FI84398B - Method for monitoring and controlling heat utilization in a drying machine - Google Patents

Description

1 84398

This invention relates to a method for monitoring and controlling the energy economy of a dryer and its heat recovery equipment and for monitoring the condition of the equipment and for locating faults and malfunctions by removing water from the product by ventilation. at least for the automatic control of the drying air flows, indicators to keep the water content of the exhaust air stream high and to keep the energy consumption low.

A significant part of the operating costs of the dryer is thermal energy costs. Since drying consumes energy not only to evaporate water from the product to be dried but also to heat the air used to transport the removed water vapor to the exhaust temperature, it is important for the temperature economy of the machine that the machine's air equipment is optimally controlled.

Thus, in both the paper machine and any other water evaporator based dryer, minimizing energy consumption requires the highest possible water content in the exhaust air, which is achieved by keeping the exhaust air flow as low as possible. This also substantially improves the heat recovery efficiency of any heat recovery equipment used and reduces the power absorbed by the fans.

The following is a first description of the dry matter and water flows <kg / h> through the drying section of a paper machine by way of example: 2 84398

Item on paper machine Dry Water Total

After the press section 17,600 26,400 44,000

Evaporation with pre - dryer - 21 850 - 21 850

Before gluing press 17 600 550 18 150 Addition of filler with press + 1,000 + 7,450 + 8,450

After gluing press 18,600 8,000 26,600

Evaporation with a post-dryer - 6 600 - 6 600 After a post-dryer 18 600 1 400 20 000

In the example case, the evaporation is thus a total of 28,450 kg / h. If the water content of the air entering the drying section was 0.01 kg / kgDA (kg water per kg dry air) and the water content of the exhaust air of the drying section was 0.16 kg / kgDA, the exhaust air flow would be n.

190,000 kg / h, ie about 6.7 times for evaporation and approx.

9.5 times compared to production. If, on the other hand, the water content of the exhaust air were only 0.13 kg / kgDA, the exhaust air flow would then be approx. 237,000 kg / h.

In the exemplary case, the heat output requirement at a lower exhaust air flow is approx. 26,400 kW and at a higher approx. 27,800 kW if the drying air outlet temperature is 85 C and the outdoor air temperature is 0 C. The difference of 1,400 kW is due to the heating of the extra exhaust air flow. Heat recovery is also about 1,500 kW less efficient with a higher exhaust air flow.

The economics of drying are best illustrated by the "specific displacement airflow" kg / kgH20 (exhaust air per kg of water evaporated), which in the example is 6.67 or 8.33 kg / kgH20 and the "specific energy consumption" kWh / kgH20 (energy consumption possibly per kg of water evaporated). If the full recovery capacity of an existing heat recovery plant can be utilized, the reduced heat recovery capacity caused by an unnecessarily large exhaust air flow will have to be replaced by primary energy, which will degrade the thermal economy of the machine.

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The lower the specific exhaust air flow of the machine, the better the thermal economy of the dryers. The limit for reducing the exhaust air flow is set by the structural aspects of the machine, such as the thermal insulation and tightness of the machine casing. On average, the air leaving the machine casing cannot be very close to the dew point, because in different parts of the machine casing the water content of the air varies within wide limits. Also, the temperatures of the structures inside the machine enclosure vary and are in some places lower than the temperature at which they are removed. The water content of the exhaust air must be kept at least at its design value, so that the exhaust air flow must be proportional to the current evaporation. This varies, especially on fine paper machines, over a wide range with different grades of paper. It is therefore likely that the drying parts of several paper machines operate far from the optimum in terms of thermal economy.

Another important aspect of the operation of the dryer is the difference in air pressures inside and outside the machine hood. The density of warm and humid air inside the mantle is about one-third lower than the density of the air outside it.

This causes an increasing overpressure inside the enclosure as it moves upward, causing hot and humid air to flow into the data center from the track outlet and other openings in the enclosure wall, causing condensation damage in cold structures outside the enclosure. This drawback is prevented by lowering the casing inner side of the casing by adjusting the pressure compensation flow is suitably off-toilmavirtaa less, and the difference of the adjustable air flow openings in the wall of the casing is replaced with false air-virtauksi11a. The aim is to make the adjustment so that at the desired height level, usually the height of the track outlets, the pressure difference across the wall of the cloak is zero. This so-called "Level 0 adjustment" is also important because when the zero level is too high, cold air flows from the openings of the cover into the cover, causing condensation damage there.

