Unique Three-Heater System Provides Best Environment For High Temperature Heating of Nitrogen

Problem:

The customer needed to heat nitrogen gas from a 10°C (50°F) inlet temperature to a final outlet temperature of 760°C(1400°F) -- an increase of 750°C ( 1,350°F). An operating pressure of 100 PSIG (16.6 kPa) and flow rate of 0.105 to 0.15 pounds (0.048 to 0.068 kg) per second were also required. Due to the environment of the heating, Division 2 hazardous location safety requirements had to be met, as wells as ASME specifications.

Solution:

Because of the heat characteristics of gas, Watlow designed a circulation system with three separate heaters. To provide maximum watt densities and keep costs to a minimum, Watlow separated the 65 kW of electricity needed into three separate piped-in series heaters with decreasing kilowatt ratings and watt densities. Baffles were used to obtain cross flow, which helps to reduce sheath temperatures, improve heat transfer and allow for higher watt densities than would be possible in a heater with parallel flow.

Watlow designed the system with 316 stainless steel for the first vessel and 316 H-grade stainless steel for the second and third vessels to ensure corrosion resistance and long life at temperatures above 1000°F (540°C). Each heater was insulated with six inches of high temperature insulation.

A control panel features a SERIES 985 controller, SCR and SERIES 340 high-limit device for each heater. To control heat generated by the SCRs, an air conditioner, modified for Class 1, Division 2 hazardous location, was added to the control panel along with a Type Z purge system for Division 2 hazardous locations.

For ease of installation, Watlow mounted the heaters and control panel on a common skid and interconnected all wiring. Only power and inlet and outlet piping had to be provided by the customer. All electrical enclosures are NEMA 4 rated for outdoor or hose down areas, and each circulation vessel is ASME code certified. Watlow's design offered the customer the best heat transfer possible while keeping physical size and cost to a minimum.