Principle Analysis on Honeycomb-rotor Dehumidifying

 

Composition of Honeycomb Rotor

 

Wheel body, fixed mount, central bearing, gears (including chain wheel and pulley) and motor compose honeycomb rotor. Wheel body is composed of ceramic fiber and ceramic wheel body with organic additive. Both Molecular sieve and silica gel are sintered with high temperature to harden surface and to adhere themselves to honeycomb inwall. And the tiny holes on the surface of ceramic fiber rotor have the functions of broadening contact area with wet air to improve the capability of moisture absorption.

 

The adopting of ceramic fiber renders wheel body difficult to fall off, powderless, anti-ageing and repeatable to clean. Clapboards of fixed mount, made from silicon rubber with highly sealing property, divide the rotor into three sectors: dehumidifying, regenerating and cooling. The running of gears driven by motor powers wheel body rotate around central bearing inside sealed area of fixed mount. In such a way, three sectors are under alternate operations.

 

 

Picture 1: Wheel body of honeycomb rotor         Picture 2: Honeycomb rotor

 

Working Principle of Honeycomb Rotor

 

 

Picture 3: Working Principle of Dehumidifying

 

When damp air (called disposed air) need to be dehumidified enters into dehumidifying area, it flows slowly to upper part of rotor through tiny holes after being cooled. Steam of moist air is absorbed by active silica gel and molecular sieve and becomes dry air, which discharges through blowers; as the increase of absorbed moisture, processing sector inclines to saturation. It is necessary to regenerate and restore absorbent to keep dehumidifying performance of sector stable. Meanwhile, dehumidifying area to be saturated turns slowly to regenerative area driven by motor, and then regeneration starts. (The process is actually moisture is drawn out of adsorbent and into regenerative air)

 

With the opposite air direction of dehumidifying area, air in regenerative area passes through rotor after being filtered and heated. Saturated sector is heated by high temperature air and its adsorption ability greatly weakens as moisture rapidly evaporates. Then moisture released from sector discharges through regenerative blower; because temperature of wheel body rises during regeneration, wheel body needs to be cooled down to maintain dehumidifying capacity. At this moment, regenerative area turns to cooling area to start the cooling. In cooling area, part of dry air after being filtered, cooled and dehumidified is used to cool regenerative area where regenerative air is disposed, and the air with residual water is exhausted. In the way, moisture absorbing area restores its absorbing ability and air discharging from cooling area enters into circulation again.

 

 

Picture 4: Airflow in Dehumidifying area        Picture 5: Airflow in Regeneration area

 

Factors Affecting Rotor Dehumidifying Performance

 

Effect of Rotor Parameters

 

• Quality score of absorbent

 

Dehumidifying rotor is composed of support materials without moisture absorbility and absorbent. Percentage of absorbent accounting for total quality is called quality score f. Research indicates that, in the same quality, f value increases and absorbent quality increases, air humidity of dehumidifier exist will be reduced. F value between 0 ~ 0.6 affects the dehumidifying performance most, and scores of more than 0.6 the effects greatly weaken. In application, f value is often between 0.8~0.85. And reducing proportion of metal support materials can effectively lower the total heat capacity, helpful for improving dehumidifying performance of rotor system.

 

• Superficial area of absorbent

 

Superficial area refers to the gas contact area. The more tiny holes, the smaller hole diameter, the larger contact area, which are conducive to moisture absorbing and reduce internal diffusion distance of absorbent, shorten the regeneration time, making airflow penetration resistance increase.

 

• Temperature of absorbent

 

Temperature of absorbent: during dehumidifying process, the temperature of absorbent needed is relatively lower. With the same absorbent, the lower temperature is, the stronger absorption ability will be. During regeneration process, the higher temperature is, the more conducive to raise pressure of surface steam of absorbent, accelerating the vaporization of absorbent moisture.

 

• Rotate speed of rotor

 

Raising rotate speed can enhance the effect of the heat transfer while it will make dehumidifying effect less efficient because absorbent stays shortly in regenerative area without being fully used; lowering speed will make absorbent stay long in dehumidifying area, resulting in that absorbents close to regenerative area are not capable of absorbing moisture due to over-saturation, and dehumidifying effect will be harmed. Thus a proper selection of rotate speed is crucial based on reality, general speed is between 8~22r/hr.

 

• Fanlike angle of regenerative area

 

Fanlike angle mainly functions as a reflection of proportion accounted for absorbent of dehumidifying area and regenerative area. With a certain amount of airflow, too small fanlike angle makes absorbent not fully regenerated and dehumidifying effect will be lowered; too big angle will decrease areas of dehumidifying and cooling, making absorbent not be fully absorbed and dehumidifying effect be lowered. Thus a optimal size of fanlike angle is very significant.

