates of 1.6 l/s, control valves found on the market produce design pressure differentials of 13, 30 or 70 kPa without intermediate values. The calculated values ​​are generally not found in the market. Therefore, control valves are generally oversized. Therefore, a balancing valve should be installed to obtain design flow under design conditions and improve the characteristics of the control valve without increasing the pressure drop.
After selecting the control valve, we must verify that its valve weight ΔpVc / ΔHmax is sufficient. If not, the design of the system must be reconsidered so that a higher Δp can be formed across the smaller control valve.
Some special designs for solving local problems
For some special cases, it is better to have a separate treatment than to let the rest of the system react to abnormal conditions.
When the selection of the control valve is in a critical state, or when a large ΔH change occurs in the circuit, a partial differential pressure controller can be used to stabilize the differential pressure across the control valve, as shown in Figure 4a. This is the case where the general control valve minimum valve weight is reduced to 0.25.
The principle is 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 differential pressure increases, the force on the diaphragm increases and the STAP is closed proportionally. In this way, the pressure differential across the control valve can actually remain constant. This differential pressure is chosen so that the design flow can be obtained at STAM when the control valve is fully open. The control valve will never be oversized and its valve authority will remain close to one.
All extra differential pressure is applied to the STAP. Compared with temperature control, the control of differential pressure is easier, and an appropriate ratio can be used to avoid irregular oscillation.
Combining a partial differential pressure controller with a variable speed pump ensures optimum control conditions, improved comfort, and energy savings in the pump and noise in the system.
For economic reasons, this solution is usually suitable for small systems.
For larger systems, the ΔH varies greatly, and a differential pressure sensor connected to the balancing valve can be used to limit the flow. When the measured differential pressure matches the design flow, the control valve is not allowed to open further. This solution is well suited when the BMS system requires that the measured flow value be near the design value.
When the end device is controlled by a switch control valve or a time proportional control valve, the restriction of the differential pressure can reduce noise and simplify the balancing procedure. In this case, the differential pressure controller can apply a stable differential pressure across a set of end devices.
This solution can also be applied to a small set of devices controlled by an adjustment control valve while also increasing its valve authority.
These examples are not limiting, but merely indicate that some specific problems can be solved by a specific solution.
Stable pressure difference in heating equipment
Variable flow distribution
In a heating installation with a radiator, the preset of the thermostatic regulating valve is usually considered to have a pressure difference ΔHo = 10 kPa.
During the balancing process, the balancing valve STAD in the branch is set to be able to obtain the correct total flow on the branch. This confirms the preset value and the center of the branch can reach the expected 10 kPa.
If the differential pressure on the thermostat can exceed 30 kPa, noise may be generated in the unit, especially if some air remains in the water. In this case, it is best to use a STAP to stabilize the differential pressure.
On each branch or small riser, a STAP stabilizes the differential pressure.
The flow rate qs can be measured by the measuring valve STAM.
Constant flow distribution
In residential buildings, the water supply temperature is regulated by the central controller based on outdoor conditions.
The head of the stimulation pump can be very high (as opposed to a thermostat valve), which can cause noise. If there is no limit to the return water temperature, a fixed flow distribution can be used.
One solution is to equip each home with a bypass pipe AB and a balancing valve STAD-1. This balancing valve takes away the resulting ΔH. Each home is also equipped with a secondary pump with a suitable pump head (less than 30 kPa). When the thermostat valve is closed, the Δp applied to the thermostat valve is maintained at an acceptable level to avoid noise in the system. The secondary flow rate must be designed to be slightly lower than the primary flow to avoid backflow in the bypass AB, creating a mixing point at A and lowering the water supply temperature. This is why the balancing valve STAD-2 is placed on the secondary circuit.
To avoid the use of secondary pumps and STAD-2, a proportional pressure relief valve BPV can be placed for each home. The BPV is connected to the balancing valve STAD-1 to obtain the required primary flow. The choice of BPV set point should meet the requirements. When the thermostat valve is closed, the differential pressure between A and B increases and there is a tendency to exceed the BPV set point. The BPV is opened to keep the pressure difference between A and B constant.
General design advice
The design of a circulatory system depends on its specific characteristics and working conditions. For example, in the design phase of a variable flow distribution system, consideration should be given to:
The trip is still the same.
The constant speed pump is also a variable speed pump.
Whether the adjustment device or the switch control is on the end device.
Some general recommendations are valid in all cases:
1. The system must be hydraulically balanced under design conditions to allow the installed power to be delivered. From this point of view, there is no difference in whether the end device is controlled or switched.
2. The compensation method or “TA balance†must be used to optimize the balancing procedure. This avoids repeated testing of the entire system, which greatly reduces labor costs. Both methods can be used to clearly determine if the pump size is too large and modify the pump, thus reducing pumping costs. The balance program provides the possibility to detect most of the irregular loop points. The manual balancing valve always measures the flow for diagnostics.
3. Care must be taken to adjust the two-way control valve
The correct characteristics (mostly EQ% or EQM).
Correct size: The control valve must be able to withstand most of the available circuit differential pressure under design conditions at full open and design flow.
Control valve authority should not fall below 0.25.
4. If the last condition above cannot 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 the variable speed pump, the Δp sensor must be placed in the correct position to reduce the pumping cost and reduce the Δp change at both ends of the control valve to achieve the best compromise between the two. Using computer simulation, the best position of the sensor can be easily found.
in conclusion
A HVAC system is designed for a certain maximum load. If the system is not balanced under the design conditions and the full load cannot be reached, then the total investment of the entire device will not be rewarded. When the maximum load is required, the control valve is fully open and still cannot handle this condition. It is difficult to determine the size of the two-way control valve, and the calculated valve is generally not available on the market, so it is usually too large in size. This balance of the circulatory 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 downtime, you need to mobilize all your power to restore comfort as quickly as possible. A well-balanced system can be done very quickly. If the start-up time can save 30 minutes, compared to 8 hours of work time, you can save 6% energy consumption per day. This is more than all the pumping costs.
In variable flow distribution, pumping energy consumption typically accounts for less than 5% of the cooling unit's seasonal consumption. The cost of reducing the indoor 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 the pumping energy consumption so that they do not adversely affect the operation of the end device control loop.
Pumping costs can be reduced by maximizing the design water temperature difference.T and using a variable speed pump that optimally positions the Δp sensor. At moderate loads, stable regulated PI control is less than the flow required for switch control, thus also reducing pumping costs.
But the most important thing is to compensate for the large size of the pump. A balancing valve set by the compensation method can show such a large size condition. It can be found that all of the overpressure is located on the balancing valve close to the pump. This balancing valve can be reopened after correcting the pump.
Cycle balancing requires the right tools, the latest procedures and an efficient measuring device. The manual balancing valve is clearly the most reliable and simple product to achieve the correct flow rate under design conditions, and the flow can always be measured for diagnostics. It can also be connected to the differential pressure controller if necessary.
