PAPERmaking! Vol7 Nr3 2021

Processes 2021 , 9 , 1707

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With the data in Figure 4, the conclusion that the total energy consumption and the energy losses display similar tendencies is drawn. As we observed in Figure 3, lower losses imply a greater availability of energy and consequently an increase in production. Note in Figure 4 that the % of total losses increase with the % of factory emissions due to global factory emissions reduction. 3.2. Temperature of Air Blown (Ebt) into the Dryer Section Hood The maintenance state of the drying enclosure hood (Figure 1, process flow) and the characteristics of the airflows within it significantly affect both the energy consumption and total amount of water that can be evaporated from the paper, and consequently, considering that the drying section is the bottleneck of paper production, the production obtained is affected. The enclosure hood must maintain optimum thermal conditions to favor water evapo- ration from the incoming sheet. The elimination of water occurs at a temperature lower than that of the water evaporation point (85 ◦ C) by transferring the moisture contained in the paper to the circulating air inside the hood. When the air reaches a sufficient moisture content, it is expelled from the hood, sending the water content to the atmosphere. The extracted air must be replaced by another equal volume from the atmosphere, which is preheated by air–air exchangers with the extracted air stream, taking advantage of the condensation energy. The energy contained in the airflow depends on its temperature and moisture content. The energy extracted from the extraction flows comes from the energy contained in the water vapor, of which most of the water is contained as condensate in the exchangers. Following the methodology described in Section 2, the main characteristics of each airflow and its energy before and after maintenance actions are obtained, as shown in Table 1.

Table1. Data of air before and after the repair of the exchangers.

gH 2 O/kg DryAir

FlowRate (m 3 /h)

Enthalpy (kJ/kg)

Energy (kJ)

Input and/Output Air

Temperature ( ◦ C)

Barometric Pressure (Pa)

Density (kg/m 3 )

RH(%)

Type

Extraction Blownair

Outlet

49 97 21

93

78.1

101,325.0 101,325.0 101,325.0 101,325.0 101,325.0 101,325.0

0.9 0.9 1.0 0.9 0.9 1.1

64,506 32,892 31,000 85,000 59,500 25,500

238 124

15,322,110 4,078,279 1,844,500 20,706,000 6,583,675 1,530,000

Before maintenance

Inlet

3

15

Compensation Inlet

18 66

8

60

Extraction Blownair

Outlet

54.7 92.4

72.5

244 111

After maintenance

Inlet

2

15

Compensation Inlet

21

18

8

60

The main thermal streams involved in the drying process into the enclosure hood include the paper from the press section (32 ◦ C, moisture content 52%) and paper leaving the enclosure hood (85 ◦ C, moisture content 6%); the water content is extracted by airflow to the atmosphere. Table 1 shows the characteristics of each airflow. The initial engineering designed flow conditions of the enclosure hood streams are 85,000m 3 /h of extraction and 59,500 m 3 /h of blowing, and the difference between the extraction and blowing may be 25,500 m 3 /h. Inside the hood, as indicated by Calvo and Domingo [62], despite the external tem- perature conditions of the blowing air and the temperature of the incoming paper into the dryer section, the outlet air and paper temperature are constant at approximately 95 ◦ C. Considering that the extracted air temperature is 85 ◦ C, the energy added to all streams (paper and air) comes from the radiation of the dryer cylinders heated by steam supplied to the dryer section (steam in Figure 1). To determine the actual state of the dryer system and enclosure hood, the data of the main associated variables of the airflow are collected to determine the air flow enthalpy before (taken in February of year 8) and after the cleaning and conditioning of the exchang- ers. The difference between the enthalpies determined by the Mollier diagram [63] gives us the energy saved after repairing and cleaning the exchanger.

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