Heater apparatus and process for heating a fluid stream with PTC heating elements electrically connected in series

Heater apparatus comprising a housing defining a fluid passage therethrough,

• heat transfer means mounted in the passage of the housing,
• a plurality of standard self-regulating electrical resistance heater elements each in heat transfer relation with the heat transfer means and having conductive grease placed between the heater elements and the heat transfer means,
• electric circuit elements for connecting the heater elements to an electric source,
• at least two of the heater elements electrically connected in series so that when fluid flow rates through the passage are above a selected rate which varies with the temperature of the fluid, the voltage drop across each of the at least two heater elements is generally equal with the resistance of each element being relatively low, and as fluid flow through the passage decreases below the selected rate the resistance of one of the series connected heater elements increases until most of the voltage potential of the electric source is dropped across the said one of the series heater elements thereby effectively shutting off the other of the said series connected heater elements and reducing the heat output of the heater apparatus.