Precise Control of Vacuum, Temperature and Rotational Speed During Vacuum Microwave Drying (VMD)
Abstract: Focusing on the current local vacuum microwave drying technology research and equipment, the drawbacks are simple control equipment, poor vacuum degree control effect, and low product quality. This article proposes the corresponding solutions and integration of vacuum microwave drying using FC series pneumatic proportional flow control valve. The vacuum degree, temperature and speed controller of the type are introduced, and the corrosion-resistant pneumatic proportional flow control valve driven by stepper motor for gas flow adjustment are introduced, which can precisely control the vacuum microwave drying process.

1.Soru
Vacuum microwave drying is a new type of microwave energy application technology that combines microwave heating technology and vacuum technology. In the drying process of materials, the drying temperature range is generally 30~60℃, and vacuum degree range is generally 0.01~95kPa, which has advantages of high drying output, good quality, low processing cost and low energy consumption. In addition, when drying at low temperature and low pressure, the oxygen concentration is low, and the oxidation reaction of dried material is weakened and slowed down, thereby ensuring the flavor, appearance and color of material, and it is easy to dry heat-sensitive materials, especially for traditional Chinese medicine extracts, meat food, chemical raw materials, low-temperature drying of fruits and agricultural products has unique advantages.
Since vacuum microwave drying technology utilizes the principle of lowering the boiling point of water under low pressure, and combines the characteristics of rapid, homogeneous and low energy consumption of microwave heating. Vacuum degree, temperature and rotational speed are the three key control parameters in application of vacuum microwave drying technology. In the current local application of vacuum microwave drying technology, there are generally problems such as rough control equipment, poor vacuum degree control effect, and low product quality, which are mainly reflected in the following aspects:
(1) For different materials, the vacuum degree, temperature and rotation speed required in the drying process are completely different. The variation range of vacuum degree is very wide, such as from 0.01kPa to 95kPa, so it is necessary to have accurate measurement and measurement of vacuum degree in a wide range. For different vacuum degree needs different control methods.
(2) The change of vacuum degree will have a serious impact on evaporation temperature, and temperature change after microwave heating will cause the rapid increase of vacuum degree. Vacuum degree and temperature are a set of variables that affect each other. In order to ensure product quality, the vacuum degree control is required to have a fast response time, so that vacuum degree in drying process is always constant and not affected by temperature changes, and the fluctuation rate of vacuum degree after homeostasis is small. Experiments have proved that the use of pneumatic proportional flow control valve can achieve the desired effect.
(3) In addition to the need to control vacuum degree and temperature (microwave heating power) at the same time, in order to ensure the stability of microwave heating materials, it is also necessary to select and control rotation speed.
(4) Because different materials require different PID control parameters and process parameters, the controller is required to have multi-program and multi-parameter storage functions, which are easy to call at any time during the process, so as to improve automation level of the entire drying process.

2. Vacuum microwave drying control scheme
The vacuum degree, temperature and turntable speed need to be precisely controlled in vacuum microwave drying process. At present, the commonly used control schemes in Taiwan are generally relatively simple. The improved scheme proposed here is shown in Figure 1.

In the vacuum degree control system shown in Figure 1, a method different from the previous vacuum degree control method is adopted, that is, an air intake channel is added to vacuum chamber, and a numerically controlled electric needle valve, or so-called pneumatic flow control valve, and high precision multi-channel controller with fast response speed are used. This program has two outstanding features:
(1) It can reach precise control of vacuum degree in the range of 0.01~95kPa, with a fluctuation rate of less than ±1%. The specific implementation is that when vacuum degree is less than 10torr, the control adopts the upstream mode. When vacuum degree is greater than 10torr, the control adopts the downstream mode. At the same time, fast-response valves and controls ensure that temperature changes have minimal effect on vacuum.
(2) Equipped with 2-channel integrated PID controller, which can reach simultaneous control of vacuum degree, temperature and turntable speed. 2 independent channels are used for the measurement, control and display of vacuum and temperature, and the alarm output channel can be used to control the speed of turntable or start-stop of rotating motor.

3.24 bit yüksek hassasiyetli çok fonksiyonlu kontrolör
In order to reach the precise control of vacuum degree, temperature and rotation speed in vacuum microwave drying, KaoLu have developed FC series 24-bit high-precision programmable PID universal controller, as shown in Figure 2. This series of PID controllers are very useful and cost-effective.

