Fig. 1. Load of large steel paper rolls being cooled after normalizing in a car-bottom furnace

We process large-size components and heavy loads in our gas-fired car-bottom furnace. What can we do during the cooling portion of the cycle to protect the area from all the heat that’s being generated?

Considering the thermodynamics of the problem, it is readily apparent that parts cool more rapidly if a greater temperature differential can be created. However, this may or may not be advantageous to your process. Stress relief is an example of where you want to maintain a slow cooling rate as opposed to an accelerated one. Remember too that the ability to remove heat is a function of mass, specific heat and difference in temperature. Consequently, the cooling rate will be much faster at high part temperature and (very) much slower at low part temperature.

If the load is being pulled out into the room (Fig. 1), the amount of heat given off by the load and car constitutes a huge heat load (BTU rejection rate) being sent into the room plus, at the higher temperatures, terrific radiant heat. The use of an overhead hood combined possibly with a side shroud should be considered. The design of the ventilation system in this instance becomes very important and exhaust fans and dampers become important elements in the overall design. Be aware that if these are placed too close to the load they may warp or restrict proper airflow. It is not unusual in these situations to consider an air stream capable of dropping the surface temperature to about half the work temperature almost instantaneously (the volume of air is a function of circulation and velocity).

If the load is being cooled inside the furnace, you might want to increase the current means of cooling. In that instance, the following might be considered:
  • Calculate the actual amount of new air that would be required to drop the load temperature based on the estimated cooling rate of the work charge, furnace and fixture.
  • Calculate the required velocity of air converging on the work surface to cool it at the rate (°F/minute) desired.
  • Calculate the velocity of the increased air out of the existing burner nozzles and over the work surface. Also consider the current path of existing cooling air via the gas/air piping at the nozzle. It might be more advantageous, cooling-wise, to add a new blower and new nozzles in the roof, dedicated strictly to high-velocity cooling gas.
Finally, there are other, more radical approaches to achieving improved cooling rates. If cooling outside the furnace, using a water mist introduced into the air stream at the appropriate point in the cooling cycle (typically as you begin to reach that point on the cooling curve where the time extends out significantly) will dramatically improve the cooling rate. If cooling inside the furnace, the addition of a small amount of liquid nitrogen into the cooling air stream – be it a larger blower with current piping, a new blower with existing piping or new blower with new air ingress – will also accelerate cooling. The liquid nitrogen is typically in the -300°F range, and a fine mist in the cooling air stream will add considerably to the cooling effect of the circulated air. The liquid nitrogen becomes gaseous nitrogen and can be easily exhausted from the furnace.