Generally multi-efficiency vacuum evaporation is used in the vacuum production of edible sea salt, lake salt and mineral salt. Salt making vacuum system is one of the most important process. It directly affects production capacity and cost. Currently, the domestic salt industry mostly using open-water cooling vacuum technology: water from circulating water poor goes to the atmospheric mixing condenser, and then it is discharged to water seal tank, and pumped at last into circulating water poor to be cooled. It is, to some extent, able to meet the desired vacuum degree of salt making, but this technique has its limit. Final efficiency vacuum degree is not high enough. In this case, steam consumption will be very large for the only way to maintain high thermal transmission force is to increase the first efficiency steam pressure. Besides, circulating water poor overworks and couldn’t offer cooling enough water because the water discharged form atmospheric mixing condenser is at a little higher temperature, especially in summer when water from circulating water poor is at about 40℃. It affects vacuum system and vacuum degree is reduced again. We designed a new type of high vacuum system to overcome the above shortcomings and it performances very well in production applications.
1, Process devices and operation principle
1.1, Process devices
In the salt production by vacuum, the key factors affecting vacuum degree are: coming cooling water temperature and non-condensable gas extraction. To improve vacuum degree, it is a must to lower cooling water temperature and use vacuum equipment with a low steam consumption, high efficiency, and large extraction. At present open water cooling vacuum technology is used in salt production (process shown in Figure 1), which relies entirely on direct contact of water vapor to produce a negative pressure. In this case non-condensable gases accumulate and vacuum degree drops. Some domestic salt companies install steam jet pumps next to atmospheric mixing condenser in order to exclude non-condensable gases. However, it requests steam pressure 1MPa, which causes large steam consumption, noise, unstable steam extraction. This way is not worthy.
Our company add a cooled water tower and vacuum pumps at the base of open water cooling technology, see Figure 2.
1.2 Operation principle
The water discharged from atmospheric mixing condenser is pumped into cooled water tower, and then discharged in two channels: one to the water injecting tower and the steam injecting tower of vacuum pump, and the other to the atmospheric mixing condenser as cooling water supply. When the final efficiency second steam goes into atmospheric mixing condenser, non-condensable gases are pumped by vacuum pump and the rest steam mix with a large amount of cooled water to form vacuum. The entire cooling water circulation system becomes independent. Therefore, it is not necessary to draw water to the circulating water poor.
2, Design and function of main equipments
2.1, cooling water tower
The cooling water tower is WFL spraying cooling water tower, which is made un of tower body, hairdryer, watering screens, water collector and spray propulsion atomizer etc.. Spray propulsion atomizer is the key component. It replaces traditional fillers and fans, and it is a new spray and jet element with liquid drive. When the atomizing spray nozzle produces the liquid which the spray propulsion atomizer needs, the reaction force of mist flow promote the nozzle and the fan to rotate in return. Meanwhile centrifugal force in such rotary boosters water flow, which help the nozzle rotate more quickly and increase liquid flow. With the nozzle rotating faster, blowing rate increases. In this case mist flow is intensified. This design allows the sprayer operating parameters to always be in a matching state so as to transfer heat and mass in a hot and cold mist conditions. In addition, new fan impellers, air passage, water screen and water collector coordinate with spray propulsion atomizer, which brings up high circulating water cooling efficiency.
2.2, Vacuum pump system
The vacuum pump system takes steam injection tower as a main pump and takes water injection tower as a backing pump. It combines the advantages of both water jet pump and steam jet pump, that is high efficiency and high suction amount under high vacuum. The water injection tower is designed as a backing pump, which not only increases the stability of the entire vacuum extraction steam, but also overcomes the shortcomings of steam jet pump sush as large steam consumption, high working steam pressure, and noise etc..