Versatile Electric Strip Heaters and Solid State Power Control: A Case Against Gas Fired Steam

Custom Solutions SCR Panel 2

The Challenge
This customer uses piping and metal parts in the fabrication of high speed turbines and compressors. In addition, they also rebuild used turbines and compressors. Parts from these units require degreasing and corrosion removal.

Previously, parts were soaked individually in small tanks containing stripper, degreaser and rust preventative solutions which make up a multi-step pickling operation. Each tank was maintained at elevated temperatures with electric over-the-side immersion heaters.

The customer recognized that processing parts individually was inefficient and time consuming. Also, due to an increase in production demand and the introduction of large parts to be processed, new larger tanks became necessary. It was decided that one large tank for each step in the pickling operation and one for rinsing, (for a total of four tanks) were required. The three tanks, in addition to the rinse tank, were to be maintained at elevated temperatures of 190°, 160°, and 160° F. Heating costs were a major concern.

The Solution
The customer assigned a full time engineering team to investigate replacement system requirements. The number of stages, chemicals to be used, tank size and shape, entry/exit logistics, initial cost, cost to operate, etc. were studied. Major emphasis was devoted to researching the costs of gas fired steam heating versus electricity. Considered were equipment, installation and energy costs as well as ancillary operation requirements and maintenance costs.

As a result of the investigation, it was determined electric heating was the superior heating method for the application with numerous advantages over gas fired steam.

After deciding to build four tanks each measuring 4' x 4' x 20', they asked a Chromalox sales engineer for guidance in selecting the most practical electric heating method. He suggested Chromalox OT3601 strip heaters which could be mounted vertically between the tank ribs on one side of each tank. Heating from the side rather than from the bottom of the tank prevents the layer of sludge at the bottom of a tank from effecting heat transfer. It also eliminates obstructions inside the tank created by immersion heaters and therefore permits easy parts removal.

Based on calculations of heat loss and allowing only a thirty minute heat-up time the Chromalox field sales engineer determined each tank would require 185 strip heaters, 1.5 kW each, for 278 kW total for each tank. He recommended the heaters be covered by a layer of sheet metal then covered with insulation and another layer of sheet metal and the other three sides be insulated and covered with sheet metal as well. He also recommended an air agitator for even heating in the tank.

The customer requested Chromalox engineers to supply a central control panel which would handle distribution control and shutdown for all three heated tanks. They also wanted a selector switch for each tank which would allow them to maintain a lower temperature during non-working hours and an operating temperature during working hours without changing controller settings.

Chromalox supplied a custom wired control panel complete with disconnects, power distribution blocks, fusing and SCR power controllers. The controllers and switches necessary to provide the operation and maintenance temperatures were also included along with over-temperature protection for each tank. SCR power components (Chromalox models 4120 and 4130) were selected to provide a continuously adjustable power output (100%) to match changing load requirements precisely.

Two separate digital indicating Y4 DIN temperature controllers were selected (one for operating and one for maintenance temperatures) for each tank. (Chromalox model 3914, PID controller for precise operating temperature and model 3913 proportional controller for maintenance temperature).

Benefits

  • As the plant did not have excess steam available, a new boiler would have been required to implement the gas fired  
    steam method of heating. Installed cost of the boiler versus the complete electric system was over 200% higher.
  • The electric control panel is pre-wired so only the heaters had to be mounted and the power lines brought into and out  
    of the panel. The boiler approach required heavy rigging and many man-hours of steam fitting, in addition to the need for bringing in electric power and fuel lines.
  • The customer's large electricity demand already resulted in a very low energy usage rate from the utility. BTU  
    calculations quickly verified electric heating costs were substantially less than energy cost incurred with steam heating (partly due to the relative efficiency).
  • Ancillary costs associated with steam heating are prohibitive. A licensed boiler operator has to be present for starting.  
    Personnel would be required to start very early to get up steam for heating versus the flip of a switch by a maintenance man or a timer to start the electric system. A boiler also requires state inspections, scheduled blow-downs, regular 
    cleaning, etc. notencountered with electric heating.
  • All of the electric heating elements and controls are readily accessible for service, maintenance or replacement if 
    required. A steam system could require shutdown for long periods for cool down before service to pipes, boiler burners, etc. could be performed.