【 Weak current College 】 indoor clean air conditioning system, as well as pressure control

1 Overview

Pressure control in clean air conditioning system is a very important aspect. Only through the purification of regional control of pressure differential, ensure reasonable airflow to achieve clean and process requirements. For example, Cleanroom must maintain a positive pressure so that the outside world without purification of air purification are not into people, guarantee the cleanliness level of the region; and to purify the region different pressure control, achieve the function of purifying partition, GMP requirements in different purification level region of the control pressure difference should be not less than + 5Pa. in biosafety in cleanrooms, pressure control is guaranteed safety barrier of key indicators, in the laboratory building specification that must make the laboratory of negative pressure gradient is stable and reliable control. Therefore for a clean air conditioning system, the pressure control is very important.
Pressure control in implementation is difficult, especially in biosafety laboratory, to obtain and maintain accurate and stable pressure differential for control engineers, absolute is a challenging task. So the design pressure control system, must be based on the actual situation in the following areas for analysis and determine:
① risk analysis and assessment;
② setting air flow system and select the VAV system;
③ pressure control and flow control method;
④ control signal and noise;
⑤ system stability and speed of response;
⑥ architectural structure on the influence of pressure differential control; air duct leakage on the influence of pressure control.
First, you must control the pressure differential for analysis of risk, such as high levels of biological safety laboratories, because it has a high risk of biological contamination, various related standards to which they have remained stable pressure gradient pollution prevention requirements strictly leakage of, control systems mustbe able to stably achieve such control objectives.
2 pressure control method

For pressure control system, the result reached by essentially on permeability or exudative air control, for the purposes of its control strategy can be divided into passive and active control.

Fixed amount of wind (CAV) is a passive control method, it uses a manual air control valve, and a simple air supply and exhaust air balance, air-exhaust less (or more) a certain amount (more air flow), to achieve the desired pressure. In the select will air this control policy must be carefully considered because there is a constant air flow system highlights the limitations. There are the following:

(1) all the time, the equipment must be maintained constant air supply and exhaust air volume.

(2) does not have any exhaust device (such as a biological safety Cabinet, etc.) to increase or decrease, bad flexibility. Future extensions because the system capacity limitations restricted.

(3) must be at full load design, to the larger allowance to cover because the filter and so air supply and ventilation system performance decreases, continuous full load operation so that the energy consumption greatly, so the running cost is very high.

(4) since the fan system, filter systems, performance degradation or air valve position changes, and other cases, the system will often need to retake the air balance adjustment, requires a large amount of maintenance.

(5) since, at all times is a big air volume, noise is too high. So if you cannot accept these limitations, you should select this control strategy. At present, through the air and exhaust air pipe, pressure-independent type of air flow control devices (such as the venturi valve) for air flow system, to a certain extent can proactive, dynamic adjustment of flow, eliminate system of static pressure fluctuations on flow, thus ensuring a constant flow rate and control of stability.

VAV system (VAV) is a proactive strategy of pressure control, flow control valve with electric constant on air or exhaust volume adjust pressure to maintain hope. Active VAV pressure control methods can be divided into two kinds: pure pressure control (OP) and air (also known as flow tracking) control (AV).

2.1 pure pressure control method

Pure pressure control method is relatively straightforward, its basic principle figure 1. its control principle is: pressure sensor for measuring indoor and reference areas of differential pressure (OP), and the set point (i.e. the expected pressure difference) comparison, the controller according to deviations by PID algorithm on the air (or exhaust volume) control, which meets the requirements of the pressure differential. You can see that the air (or exhaust volume) is a pressure differential (Δ p), set point and PID constants (α, β) function.

