Application of the hottest countercurrent cooling

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Application of counter current cooling technology on roots blowerabstract:theoretical analysis is carried out for application of counter current cooling technology used on roots lculating formula of discharge temperature is derived design method of Roots blower with counter current cooling is ta of contrast test is analyzed

key words:roots blower air flow cooling application I. Introduction roots blower is a positive displacement compression machine, which does not compress the gas inside and generates pressure rise by relying on the instantaneous return of high-pressure gas in the system. Therefore, it has the characteristics of forced exhaust. In addition, the transmitted medium is not polluted by oil. It has simple structure and convenient maintenance. It is widely used in various departments of the national economy

the operating principle of Roots blower determines that it has a high exhaust temperature, which increases with the increase of exhaust pressure. Excessive exhaust temperature will increase the thermal expansion of impeller and casing, reduce the working clearance between impeller and impeller, impeller and casing, impeller and front and rear wall panels, and affect the safe operation of Roots blower. In addition, the temperature of lubricating oil, bearing and gear will rise through heat transfer, which will reduce the reliability of Roots blower operation. Therefore, the pressure rise of Roots blower is often limited by the exhaust temperature. The counter current cooling technology can effectively reduce the exhaust temperature of Roots blower, increase the exhaust pressure (within the allowable range of strength and stiffness), and expand the application range of single-stage roots blower. At the same time, it can also reduce the noise of Roots blower. 2、 Theoretical analysis: countercurrent cooling is to draw some gas from the high-temperature and high-pressure gas discharged by roots blower. After being cooled by the cooler, it enters the closed chamber composed of the casing, impeller and front and rear wall panels through the return groove of the casing, so that the gas pressure in the chamber instantly reaches or approaches the exhaust pressure, so as to avoid the impact of the instantaneous return of high-temperature and high-pressure gas when the chamber is connected with the outlet system, So as to reduce the exhaust temperature and pulsating noise of Roots blower. Its principle is shown in Figure 1. Fig. 1 working principle diagram of reverse flow cooling roots blower at position (a), the gas in the inlet state (pressure Po, temperature to, mass flow Mo) enters the suction chamber of the fan with the operation of Roots blower, and its pressure drops to PS. due to a certain pressure difference between the suction and exhaust chambers, a small amount of high-temperature gas (temperature TD, mass flow MD) leaks into the suction chamber and mixes with it

at position (b), the inspiratory process is completed, and its temperature rises to ts. at this time, the gas state parameters are: pressure Ps, temperature ts, mass flow mo+md

at position (c), the suction chamber is connected with the counter flow channel (casing return tank), and the cooled high-pressure gas (pressure PD, temperature TR, mass flow MR) passes through the casing return tank to make the gas pressure in the closed chamber rise to PD (or close to PD) and the temperature rise to TD. Gas state parameters at this time: pressure PD, temperature TD, mass flow mo+md+mr

at position (d), the closed chamber is connected with the exhaust chamber, and the gas is pushed to the exhaust port along with the roots blower and enters the conveying system

the main purpose of Roots blower adopting countercurrent cooling technology is to reduce exhaust temperature and increase exhaust pressure. The exhaust temperature calculation formula is derived as follows:

it is assumed that the roots blower has the following conditions during operation:

(1) there is no heat exchange between the fan and the outside world

(2) gas reflux is completed instantaneously

the process state equation is:

povt=mtrto (1) cpmoto+cpmdtd=cp (mo+md) ts (2) psvt= (mo+md) RTS (3) CV (mo+md) td+cpmrtr=cv (mo+md+mr) ts (4) pdvt= (mo+md+mr) RTD (5) η V=mo/mt (6), where η V - volume efficiency

when selecting, it is necessary to consider that MT - theoretical mass flow

makes Erdogan's popularity rise rapidly. From (1) to (6), it can be deduced that the theoretical exhaust temperature formula of the fan is:

td=( η v+ ε- 1)kTo/〔k η v+( ε- 1) To/tr] (7), where ε= Pd/po, K -- adiabatic index

the theoretical exhaust temperature formula of Roots blower without countercurrent cooling is:

td ′=( ε- 1)(k-1)To/(k η v) +to (8) take the rd-125 fan of our factory as an example, the experimental report can also be programmed and printed according to the format required by the user; For example, suppose that the rotating speed is 1750r/min, the inlet state is the standard suction state, and the temperature after gas countercurrent cooling is 40 ℃. Theoretically, some electronic tensile testing machines in the market adopt reducer exhaust temperature comparison (see Figure 2). It can be concluded that under the same pressure rise, the theoretical exhaust temperature (curve I) when countercurrent cooling technology is adopted is lower than that without countercurrent cooling (curve II). The higher the pressure rise, the more obvious. Fig. 2 pressure rise and theoretical temperature curve III. key points of design when the pressure rise of Roots blower is limited by the exhaust temperature, and the strength, stiffness and bearing life allow the pressure rise to be further improved, the counter flow cooling technology can be used to reduce the exhaust temperature and improve the pressure rise. The design steps are briefly introduced as follows

1. Calculate the strength, stiffness, bearing and gear life of the shaft and impeller, and determine the allowable pressure rise range

2. Set the temperature of the countercurrent gas after cooling, calculate the exhaust temperature at each pressure rise point according to formula (7), and then comprehensively determine the maximum pressure rise in combination with the allowable pressure rise of strength, stiffness and bearing and gear life

3. Calculate countercurrent gas

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