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Efforts have been made with paper machines to control the water content of the effluent air described above and the adjustment of the zero level of the robe, but with poor success. The water content of the exhaust air is adjusted by measuring the moisture content of the exhaust air by means of a suitable individual sensor and then directly adjusting the exhaust air flow directly by means of a fan or control damper so that a predetermined water content is reached. In practice, the zero level is not directly "adjusted", but the position of the replacement air dampers is set on the basis of the speed of the exhaust air fan or the position of the exhaust air dampers, using a pre-made series of test measurements, te. on the basis of individual experience.

The known methods have considerable drawbacks. Zero-level adjustment based on test series alone is inaccurate in itself and also unreliable because many factors in the ventilation and heat recovery plant may have changed after the test measurements. In addition, there is no reliable information on the operation of heat recovery devices and in particular on whether they operate under optimal conditions.

Energy management is an essential quantity in papermaking, where the amounts of water evaporated per product are high. Measuring the moisture of the product, on the other hand, is not a problem because the moisture in the paper web can be accurately measured and adjusted on site as a continuous process.

For example, the moisture humidity control of the product is known from FI-73819, FI-31141 and FI-58021. They thus aim to bring the moisture content of the product to the desired value in a situation where the moisture content of the product in question cannot be measured. The publications mainly concern the control of the humidity of timber or tobacco leaves or cellulose pulp, none of which can be reliably measured in the process. Thus, at best, these publications have sought an indirect way to determine the moisture content of a product. Compared to the starting point of this application, moisture-bound energy or energy efficiency is of very little importance in the field of application of said publications, since the amount of water to be evaporated is small in relation to the weight of the product.

FI-73819 measures the humidity of the incoming and outgoing product, the humidity and temperature of the circulating air and the speed of the veneer. Correspondingly, the publication regulates the speed of the recirculated air, exhaust air and veneer, in which case the goal is the correct humidity of the outgoing product. No attention is paid to the energy economy and it cannot be optimized with the presented measuring orure control arrangement.

FI-31141 and FI-58021 also deal with the control of the humidity of the product by measuring the humidity of the air in the drying room using either the "limit cooling temperature" or the "pressure" and keeping the humidity at the desired value, so that the control technology is extremely straightforward. minimum search, and does not even measure the quantities required for such an adjustment.

in addition, it should be noted that at least on a paper machine, the measurement of airflows is unreliable for a variety of reasons, so arrangements that require their direct measurement are not suitable for operations that require good accuracy and reliability. Mm., Therefore, needs to know to obtain reliable measuring results and correct adjustment, e.g. machine casing interior and the exterior of the pressure differences. FI-63631 discloses an arrangement for detecting a pressure difference at one point. Such a measurement does not provide accurate information on the headland ratios at different points.

The object of the invention is thus to provide a method for continuously adjusting the water content of the exhaust air of a drying machine or the margin between the dew point and the temperature of the exhaust air. A further objective is to provide a process for obtaining 6 84 398 continuous drying machine in the casing interior and the exterior of the pressure difference between the zero height provided at a predetermined position. Still another object is to provide a method for obtaining information on the energy efficiency of the dryer and its heat recovery equipment so that the energy consumption of the plant can be kept as low as possible and information on the different heat recovery stages so that the operation and condition of the various main functions can be monitored for maintenance. No attention is paid to the final moisture of the product in this context.

The method according to the invention indeed provides a decisive improvement in the above-mentioned drawbacks. To achieve this, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The most important advantage of the invention is that the energy consumption of the dryer and the process values of its ventilation and heat recovery devices continuously provide information describing their actual state, on the basis of which air flows and other process variables can be adjusted in the best possible way. Especially in terms of energy consumption, the entire equipment can be adjusted to give the optimum result when both technical and economic factors have been taken into account in a controlled and desired manner. In addition, the invention makes it possible to print information such that the actual operating values of the various main components of ventilation and heat recovery equipment can be compared in detail with their target values, that the suitability of these components can be monitored and adjusted and that any malfunctions and defects can be accurately reported.

The method according to the invention completely avoids the continuous measurements of the airflows of air devices, the performance of which is in most cases very difficult and unreliable due to the size of the air ducts and their missing sufficiently long straight sections.

