Roll Cooling. Effective System

To have the effective use of water and to avoid frequent roll changes and breakages, the roll cooling system should be designed upon the principle of rapid heat extraction from the roll surface. Cooling should start immediately after the hot rolled metal loses its contact with the roll.

The picture shows rolling mills and an integrated roll cooling system

Cooling system should be covered to the all segments for the shaped profile. Spray density and water pressure should be selected such to suite the pass configurations, rolling rate, roll material and stock temperature. Significant improvement in roll life has seen in the wide hot strip mill by only optimizing the distribution and proper orientation of nozzles and water pressure.

In structural mills, wear of the rolls on side walls of the passes plays an important role in achieving dimensional tolerances as well as to regain the pass profile after roll turning. Therefore, cooling headers should take care of effective cooling of full pass width as shown in Fig. 3.8.

Effective Roll Cooling System for Hot Rolling Mill

Fig. 3.8 Effective Roll Cooling System for Hot Rolling Mill.

Cooling of rolls from entry side is not generally recommended for mills producing long products. Roll cooling headers for high speed mills like Bar and Rod, Wire Rod and Merchant Mill, should not only cover the full width of the pass, but also cover the maximum possible arc of the rolls on exit side.

The figure shows the  roll cooling during operation

However, in case of hot strip mills, the cooling should be carried out from the exit as well as from the entry side also, due to more heat input to the rolls given by the rolled stock as shown in Fig. 3.9. But it should be noted that water on exit side should be substantially more than the entry side that for the effective roll cooling.

Effective roll cooling for Hot Strip Mill

Fig. 3.9 Effective roll cooling for Hot Strip Mill.

Roll Cooling Parameter

Details of roll cooling parameters for each stand of different mills are given in the Table 3.24 below.

Table 3.24 Distribution of Water for Effective Cooling in Different Mills

Type of mill

Type of Cooling

Water flow rate (m3/hr)

Water pressure (kg/cm2)

Heavy Structural and rail mill

Segmental 70° on exit side

6.0-10.0

3.5-5.0

Wire rod and Bar mill

Segmental 90°-100° on exit side

1.0-2.0

2.5-4.0

Medium section mill

Segmental 70°-80° on exit side

5.0-7.0

2.5-4.0

Narrow hot strip mill

Concentrated water jets

15.0-20.0

5.0-7.0

Wide hot strip mill

Concentrated water jets

400-750

5.0-7.0

Basic Requirements of Effective Roll Cooling System

Following are the basic requirement for effective roll cooling.

  • • Flow rate,
  • • Pressure,
  • • Temperature,
  • • Design of roll cooling pipe,
  • • Quality of water,
  • • Mill speed,
  • • Control valves,
  • • Mill scale removal,

Flow Rate

Recommended flow rate for roll cooling is shown in above Table 4.24.

Water consumption in a rolling mill depends upon water circulation rate, atmospheric temperature and relative humidity, depending upon these factors. It varies between 1 to 3% of the circulation rate.

Example: For a bar mill of say 400,000 tons per annum. Water circulation rate is expected to be 650 inA'hr and the evaporation loss shall be about 13 m3/ hour. That means mill needs about 260 m3 of makeup water per day.

Water Pressure

Recommended water pressure for roll cooling is shown in Table 4.24. Water pressure depends upon quality of roll. It is desired to have double the pressure in case of tungsten carbide rolls.

Water Temperature

Recommended water temperature for roll cooling at the point of application shall not exceed 80° C.

Steam Formation during Rolling

Fig. 3.10 Steam Formation during Rolling.

When hot metal (at a rolling temperature exceeding 1000° C) conies in contact with roll surface then there is a direct transfer of heat by conduction.

But when cooling water temperature is higher, its adverse effect will be seen. It will have a greater tendency of reaching the boiling point and of formation of steam layer. This steam acts as an insulating layer between roll surface and the water stream. The dispitation of heat from the roll surface is thus badly affected.

Design of cooling pipe

Followings points are to be noted, while designing the cooling pipe:

  • • Pressure Gauge on supply header.
  • • Maximum flow at delivery point.
  • • Fixing the pipe on delivery guide box will automatically sets the alignment.
  • • Spray nozzle to deliver 30% of the water on the groove.

Quality of Water for Roll Cooling

Following quality norms to be adhered, while maintaining the quality of water:

  • • Total suspended solids (TSS), should not exceeds 200 nrg/1 for normal rolls and 80 nrg/1 for tungsten carbide rolls.
  • • Maximum particle size at application should not exceed 200 micron.
  • • Total dissolved solids (TDS) should not exceeds 1500 nrg/1.
  • • Oil and grease content should not exceeds 40 nrg/1.
  • • pH of water should be between 8 to 8.5 in case of Tungsten carbide rolls.
  • • Install oil skimmer in the sedimentation tank so that oil/grease do not clog the fills and spray nozzles.

Peripheral Speed of Roll

In no case, pheripheral speed should go below 0.2 rn/sec, as due to longer period of contact of metal with rolls, fire crack get developed as shown in Fig. 3.11.

Fire crack Formation

Fig. 3.11 Fire crack Formation.

Use of Additives and Effective Roll Cooling System for Improvement of Roll Life

  • 1. Advantages derived with the introduction of roll lubrication technology, alongwith effective Roll cooling system are:
    • • Improvement in the surface quality of product.
    • • Improvement in the dimensional tolerance of finished product.
    • • Improvement in the operational environment in the mill due to less pass/roll changes.
    • • Mill loading also get increased.
  • 2. The roll pass lubrication system is unlikely to give positive impact, in the absence of efficient roll pass cooling; rather it will have the detrimental effect. The efficient roll cooling system will cause the following:
    • • The roll temperature should not exceed beyond 80°C.
    • • Roll cooling headers designed to cool the pass groove, to be set as close at the exit of the pass and also to be kept with proper orientation and distance from the roll.
    • • Cooling water pressure should be between 2.5-4.0 kg/cm2 with water flow rate of 5-7 M3/hr in section mill, having rolling speed <10 m/sec. It keeps the temperature of the roll within the desired limit by controlling the burning across the roll groove, generation of fire cracks; thermal shocks. It will also break the oxide film which formed on the surface of roll.
  • 3. Another aspect which needs attention while designing of a roll lubrication system is the logic to control duration of lubricant application. The continuous application of roll lubrication, during when there is no bar in the pass, may lead to biting problem due to loss of friction co-efficient across of roll groove. It will also lead to wastage of lubricant oil. Automation of system through the monitoring of current loading of drive motor is not feasible because of the generation of fault signal generation will be very high. Therefore, photo sensors should be installed to sense the presence of hot bar.
  • 4. It also needs utmost care in deciding the concentration of lubricant in the emulsion. Lubricant is chosen on the basis of then ability to bum off during the pause. But in case of high speed of rolling mill, the time gap between the stoppage of emulsion with exit of bar and energisation of lubricant application with biting of next bar is veiy small. The lubricant gets very little time to bum off or to get washed away. It leads to burning of lubricants, when it comes in contact with the hot bar. Roll surface then becomes black and it gets roughen in veiy short time. Thus, it adversely affects the roll life. Therefore, use of lubricants should begin with oil concentration should be as low as possible.
 
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