 

In practical applications, the following factors below about fanlike angle should be considered: absorbent is easy to be fully regenerated; air humidity of disposed air in outlet can drop very lower; dehumidifier has a high coefficient of performance. Only by taking these factors into account can angle be confirmed properly. Generally, the temperature of regenerative air is high and rotor regenerative area accounts for a quarter of the total area of rotor, that is to say fanlike angle are 90°. If it is need to change the temperature and air flow of regenerative air, in order to effectively regenerate absorbent, fanlike angle of dehumidifying rotor needs to be changed.

 

Effect of air parameter on dehumidifying performance

 

The air of rotor dehumidifying system includes disposed air and regenerative air. Parameters of disposed air such as temperature, humidity and flow rate directly affect dehumidifying performance of rotor dehumidifier. Parameters of regenerative air directly affect regenerative performance, and even affect absorbility and dehumidifying performance. Therefore, these two kinds of air are restricted mutually. A better know-how on effects from these airs is beneficial for model selection and machines stable operation.

 

• Effect of disposed air temperature in exist

 

Analysis of adsorption isotherm in different temperatures concludes that the same adsorbent in the same pressure the higher the temperature is, the weaker absorbent adsorption ability is; Absorption performance of absorbent weakens as the air temperature rises. In practical project, the performance of dehumidifying rotor is improved by lowering import air temperature. And it is possible to reduce disposed air temperature in rotor outlet through precool measures to make rotor dehumidify low temperature air.

 

• Effect of disposed air humidity in exist

 

With the same dry ball temperature, the more air relative humidity is, the more moisture content it has. Partial pressure of air water vapor approximates to that of saturated water vapor, then pressure difference with surface air of absorbent increases to amplify dehumidifying impetus, finally making the dehumidifying amount increase; with same moisture content, partial pressure of air water vapor is constant value. The more air relative humidity is the lower dry ball temperature is. So partial pressure of saturated water vapor in the rotor surface air becomes lower, making a smooth process of dehumidifying.

 

With same relative humidity, the higher air moisture content is, the higher the air dry ball temperature is. Temperature rising of disposed air will make the saturated air temperature rise, thus partial pressure of saturated water vapor rises accordingly. This is harmful for air dehumidifying; but the increase of air moisture content will make the partial pressure of water vapor rise, which is beneficial for dehumidifying; therefore, for the influences on dehumidifier process, both of factors above should be considered. In practical application, air dew-point is taken as control quantity for air humidity.

 

• Effect of disposed air flow rate

 

The lower air flow rate is, the more contact time between air and absorbent will be, also the more fully their heat and mass transfer will be. However, the disposed air quantity in unit area will become smaller. Acceleration of air flow rate can increase both coefficient of heat transfer and mass transfer coefficient, which is beneficial for counter-flow mass transfer between air and absorbent; But flow rate increase will shorten contact time and make disposed air flow out of rotor without being dehumidified. That could result in the failure of air reaching the predetermined humidity. Therefore a proper air flow rate is an important parameter for the operation of rotor dehumidifying system. Influences of disposed air flow rate on practical application embody in the determination of disposed air quantity. After the specification of dehumidifying rotor is ascertained, air flow quantity should not exceed the rated flow of rotor.

 

• Effect of regenerative air parameter

 

In practical application, regenerative air parameter is easy to control so influences of regenerative air on dehumidifier performance take more concerns: when air moisture content keeps constant, rising regenerative air temperature can strengthen vaporization and the capacity of taking away moisture, also can heat up absorbent to raise water diffusion rate in exterior and interior of absorbent, which is favorable for constant speed drying stage and slow down drying stage. But each kind of absorbent has the allowed maximum temperature; the lower air moisture content is, the stronger the capacity of taking away absorbent moisture will be, and the better the impetus of drying process will become, thus drying efficiency is higher; regeneration air flow rate directly influences the regenerative speed of absorbent. The coefficient of heat transfer for counter flow will increase as flow rate accelerates and the same as for increasing of coefficient of heat transfer. In this way, the time of regenerative process has been shortened; in addition, the regenerative air flow rate can be adjusted to adapt changes of disposed air quantity and parameters.

 

In short increasing of regenerative air flow rate strengthens regenerative process and accelerates the rotor regeneration, but there is no change for the regenerative fanlike angle. Probably, the regenerated rotor would be heated and absorbent temperature rose, thereby affecting moisture absorption process; as far as the system energy consumption is concerned, flow rate acceleration can lead to the growing demand in the regenerative heat quantity, which will lower heat exchange efficiency at rotor regenerative side. Therefore under the rated working conditions changes of air flow rate should be considered seriously. If regenerative air flow rate is changed, parameters such as fanlike angle and regenerative air temperature should be adjusted accordingly. Actually, the fashion of changing fanlike angle in regenerative area is not feasible so adjusting regenerative air temperature is the alternative.