The exhaust air flow is controlled either by the speed of the exhaust air fan or by the fan duct or a separate control point, alternatively either so that the dew point and temperature of the exhaust air have the desired margin or so that the water content of the exhaust air has the desired value.

The replacement air flow is controlled either by the speed of the replacement air fan or by the fan guide vane or by a separate control head 11i so that the zero level of the pressure difference in the machine cover is at the desired height level. The adjustments are handled either by the machine with an existing automation system or by separate unit controllers and a separate microprocessor that performs the necessary calculations.

The method is based on measurements of the exhaust air and the room air condition (air temperature and dew point or alternatively wet temperature). From these values, the water and heat contents of the air as well as the specific exhaust air flow and specific energy consumption are calculated. Based on the existing automation system of the machine and / or other instrumentation of the machine and partly species-specific production and dry matter values, the total evaporation of the machine is calculated, from which the exhaust air flow and energy consumption of the machine are calculated. If the machine has a heat recovery system, measurements of the air temperature and water content of the machine and the ventilation air flow of the machine room, as well as the air and temperature balances of the machine and the machine room can be calculated on the basis of air temperature and water content measurements at suitable points.

The adjustment of the zero difference of the pressure difference of the machine scraper of the machine section of the drying section is based on the information on the height of the zero flow obtained from the air flow direction measuring sensors (e.g. 10 - 20 pcs., Spacing e.g. 20 - 10 cm) located at equal intervals in the scraper wall.

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The following invention will be explained in detail with reference to the accompanying drawings.

Fig. 1 schematically shows the air devices of a typical dryer as their measuring points, Figs. 2A and 2B show in principle the differential pressure sensor of the casing and its connection by which the "zero level" is indicated, Fig. 3 shows the printout of air and temperature balances in the air device system of Fig. 1.

The taking of a representative air sample from the air ducts to be measured for temperature and water content measurements is crucial because the temperatures, water contents and flow rates at the cross-sections of the ducts vary within wide limits. The sample air flow is sucked from the air ducts by horizontal or vertical or diagonal sampling pipes, as the case may be, in which the size of the suction holes is taken into account in the air velocity differences in the cross-section of the duct. This is important e.g. due to the fact that air / air heat exchangers generally operate on the cross-flow principle, which causes large temperature differences in the cross-section of the duct.

Figure 1 shows a conventional aerial diagram of a dryer and a computer room. Dehumidifying air is removed from the machine formula 2 of the dryer 1 (not shown) as exhaust air flow 3 through heat recovery devices LTO-1, LTO-2 and LTO-3, where part of the exhaust air heat content is recovered in dryer replacement air flow 4 in LTO-1, computer room ventilation air flow 5 LT0-2 and process and other waters in 6 LTO-3. In addition, the replacement and ventilation air is also heated with fresh steam if necessary. 7. The following measuring points for temperatures (tl - tll) and water content (xl - x6>) are marked on the diagram: 9 84398 tl, xl = outdoor air t2, x2 = indoor air t3, x3 = t4, x4 = exhaust air after LTO-1 t5, x5 = exhaust air after LTO-2 t6, x6 = exhaust air after LTO-3 t7, x7 = x2 = replacement air after LTO-1 t8, xö = x2 = replacement air after evaporation t9, x9 - xl = ventilation air after LTO-2 tlO, xlO = xl = ventilation air after steam tll, xll = xl = ventilation air in the computer room

In other key figures, the above-mentioned reference numbering is followed to indicate the place of the major in the process. The water contents of the air are measured either with a LiCl dew point sensor (e.g. a DEWCEL sensor developed by Foxboro Co. in 1948) or psychrometrically with a dry and wet temperature sensor. In the previous case, the water content of the air is calculated from the temperature measured by the sensor from the equation: (t / a - b) <1> x = 10, where a and b are constant

Psychrometric measurement is used in places where the water content of the air is so high (e.g. exhaust air from the machine scraper) that the psychrometric measurement method gives better measurement accuracy than can be achieved with a LiCl sensor. Then the water and heat content of the air is calculated from the following equations: (c - (d / e + t ')) (2) p' = 10 (3) x '= f * p' / (po - p ') ¢ 4) x = x '- (Ca + Cv * x') * (t-t '> / (Mo + Cv * t-Cw * t') <5) i = Ca * t + x * (Mo + Cv * t) tk = g * (log x + h), where i - heat content, t = dry temperature, t '= wet temperature, Po = total pressure, p' = partial pressure of saturated water water vapor, 10 84398 x '= saturated water water content, x = air content of air, tk = dew point of air, Ca = specific heat of air, Cv = specific heat of water vapor, Cw = specific heat of water, Mo = heat of vaporization of water and c, d, e, f, g and h are constant.

