Quench system design – heating and cooling

Numerous factors should be considered before deciding on the right quench system to improve a metal’s performance.

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In this month’s column, we will discuss the heating and cooling of a quench tank.

All quenching fluids have an optimum operating temperature range requiring some form of temperature control. Maintaining the proper temperature control will provide consistent quenching.

Heating

Quenchant heating can be achieved by several means, including electrical resistance heating elements, gas or oil-fired radiant tubes, or waste heat from the furnace exhausts. For some systems, the quenchant is heated by quenching a dummy hot load of parts. However, this is not an effective or efficient means of heating a quench tank.

The radiant tubes or electrical heating elements should have an energy density of no more than 10-watts per square inch (1.5 watts/cm2). This prevents the heaters from preferentially oxidizing the oil and depleting the oxidation additive package.

If the heating element energy density is too high, the sheath temperature will be excessive, and the oil will rapidly oxidize by depleting the anti-oxidants present (Figure 1). The excessive sheath temperatures will also cause premature failure of the heating element (Figure 2). For most oils used for quenching, the recommended maximum energy density is 1–1.5 W/cm2. Higher energy densities may be used on the condition that adequate quenchant flow is provided around the heaters to minimize the local oil temperature.

Figure 1: Sheath temperature as a function of energy density for different substances [1].
Figure 2: Damage to immersion heater due to build-up of oxidation production on elements. (Courtesy: Hofen Tech Co. Ltd.)

It is also important, should higher energy densities be used, that proper flow is maintained around the heater elements. If heat is to be maintained on the oil during weekend shut-down for instance, it’s important that the agitation is maintained to reduce the sheath temperatures. The heaters should always be interlocked with the agitation system to prevent the heating elements from being energized when the agitation is off. However, regardless of the agitation, the sheath temperature should be limited to approximately 50°C below the flash temperature of the oil. Agitation should be strong enough that a persistent vapor phase does not form around the heating element.

For polymer quenchants, the energy density recommendations should also be followed, with an additional provision. The heater sheath temperature should not exceed approximately 160°F (71°C) to prevent exceeding the cloud point of the material. The heaters should also be interlocked with the agitation system. The heaters should shut down in the event the agitation system is shut off or fails. It should also be impossible to turn on the heating system without the agitation operating.

Cooling

There are various methods available to cool quenchants:

  • Submerged pipes containing water.
  • Cooling jackets around the quench tank.
  • External water-cooled or air-cooled heat exchangers.
  • Cooling towers.
  • Refrigeration systems.

Submerged water-cooling pipes and cooling jackets are only suitable for small systems. There is always a risk of water contamination of the quenchant.

External water-cooled heat exchangers and air-cooled radiators are very efficient and are used widely for cooling large quenching systems (Figure 3). However, the potential for water entrainment from a leak, and a potential for fire due to water entrainment, limits the applicability of this type of heat exchanger for quench oil service. Often, it is the only type that will work effectively with polymer quenchants because of the much lower operating temperature.

Figure 3: Schematic of an air to oil heat exchanger commonly used for cooling quench oil. (Courtesy: Dry Coolers, Inc., Oxford, Michigan)

Air-cooling towers, where the quenchant is exposed directly to the air, are not recommended since contact between the quenchant and the air blast promotes oxidation, quenchant loss, and biological activity, and reduces quenchant life.

Refrigeration systems are effective but are costly to install.

To obtain maximum efficiency from the cooling system, the direction of circulation should be such that the hot quenchant is removed from the top of the tank either by direct outlet or over a weir, then passed through the heat exchanger and returned via the bottom of the tank.

When measuring the temperature rise of an unheated quench tank, the equation used to determine the size of a quench tank is:

Where Mmetal is the mass of the metal, Cpmetal is the specific heat of the metal, and ∆Tmetal is the decrease in the temperature of the metal being quenched. Similar values of the quenchant are also needed. This equation can also be used to determine the amount of heat that must be removed from the system to return the quenchant back to the operating quenchant temperature. In this case, only the left side of the equation needs to be solved, as this is the amount of heat that is removed from the quenched parts and transferred to the quenchant.

For instance, if I have a 5,000-pound workload (including racks and fixtures), heated to 1,600°F, and it is quenched into a 5,000-gallon quench tank at 180°F, then the heat that must be removed is

5,000 lbs * 0.17 BTU/lb/°F * (1,600-180°F) = 1,207,000 BTU.

If the shop is quenching two of these loads per hour, then this value would need to be multiplied by two to get the heat extraction rate of 2,414,000 BTU/hr that needs to be removed from the oil. This sets the minimum value of heat extraction for sizing the heat exchanger.

In a continuous operation, a constant influx of heat from the parts is occurring. This absorbed heat from the parts initially heats up the oil, but over time is radiated to the outside. If the heat input from the hot parts is equal to the heat recovered by tank losses and heat exchanger, then the quenchant temperature will maintain a steady state temperature. In this case, the operating temperature is controlled as a function of the kilograms per hour of workload, and the amount lost through the tank or recovered via the heat exchanger.

For most continuous furnaces using cold oils, the initial load heats the oil to the operating temperature, and the heat exchangers remove the balance of the heat. The heat exchanger is sized (plus safety margin) to remove the heat of the weight per hour.

In this case, the equation above is modified, to replace Mm to be the pounds per hour instead of pounds quenched at one time. The units then become BTU/hr for the heat that needs to be removed from the system. Your heat exchanger supplier can get much more in depth with the calculation of the heat exchanger, and properly size the heat exchanger, pumps, and filters needed for the heat exchanger installation.

Conclusions

In this column, the heating and cooling of quenchants was discussed. The energy density of heating elements should be limited to
1-1.5 W/cm
2 for most quenchants. It should be impossible to energize the heaters without agitation.

A very short review of the types of heat exchangers was conducted. A method was described to initially size heat exchangers for batch and continuous furnaces.

Should you have any questions regarding this column, or any other column, or have any suggestions regarding future articles, please contact the editor or myself. 

References

  1. Watlow Electric, “FireBar Tubular Heaters Datasheet,” St. Louis, MO, 2015.