Three different waste heat recovery devices in paper industry

The air exhaust temperature of the airtight hood is generally around 82 "C, the moisture content is about 0.16kgh20 / kg. D.A, and the enthalpy per kilogram of air exhaust is 507kj. For a paper machine with an annual output of 100000 tons, the air exhaust volume of the airtight hood is about 150000kg / h, and the paper opportunities of different paper types are different.

The heat enthalpy in the exhaust air per hour is about 76050000kj, which is equivalent to the condensation heat release of 33 tons of steam. The heat enthalpy value is quite considerable, and the energy is wasted. Due to the use of the airtight hood, the exhaust temperature and moisture content are higher, which is very expensive. Due to the use of airtight hood, the exhaust temperature and moisture content are higher, which creates conditions for the heat recovery of exhaust. Finding a good use for the recovered heat is the primary problem of heat recovery.

For example, in cold areas, the world's manufacturers usually divide exhaust air into three levels for heat recovery. The first stage of waste heat recovery unit is gas heat recovery, the second stage is water heat recovery and the third stage is glycol solution heat recovery. Other regions can choose primary or secondary heat recovery according to climate conditions.

The first stage is gas heat recovery. Exhaust air is used to preheat the air supply system in the hood. The recycling value is high, and the heat is recycled all the year round, which is necessary for any closed hood exhaust. Because such a set of heat recovery device can be recovered in less than one year. The first stage gas heat recovery device adopts the tubular heat exchanger. Due to the gas heat exchange, the heat exchange coefficient is small and the heat recovery is limited. The heat recovered by the first stage heat recovery device only accounts for about 6% of the total heat enthalpy of the exhaust air.

After the first stage gas heat recovery, the air supply temperature can reach about 60 ° C, the exhaust temperature is about 62 ° C, and the exhaust heat enthalpy is still very high. The second stage water heat recovery device can be used to recover heat. " The second stage heat recovery device generally uses exhaust air to heat the process water, and a large amount of hot water is used to wash the blanket in the process. If steam is used to heat the clean water, a large amount of steam will be consumed, and the exhaust air can be fully used to heat this part of water.

The secondary heat recovery of  waste heat recovery unit is water heat recovery. Stainless steel heat exchange tube is used. The gas water heat exchange coefficient is high and the heat recovery is high. It is operated all year round. As long as there is demand, the investment of secondary heat recovery can be recovered in half a year. After the secondary heat recovery device, the water temperature can reach about 45 ° C, the exhaust temperature is about 45 ° C, and the recovered heat accounts for about 20% of the total heat enthalpy of the exhaust. After the secondary heat recovery, the heat enthalpy of the exhaust air is still large, but due to the low temperature, the value of recovery is not great. Only in the area where the calculated temperature outside the heating room is lower than - 15 ° C, the third level of heat recovery is considered.

The third stage heat recovery of waste heat recovery unit is glycol solution heat recovery. Exhaust air is used to heat about 20% glycol solution to preheat workshop air supplement. In order to achieve the air balance of the paper workshop, a large number of outdoor fresh air needs to be added to the workshop. In the north area, the air volume from room to workshop is balanced in winter, so a lot of outdoor fresh air needs to be added to the workshop. The outdoor wind temperature is very low in winter in the north area, so it needs a lot of heat to heat this part. Therefore, for the cold north area, the external wind temperature is very low, and heating this part needs a lot of heat. Therefore, for the cold north area, about 25% of the heat enthalpy is difficult to recover after three-stage heat recovery.

The above three heat recovery devices are equipped with automatic water spray cleaning device to prevent paper wool from accumulating in heat exchange.