VPC serisi kontrolörlerin ana performans göstergeleri aşağıdaki gibidir:
(1) Hassasiyet: 24 bit A/ D, 16 bit D/A.
(2) Maksimum örnekleme hızı: 50 ms.
(3) Çeşitli giriş parametreleri: 47 giriş sinyali (termokupl, termal direnç, DC gerilimi) ölçüm, görüntüleme ve kontrol için çeşitli sıcaklık ve vakum sensörlerine bağlanabilir.
(4) Çeşitli çıkış formları: 16bit analog sinyal, 2A (250VAC) röle, 22V/20mA katı hal rölesi, 3A/250VAC tristör.
(5) Çok kanallı: Bağımsız 1 veya 2 kanallı çıkışlar. İki kanal, sıcaklık ve vakumun eşzamanlı ölçümünü ve kontrolünü gerçekleştirebilir ve alarm çıkış kanalı, dönen elektrikli makinenin başlatılmasını ve durdurulmasını kontrol etmek için kullanılabilir.
(6) Çok fonksiyonlu: ileri, geri, ileri ve geri çift yönlü kontrol, ısıtma/soğutma kontrolü.
(7) PID program kontrolü: Geliştirilmiş PID algoritması PV diferansiyel ve diferansiyel ileri kontrolü destekler. 20 set PID depolayabilir ve 20 program eğrisini destekleyebilir (her biri 50 segment).
(8) İletişim: İki telli RS485, standart MODBUS RTU iletişim protokolü.
(9) Görüntü: Digital Hall ve IPS TFT True Color LCD.
(10) Yazılım: Yazılım bilgisayarı denetleyicinin çalışmasını ve veri toplama ve depolamayı gerçekleştirebilir.
(11) Boyutu: 96x96x87mm (delik boyutu 92x92mm).

4.Step motor tahrikli korozyona dayanıklı yüksek hızlı iğne valfi
In order to reach high-precision adjustment in the process of vacuum degree control, KaoLu have developed a series of pneumatic proportional flow control valves with different flow rates based on the needle valve using stepper motors, as shown in Figure 2. The magnetic hysteresis of this series is much smaller than solenoid valves, and has a high-speed response within 1 second. In particular, the use of FKM sealing technology enables the valve to have superior corrosion resistance. The detailed technical indicators are shown in Figure 4 shown.

Model	FC-20	FC-120	FC-300	FC-1000
Valf tipi	Iğne valfi
Makara sürüklenme çapı	0. 9 mm	2. 25 mm	2. 75 mm	4. 10 mm
Uygulayıcı kurum	Bipolar step motor kontrolü
Tepki süresi	0. 8 saniye (açık-kapalı)
Standart Boyut	G1/8 "			G3/8 "
Sıvı	İnert gazlar ve sıvılar
Temas malzemesi	Paslanmaz çelik
Basınç aralığı	-1-7 bar			-1 ~ 5 bar
Azami akış	50L/ dak @ 7bar	240L/ dak @ 7bar	290L/ dak @ 7bar	600L/ dak @ 7bar

 

Doğrusallık	± 21 TP 1 t	± 0.1 ～ 1%	± 0.2 ~ 5%	± 11%
Tekrarlanabilirlik (Tam boyut)	±. 11 tp1t
Akış çözünürlüğü (adım boyutu)	0.1 litre/ dk	0.1 ～ 0.2L/dak	0.2 ～ 0.75L/dak	1L/dak
Kaydırma çözünürlüğü (adım boyutu)	12. 7um		25.4um
Çalışma sıcaklığı aralığı	0 ~ 84 °C
Conta	Standart FKM veya diğer seçilmiş contalar
Kontrol sinyali	DC: 0 ~ 10V (veya 4 ~ 20mA)
Güç kaynağı	DC: 24V (12W)

(Figure 4 Technical indicators of KaoLu’s FC electronic needle valve)

Daha fazla bilgi için lütfen ziyaret edin https://www.genndih.com/proportional-flow-control-valve/mid-flow-proportional-valve-0-130L-min.html

FC pneumatic proportional flow control valve is equipped with a stepper motor drive circuit module, which provides the required power supply and control signal for FC pneumatic proportional flow control valve, and converts the DC signal into step control of bipolar stepper motor, and can also provide RS485 direct control of serial communication. Its specifications and dimensions are shown in Figure 5.

When using pneumatic proportional flow control valve in vacuum microwave drying, open-loop control method can also be used to install needle valve at the front end of vacuum pump instead of electric ball valve, and the vacuum degree can be controlled by adjusting pumping flow, but the stability of this open-loop control method is poor. It is difficult to reach higher stability requirements. Therefore, it is generally recommended to use closed-loop control method shown in Figure 1, that is, to add an air intake control valve to vacuum chamber, and to achieve precise control of vacuum degree by adjusting air intake flow and exhaust flow at the same time.