Sizing tanks for batch immersion quenching

The type of quenchant to be used determines the volume of quenchant necessary for successful processes.

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In this column, I will demonstrate how to calculate the size of a quench tank used for batch immersion quenching.

Introduction

The size of a batch immersion quench tank depends upon the dimensions of the workload, as well as the allowable temperature rise. The temperature rise permitted is dependent upon whether the quenchant is oil, water, or polymer.

In a batch operation, care should be taken to ensure that enough quenchant covers the top of the workload. The physical dimensions of the tank should be large enough to ensure full immersion of the quench load and fixtures, and at the same time allow enough space for agitators and manipulators. Depending on the size of the workload, it is generally appropriate to have at least 150-300 mm of fluid over the top of the workload, and preferably more.

When using hot quenching oils, allowance must be made for the thermal expansion of the quench oil, either by making provisions for an overflow system, or by manual adjustment of the fluid level.

Tank Capacity

The quench tank must contain sufficient fluid to quench the load without excessive rise in temperature of the quenching fluid. In an uncooled tank, the quantity of quenchant required can be calculated from the basic equation:

Where Mm is the mass of the metal, Cpm is the specific heat of the metal, and ΔTm is the decrease in the temperature of the metal being quenched. Similar values of the quenchant are also needed.

Typical values for specific heat at 20°C are as follows:

  • Steel 0.17 cal/gm/°C (0.17 BTU/lb/°F)
  • Aluminum 0.23 cal/gm/°C (0.23 BTU/lb/°F)
  • Quench Oil 0.50 cal/gm/°C (0.50 BTU/lb/°F)
  • Polymer Quenchant 0.95 cal/gm/°C (0.95 BTU/lb/°F)
  • Water 1.0 cal/gm/°C (1.0 BTU/lb/°F)

As a general guideline for steel quenching, for a single quench, 10 liters of oil is required for each kilogram of total charge weight (1 gallon of oil per 1 pound of load). Using this rule of thumb will produce about a 38°C (70°F) temperature rise under nominal conditions. This temperature range is recommended to prevent the oil from reaching the flashpoint of the fluid. It is always recommended that the maximum temperature during quenching for oils be at least 100°F (56°C) below the flash temperature. The following example illustrates this:

However, with successive quenches, some form of cooling is necessary to prevent the oil from overheating. The heat exchanger should be sized to recover the heat produced by the quenched load within one heat treating cycle. Figure 1 shows the relationship between furnace load and quench tank size for oil (based on the one pound, one gallon rule of thumb).

Figure 1: Relationship between furnace workload and size of quenchant needed, based on the one gallon of quenchant per one pound of load rule of thumb.

Water and polymer quenchants have different limitations on temperature. This is not related to safety, as in the case of quench oils, but effective cooling for water, and maximum operational temperatures for polymer quenching. For water, the maximum temperature is 100°C (212°F); however, this limit is rarely used as the cost of make-up and cooling becomes excessive. For polymer quenchants and water used in quenching aluminum, aerospace standards specify a maximum temperature rise of 10°F (6°C) with a maximum temperature of 110°F (43°C).

For polymer quenchants, it is a bit more difficult. For polyalkylene glycol (PAG) quenchants, the maximum temperature cannot exceed the cloud point temperature. For most PAG quenchants, depending on the molecular weight of the polymer, the cloud point temperature is between 57-76°C (135-170°F). It is also recommended that the maximum bulk temperature rise be at least 20°F (11°C) below the cloud point temperature. Polymer quenchants, besides being sensitive to agitation, are strongly affected by temperature. As temperature increases, the cooling rate substantially decreases. Using small temperature rises during quenching will reduce variations in cooling rate and quench rate effectiveness. This will also reduce drag-out and produce more uniform quenching. It will prolong the life of the quenchant. This limitation of temperature rise greatly increases the quench tank size. Figure 2 shows the relationship between furnace workload size and quench tank size.

Figure 2: Relationship between the volume of water or polymer quenchant required when quenching steel.

For PVP type quenchants, there is no cloud point temperature. However, the maximum peak temperature is generally limited to 150°F (65°) or below to prevent destructive oxidation from occurring to the product. This also reduces the amount of drag-out and chemical consumption to the system.

As an example, a heat treater wants to manufacture a quench tank containing a hybrid polymer quenchant that does not have a cloud point. He wants to quench steel parts from 1,600°F (871°C) to 200°F (93°C), with a maximum load weight of 5,000 pounds. The quench tank operates at 110°F (43°C) nominally. How big should the quench tank be in gallons?

To solve this, he first needs to determine the maximum allowable temperature rise. For this quenchant, the maximum temperature is 135°F (57°C).

Since the quench tank is operating at 110°F (43°C), then the temperature rise of the quenchant (ΔTq) would be 25°F (135-110°F). From the equation above, and rearranging to determine the mass of water required:

The calculations indicate that 50,100 pounds of polymer quenchant are required to quench 5,000 pounds of steel. Knowing that a gallon of water weighs 8.33 pounds per gallon, the number of gallons required to quench 5,000 pounds of steel as described above, would be 6,000 gallons.

Conclusions

In this short column we have demonstrated how to calculate the size of quench tanks for batch immersion quenching. Should you have any questions regarding this article, or any suggestions for new columns, please contact the editor or the author.