The power of the electric heater should be greater than or equal to the heat loss power of the instrument box. For example, if the heat loss power of the instrument box is 80W, then the power of the electric heater should be selected as 100W.
Before selecting an explosion-proof electric heater for the instrument box, the known conditions required are:
■Explosion-proof area temperature class: T3, T4 or T6.
■Temperature required inside the instrument box: Anti-freezing 5℃; Maintaining normal temperature: 10, 20, 30 or 40℃; Maintaining high temperature: 50～140℃ for T3, 50～80℃ for T4._Heat loss of the instrument box (heat transfer power).
Anti-freezing and maintaining normal temperature
Conduction heaters are limited to a few applications such as heating valve groups, while convection space heaters are widely used. Self-limiting convection space heaters have relatively low power, around 100W, and are mainly used for anti-freezing protection. Constant power convection space heaters are the most widely used, suitable for anti-freezing, maintaining normal temperature 10～40℃, and maintaining high temperature above 50℃.
To prevent the electric heater from burning out when its temperature approaches the maximum allowable temperature of the temperature class, the constant power convection heater must be used together with a thermostat. The thermostat controls the maintained temperature inside the instrument box. When the maintained temperature is reached, the thermostat cuts off the power, and the electric heater stops heating, preventing the surface temperature of the heater from approaching the maximum allowable temperature of the temperature class. When the temperature drops to a certain level, the thermostat turns on the power again to continue heating, keeping the temperature inside the box within a certain range through repeated operation.
When using a convection electric heater, there should be sufficient space around the heater to facilitate air circulation, and the fins should not be covered. Otherwise, the heat generated by the electric heater cannot be fully dissipated, remaining in the heater body, causing the heater temperature to rise too high and reach the maximum allowable temperature of the temperature class.
