Duct-heater power is governed primarily by mass flow, gas specific heat, and required temperature rise. In simple terms, heat duty equals mass flow multiplied by specific heat and the difference between outlet and inlet temperature. Engineering units must be consistent, and the calculation should account for heat losses, leakage, altitude, moisture, and any variation in gas composition.

Volumetric flow alone can be misleading because gas density changes with temperature and pressure. State whether the value is actual or standard flow and provide the reference conditions. Also identify the minimum flow at which the heater may operate. A heater sized for peak flow can overheat quickly if power remains high during turndown.

Controls and element staging should follow the real operating profile. On-off control may be adequate for a stable, low-power duty, while SCR or thyristor control can provide smoother modulation for tight temperature requirements. Multiple stages can limit inrush, improve turndown, and allow partial operation during maintenance, depending on the electrical design.

For quotation, Runkel recommends providing duct size, gas name and composition, flow range, pressure, inlet and target temperatures, ambient conditions, voltage, phase, hazardous-area data, allowable pressure drop, control signal, and installation drawing. When data are uncertain, fan capacity, motor rating, duct velocity, or an existing heater nameplate may help establish a preliminary estimate for engineering review.

Moist air may require additional energy beyond a simple dry-air calculation, particularly when the process involves evaporation, drying, or a change in moisture content. Likewise, a mixed process gas may have a different specific heat from air. The system engineer should use property data appropriate to the actual composition and temperature range rather than relying on a single room-temperature value.

The calculated kilowatts should be checked against airflow velocity, permissible element loading, duct space, and pressure drop. If the requested temperature rise is too large for one section, two heating stages in series or a larger duct section may provide better uniformity. Runkel can review the thermal duty together with the mechanical drawing and propose a practical element arrangement for further project approval.

Call to Action

Discuss your application with Runkel. Send the medium or gas composition, flow or volume, inlet and target temperatures, pressure, voltage, installation dimensions, control requirements, and hazardous-area information to info@runkel.cn, or visit www.runkel.cn.