Another similar pressure control methods are based on Bernoulli's principle, use a mounted on the small tube, small wind speed probe tube placed in clean room and reference areas of open holes, clean room and reference area of the pressure difference would allow air to flow from this small tube, pipe velocity probe to sensing clean indoor and reference areas of air velocity, thus under Bernoulli principle use wind speed calculated by reference to the area of clean room and differential pressure, according to the pressure differential signals, in accordance with the above-mentioned methodology, the controller on Cleanroom air or exhaust volume control, achieve the desired pressure value, this method is called "pseudo differential pressure control method".
2.2 Yu air flow (flow tracking) control method
Cleanroom air supply and exhaust air flow between certain amount of wind (called a residual air flow), inevitably lead to the clean room have differential pressure. Residual air flow (flow tracking) to control the real-time measurement of air flow control system (air supply andexhaust volume) changes, by adjusting the air or exhaust volume, dynamic air to reach the appropriate balance between air and exhaust air flow between the air flow remains constant, thereby maintaining a constant differential pressure. Its basic principle is shown in Figure 2, the control system utilizing real-time measurement of gas flow measuring device air supply and exhaust air, exhaust volume can exhaust competent on measurement, or as in the individual row of wind measurements and summation, the controller according to the regulation of air to track changes in the exhaust volume, keep a certain residual air volume, so as to achieve the desired pressure value. You can see more thanAir volume control is an open-loop control system.

The air here is to reach the desired pressure, infiltration or exudative Cleanroom air flow (CFM). Negative balance of wind that is total exhaust volume is greater than the total air, it will result in negative pressure, while the remainder of the total air flow rate is greater than the total air exhaust, it will result in positive pressure.
In Figure 2 of the air flow equation, air flow is a fixed value. But in reality, it is changing, for example when a flow sensor offset arise, actual residual air volume will change. Therefore, you should consider choosing more than large enough to cover due to the volume envelope airtightness, air pipe leakage and the flow measurement device accuracy error, etc.
The above two pressure control method, in practical application must be in accordance with a predetermined frequency. For example, on balance airflow control per annum should be on the air volume setting.
2.3 hybrid control system
Due to the bio-safety level 3 or 4 biological safety laboratories of research and experimentation object is very dangerous, laboratory control of pressure and air flow direction control even more important, you must ensure that the pressure and air flow direction is stable and reliable control. This pressure difference control critical local, control using purification differential pressure and air flow control methods mixed control system is a good choice, it can ensure that laboratory pressure differential is stable and reliable control.
Common practice is to adopt more airflow control for basic control method, plus one differential pressure sensor and controller on residual flow control system of air volume set. When room property is changed, the wind pipe leakage and the envelope of air tightness, air change, change (usually larger), when the pressure control system can be dynamically calculated a suitable balance air to maintain a stable pressure control.
At the same time, once the remaining air volume increased to a predetermined value, the system will issue a warning, you may need to be calibrated flow measuring device, or on the wind pipe and envelope leaks are processed, the system state back to normal range. Therefore, such a system can monitor the remaining air volume of the entire laboratory of control systems, air duct system, the monitor enclosure integrity.
3 stability and speed of response

General building technology constitute a room, it can achieve control pressure differential of about 2.5Pa, measurement, this is a very small pressure differential (signal), also for measurement sensor calibration is very difficult. Due to the door switch, biological safety Cabinet Regulation valve movement, the movement of people and so many factors cause of disturbances (noise) can reach 25Pa. Thus for pure pressure control, the measuring signal and noise ratio is 1: 10. such a situation is like measuring the level of a Lake, requiring accuracy is 1 cm, but the waves of the Lake, there are 10 cm high, if you want to get an accurate measurement, it takes a long time to average wave crest and the trough. In this case, if you want fast response cannot guarantee the accuracy, precision and speed (or response time) is contradictory.
r>For pure pressure control system, the response time is within in a few minutes. Therefore, many of these control systems are sacrificing stability to meet the requirements of response time, before reaching the stability control needs to set point fluctuations around a long time. Unfortunately, the system reaches a stable control of time often occur than disturbance frequency long, the system may fluctuate all day, until the end of the work, working people, there is no longer a disturbance occurs, the system will be able to reach a steady state.

