Abstract: In an oil field fracing operation, a direct fire heater is provided that is mobile and self-contained on a trailer chassis. The direct fire heater has the maximum heat exchange possible between a burning gas and the fracing fluid by direct contact therewith. Recirculation is provided to maintain the fracing fluid at the desired temperature. A stack for the direct fire heater is removable for movement from one frac job or frac water heating site to another. Pellets in the stack helps insure a more complete heat exchange between the fracing fluid and the burning gas.