Machine production (T, kg / h) is either obtained directly from the machine's automation system or calculated from the machine's speed, surface weight and track width.

Evaporation of the machine H <kg / h) is calculated from the production of the machine and the dry matter contents of the web K (kg / kg): K1 = after the press section K2 = after the initial dryer K3 = after the gluing press K4 = after the post-dryer K = dry matter addition (kg / h)

Dry matter concentrations K2 and K4 are usually obtained from machine instrumentation. K1 and K3 are obtained from species-specific laboratory determinations. K3 and the dry matter addition with the adhesive press K can also be calculated from the flow and concentration measured values of the filler of the adhesive press.

The specific exhaust air flow OP (kgDA / kgH20> and the specific energy consumption OE <kWh / kgH20) is calculated from the equations: (7) OP = 1 / <x3 - x2) (8) OE = OP * (i3 - i2) / 3600.

The exhaust air flow in the machine shell m2 - m6 (kg / s) is calculated from the equation: (9) m = H * OP / 3600.

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The water content of the air stream at different stages of the process is calculated from Equation (4), its thermal content from Equation <5) and Dew Point from Equation <6). The airflow power (kW) is calculated from the equation: (10) P = m * 1 The heat recovery in each heat exchanger is equal to the difference in exhaust air power before and after the heat exchanger in question. The replacement air flow m7 - m8 (kg / s) and the ventilation air flow m9 - mlO are calculated from the heat of these air flows at different stages of the process and the heat 11 eeno 11 o t. From the e-hoe using equation (10).

The "cash flow" (FIM / d) for drying is calculated from the equation: (10) FIM / d = 0.024 * <P4 - P2> * FIM / MWh, where mk / MWh = marginal price of thermal energy.

As described above, the complete air and temperature balance of the drying machine 1 and the computer room is calculated, which is printed in the table <fig. 3) in the form of, for example, a video monitor. The display mainly serves experts in monitoring the temperature economy and condition of the various phases and main components of the heat recovery equipment of the dryer, as well as in locating faults and malfunctions.

For production personnel, a second display is preferably printed in the form of a diagram showing only the most important measured values. This display also shows the specific consumption and specific exhaust air flow indicators as well as the “cash flow value” to increase the motivation of production personnel in monitoring the heat economy. The above indicators should also be printed for periodic reporting as seasonal averages.

Alternatively, the measured value of the exhaust air flow control circuit 3 is either the difference between the exhaust air temperature t3 and the dew point tk3 of the exhaust air t2 84398 (= temperature margin1i) or alternatively the water content x3 of the exhaust air of the exhaust air. If the adjustments are made with separate unit controllers, the measured values are entered as a standard current message. The control is carried out, for example, as a normal P1 control, which controls either the 8 speeds of the exhaust air fan or the fan duct grille or a separate damper.

The zero level of the pressure difference of the machine cover is indicated by a set of sensors 16 mounted on the use-side wall of the machine cover, with 10 to 20 special sensors, typically at intervals of 20 to 10 cm in the height direction. The amounts and intervals are, of course, selected according to the desired measuring range.

The principle of the sensor is simple and shown in Figures 2A and 2B. A pipe 17 made of a poorly conductive material mounted through the wall 2 of the machine casing leaks air out of the machine casing or into the machine casing, depending on the direction of the pressure difference. Inside the pipe, there are, for example, two NTC resistors 18 and 19 as temperature sensing elements, which measure the temperature of the air flowing through the pipe. Two other similar e.g. NTC resistors are also connected to the sensor, the first 20 of which measures the temperature inside the enclosure and the second 21 measures the temperature outside.

According to Fig. 2B, the resistors are connected to a connection (Wheatstone bridge), the output voltage Uo of which changes direction when the pressure difference and the air flow direction change, because the internal temperature t3 of the machine casing is always higher than the computer room temperature t2. The outputs of the measuring stations are connected to the analog inputs of the machine's existing automation system or a separate microprocessor. From the measurement data, the processor converts the measurement value of the zero-level control circuit into a message that is proportional to the number of sensors in which airflow occurs inside the enclosure. As the pressure difference increases with the upward shift, the number is sufficient for zero-level detection at sensor gap resolution.