For "pseudo differential pressure control system, its" measurement object is air velocity, relative to the pure pressure control more stable, faster because current signal and noise signals with dynamic pressure is proportional to the square root of the relationship, it's about to turn signal and noise ratio to 1: 3. as can be seen, simple measuring objects change can significantly improve system performance by J. However, even so, the noise is still achieved the signal of 3 times, when a disturbance, the control system is still a need for more than 60 seconds or more for a stable output. It should be noted that, due to the measurement of air flow speed in the room with the reference area of the hole, so this kind of control system for a number of occasions by the application is not allowed, such as the cleanliness high demands on occasions, or high levels of biological safety laboratories also should not be used.

For pressure and pressure difference "pseudo" system, under certain conditions, the pressure will cause serious problems, such as the negative control, when the clean room door is open, all of the measurement signal such as pressure differential and flow will disappear. Although some controllers have scheduled lock output function to compensate for this problem. However, when the door is open for long periods of time, pressure control system will close the air supply, so that the room back to negative pressure set-point. At this point, the air is removed from the aisle (or adjacent areas) sucked people open room, hallway (or adjacent region) the inevitable decline of pressure. But if other clean room also use aisle (or adjacent areas) as the pressure point of reference, the other clean room pressure controller will close the air supply chain reaction occurs, thereby, more air is from aisle (or adjacent areas) inhalation Cleanroom discharge, measuring pressure value has not been possible to achieve the set, and the actual pressure continues to decline. Similarly for positive pressure control will also have similar problems. You can imagine, this will cause the entire clean room serious pressure problems. Of course, for those who do not require strict room pressure control, or a risk assessment on stability and stability is not fairly high requirements for facilities, and HVAC system design measures (such as a two-door interlock for isolation of buffer) to avoid such problems occur, using purification pressure control are also possible.

Relatively speaking, air volume (or flow tracking) control system signal measurement is the flow measurement device on air and exhaust air volume measurement. The air supply and exhaust air volume is usually relatively large measurement values, in this case, for example signal measurement, noise for 1000CFM (various disturbance) about energyAchieve 1000FM, signal noise ratio can be as high as 10: 1. Accordingly, in this case, the system can achieve very high precision, high stability and very fast response. Thus the pressure differential control have higher requirements of the application, usually recommend or require the use of such control method.

For more airflow control system, the flow measuring device is impact system performance of critical equipment. General common flow measurement device for hot wind speed sensor array and bi managed array. Such flow measuring device with high precision. However, once aparticle adhesion or blocked on the sensor or sensors to corrosion, its measurement occurs very large deviation. For a managed array of BI, you also have to note its low wind velocity is a great measurement error, so you should consider its application range. Flow measurement device installation location also needs to be strictly in accordance with the technical specifications for the select, otherwise will result in measurement error.

In addition, there is a kind of flow control devices appear in many application. It is a linear, pressure-independent air flow control valve, can provide the appropriate flow valve position feedback signal (e.g. venturi valve), its calibration and calibration at the factory has been completed by professional suppliers. Relative to the simple flow measuring device, the device features a more integrated, the flow control of traffic measurement can be carried out. In actual use, this pressure-independent device flow feedback accuracy, generally use backup of flow measurement device for validation. Currently this is independent of pressure, air flow control valve type already in many more high differential pressure has been successful in controlling the application.

4 impact pressure control for other factors

Building technology on pressure control of performance and results have great impact and is not a closed envelope is difficult to establish a stable pressure gradient. It requires a great amount of wind to make up for more than a lot of leaks, when you use a lot of air flow, the extract to the adjacent space (or from) a large amount of secondary air and, therefore, may cause the temperature and humidity control. Therefore you must make the clean room has a sealed envelope, can guarantee the appropriate pressure and reasonable air flow direction.

Duct leakage to balance airflow control precision and performance. If the flow measurement device and cleanroom enclosure, air leakage between the duct or into one duct, will result in flow measurement errors can cause pressure control significant deviations. If you are in the constant pressure system, the error of relatively constant; but if the system pressure was fluctuating, this error will fluctuate, so control system is very difficult to take technical measures to eliminate such errors, resulting in a deterioration of control performance. It must therefore be asked to supply air and exhaust pipes leak detection, the maximum allowable maximum leakage rate should not exceed 0.5% (see in particular the professional design of air conditioning).