To safely heat a special asphalt for corrugated pipe coating in a tank semi-circular in cross-section, 34 feet long, 6 feet wide, with a full capacity of about 12,000 pounds was a particular challenge for this manufacturer. Flame heating was not suitable, because of danger of fire, resulting in possible loss of tank and contents.
Also, better temperature control, possible with electric heat, was desirable for several reasons. First, specifications must be closely adhered to, and the use of heat sources other than electricity proved inconsistent, making product control difficult. And asphalt consumption could be more economically controlled - too little heat would cause an unnecessarily thick coating, while over-heated coating would be too thin.
When the new plant started operating, electric heat has performed according to expectations. Its choice resulted from careful calculations which indicated a competitive operating cost.
Tank is double-walled with 180 Chromalox Electric Strip Heaters - Type 5E-4301XX, 230 volts, 1000 watts - clamped to the inner wall with standard 5971 Chromalox clamps.
Mounting tabs were purposely omitted from the heaters, leaving an overall length of. 40 inches. Heaters are evenly spaced over the length and width of the outside surface of the inside wall. Space between the two walls is six inches; 1 1/2 inches of magnesium board is attached to the inside of the outside wall for insulating purposes.
OPERATING PROCEDURE: Heaters are energized under thermostatic control at all times, including nights and weekends. Plant operates two eight-hour shifts at least five days a week, and in periods when no coating or paving (filling in the bottom of the culvert with ladled asphalt to add to its wear resistance) is being done, heat losses are reduced by capping the tank with several large square magnesium board lids. One lid is removed during the paving operation, allowing access to the asphalt for ladling, yet maintaining heat retention in the remainder of the tank.
Because metal must be dry before coating, a day's supply of formed culvert is usually kept inside the building.
OPERATING TEMPERATURE is 420°F plus or minus 10 degrees. Flash point of coating is 510° F, and special protection against dangerous overheating follows.
CONTROLS: Heaters are grounded or zoned. in three banks and are delta-connected in three phase.
Each of the three zones has its own indicating thermostat, 100 ° to 650 ° F. range, with bulb in asphalt, and two 230 volt, 90 amp, 3-pole, 60-cycle enclosed magnetic contactors.
A single limit control, 100° to 1000 ° F range, with bulb in the jacket or wall, de-energizes heaters if the jacket temperature exceeds the safety limit.
An alarm control, 100° to 1000° F range, with bulb in asphalt, does likewise.
Limit and alarm controls override all others and wired with them is one 230-volt, 13-amp., 2-pole, 60-cycle enclosed magnetic contactor with 230-volt holding coil.
A temperature recorder, 100° to 650° F. range with seven day chart, is also used.
- Inherently safer for heating the inflammable asphalt coating compound.
- Accurate controls save material by eliminating under-heated, over-thick coatings.
- Quality is easily controlled.
- It permits the customer to meet specifications without guesswork.
- Temperatures are easily reached, maintained and duplicated.
- Operating cost is competitive.
- No attendant is required at any time. Dependable, high quality thermostats do the sensing - switching heaters on and off as needed-with temperatures never exceeding safe limits.
- Maintenance expense is slight.