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Particularly preferred is a structure in which the tubes 17 are shaped as thin and long tubes, e.g. of ceramic or suitable plastic, which extend through the wall beyond its two surfaces. The flow direction dependent temperature sensors 18 and 19 are located close to and approximately at the symmetry level of the longitudinal temperature profiles of the pipe and apart from the walls of the pipe at the center of its cross section. Sensors 20 and 21 for permanently measuring the indoor and outdoor temperatures are located on the outer surface of the pipe ends and isolated from the pipe and the flow through it. In this way, a separate small sensor unit is obtained from the structure, and its sensors do not give an erroneous result, e.g. as a result of changes in the internal temperature of the robe and / or the temperature in the data center.

The control is carried out as a normal ΡΙ control, which controls either the speed of the compensation air fan 9 or the fan duct grille or a separate damper 10. In the event of a line failure and downtime, to the values selected before the interruption, which remain for a pre-set time so that the measured values have time to equalize before the settings are switched on.

As the control measures described above take place continuously, external disturbances such as changes in outdoor temperature and humidity, changes in computer room conditions possibly made by changing the computer room ventilation air flow by fan 12 or adjusting its damper 14 or by changing the power of the room exhaust fan 11 or bypassing position or for some other reason, automatically.

The invention is in no way limited only to the drying time of the paper machine and not only to the air device diagram according to Fig. 1. The method is applicable to any drying process in which volatile moisture is removed by ventilation. The air circulations and heat exchangers can differ significantly from that shown in Figure 1. However, it is essential that the temperatures and water tanks of all air streams used for drying are known and that the temperatures and water contents of the moist exhaust air stream are known before and after each heat exchanger, with a heat exchanger or heat exchanger In this case, the temperatures and water contents are measured before and after such a heat exchanger.

Claims (6)

84398 A method for monitoring and controlling the energy efficiency of a dryer and its heat recovery equipment and for monitoring the condition of equipment and locating faults and malfunctions by dewatering the product without adjusting the final humidity of the product and adjusting the airflows by measuring and calculating at least automatic airflow parameters indicators to keep the water content of the exhaust air flow high and to keep energy consumption low, characterized in that the air flows and heat flows of the exhaust air and replacement air as well as the ventilation air of the hall and the indicators affecting the temperature economy on the basis of which said flow adjustments and monitoring and positioning are calculated indoor air temperatures and water contents as well as the production volume and dry matter content of the product to be dried on the basis of the measured values of the Method according to claim 1, characterized in that the temperature and water content of the exhaust air of the engine cover are measured before and after each heat exchanger located in this air flow, the replacement air of the engine cover and the ventilation of the engine room and other air flows are measured after each heat exchanger or heat supply point located in these flows. and the room air temperatures and water contents are measured, to calculate said parameters of the air and heat flows and of the dryer support. Method according to Claim 1 or 2, characterized in that the exhaust air flow is controlled in order to keep its water content high. based on the values so that the difference between the varying exhaust air temperatures and the dew point temperatures is in accordance with the preselected temperature / dew point margin1 in. 84398 BC Method according to claim 1, 2 or 3, characterized in that the exhaust air flow is controlled to keep its water content high on the basis of said values calculated continuously so that the water content of the exhaust air is equal to a preselected value. 5. The method according to any of the preceding claims, characterized in that the machine robe future korvausil-mavirtaa adjusted based on the constant of said values calculated in such a way that the inner plane of the casing and the exterior of the pressure difference between the zero level in the vertical direction at a predetermined height, and in that this zero level, as a plurality of machine casing a wall positioned in the vertical direction a series of sensors <16) with a warm engine casing interior and this pressure difference between the colder side is identified by the temperature of the air flow caused by small differences in the wall of the casing inside and outside temperatures compared. 6. The method according to claim 5, characterized in that the machine casing warm the interior and that the colder outer side of the pressure difference between the identification of four bridge coupled to the temperature sensor (18, 19, 20, 21) by means of which the bridge opposite branches of the two sensors (20, 21) permanently measure the internal temperature and the outdoor temperature of the enclosure, respectively, of which two other sensors (18, 19) in other opposite branches of the bridge are located in said wall openings to measure a flow-dependent temperature indicating the direction of said flow and pressure difference. 17 84398