the smokebox located above the boiler with piping for the safety (pop) valve, throttle, and other steam devices protruding through

The term smokebox comes from the age of steam locomotives and steam traction engines.  Their boilers were designed with the flue tubes horizontal.  As the combustion gassed exited the flue tubes, the gasses needed to be directed up a smokestack.  The smokebox accomplished this function.  Smokeboxes also incorporated screens and other devices to collect hot embers so that they would not fly out the smokestack and create fires nearby.

The Stanley boiler is a vertical flue, fire tube boiler design.  However a Stanley doesn't have a smokestack (except in one photo where one was attached for humorous purposes) and the need still exists for the combustion gasses exiting the flues to to be collected and directed to the exhaust duct.  A sheetmetal cap, the same diameter as the boiler and 4" to 6" high, mounted to the top of the boiler accomplishes this need.



The smokebox is designed as a "cap within a cap". Shown at the right is a smokebox nearly ready to be installed to the top of a Stanley boiler.  The square section at the lower right serves to connect the smokebox with the exhaust duct that is located in front of the firewall.  The circular hole offset from the center of the smokebox is for the "trap door" (shown with the door closed in the photograph).  Around the perimeter of the smokebox are various size openings which allow piping to pass through.

The outside cap or shell of the smokebox is only slightly larger than the diameter of the boiler.  This allows the smokebox to slip tightly over the boiler and form a semi-tight seal.  This cap also includes the transition ductwork necessary to connect the smokebox to the exhaust duct.  Inside the transition ductwork is a short length of copper tubing (not shown in the photograph) that extends down the center of the ductwork and is the discharge for the stack blower (see the discussion on the stack blower for information related to how the stack blower is used).

Inside the outer cap or shell is an inner, smaller, cap or shell.  This cap is several inches smaller in diameter and height from the outer cap.  The diameter of the inner cap is such that it can sit over and encapsulate all of the boiler's flue tubes as they extend through the top of the boiler.  The height of the inner cap is such that it allows several inches of insulation between the the caps as well as allows the outer cap to slip over the exterior of the boiler while the inner cap rests on the flue sheet to hold the smokebox in place.   As shown in the photograph, the difference in size between the two caps allows insulation to be placed between the outer surface of the inner cap and the inner surface of the outer cap (the photograph shows insulation already in place for the top of the smokebox with the insulation for the circular side wall of the smokebox yet to be cut for the openings around the perimeter and installed.  The inner cap has an opening that matches up with the exhaust duct portion of the outside cap of the smokebox.  The two caps are bolted together to form a single unit.

Combustion gasses exiting the boiler's flue tubes enters the inner cap of the smokebox.  There all the gasses from all the flues are collected together and directed towards the opening in the smokebox that connects to the exhaust duct.  Mounted to the front center of the firewall is the exhaust duct which travels down the firewall and under the car.  The exhaust duct carries the combustion gasses collected in the smokebox down below the car's water tank and discharges them into the atmosphere.  As the exhaust duct makes its way to the underside of the water tank it is directed towards the back of the car and shaped such that when the car is in motion the air passing across the end of the exhaust duct causes a venturi effect on the duct to actually draw gasses from the duct and mix them with the air. 

The exhaust duct for a Model 735 Stanley is shown at the left. The photograph was taken from behind and below the front left wheel looking towards the exhaust duct. The water tank is the gray tank in the photo and sits in its mounting straps which are secured to the car's frame.  The burner is along the left side of the photograph.  The steam siphon piping is at the top center of the photograph.  Two boiler blow-down discharge pipes are seen in the photo, secured to the water tank support brackets, and running nearly parallel to the exhaust duct.  A third boiler blow-down discharge can be seen just protruding below the other side of the exhaust duct.

In the top of the smokebox is a trap door.  The purpose of the trap door is two-fold.  First it allows the escape of any combustible gasses from the burner or pilot that may accumulate and be trapped in the smokebox.  When the burner is shut down by the steam automatic (or burner fuel valve when that is closed) there will be some fuel in the vaporizer tube that becomes vaporized but with no fresh fuel pushing it out of the vaporizer it will flow out of the vaporizer tube slowly and enter the mixing tubes.  The heat of the boiler will cause convection air currents through the flue tubes and draw in the vapors.  Normally the pilot will burn them off but if the pilot has gone out for some reason the gasses will pass through the burner grate and into the boiler flues.  These gasses will find their way to the top of the boiler via the flues and into the smokebox.  If the car is not in motion the gasses in the smokebox will not be drawn out and thus some small quantity of combustible gas may accumulate in the smokebox.  Opening the smokebox when stopping the car after a long drive, and especially before bringing a lighted blow torch near the boiler, will allow any accumulated combustible gasses to be released.

The second function of the trap door is to allow the easy escape of combustion gasses when firing up the car.  The burner will be producing copious amounts of combustion gasses and heat.  These gasses once in the smokebox will have trouble passing down and out the exhaust duct since the gasses are both hotter and lighter than the air.  Thus opening the trap door during firing up allows the combustion gasses of the burner and pilot an easy passage through the boiler flue tube and into the atmosphere.  Once steam pressure has been raised and the car is ready to be moved the trap door can be closed and the hood lowered.  When the car has been driven for awhile it is good practice on stopping for an extended time to open the hood and trap door to allow any accumulated gasses to escape.  After being open for fifteen minutes or so the trap door can be closed to help maintain the heat in the boiler.