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Watt Atmospheric Engine Description

This page describes the operation of the Watt Atmospheric Engine used to drive water pumps. The engine photos on this page were taken of engines exhibited at the Henry Ford Museum in Dearborn, MI. The Watt Engine is a significantly more efficient engine than the Newcomen Engine because it incorporates a separate condenser.

This Watt engine is dated 1796 by the museum. The engine uses the original steam valve configuration and a single-acting piston, even though the double-acting piston had been developed by Watt by 1796.

Manufacturer: Boulton and Watt
45 hp
12 strokes per min
Bore 46"; Stroke 96"

Here is view of the steam piston for the 1796 Watt engine. The column at the right of the photo is the main support column for the beam. In this engine, the condenser is located to right side of the main support column. The boiler sits behind the steam piston and is not visible in this photo.

The condenser and the air pump are located to the right of the main support column in this 1796 Watt engine. Note the outlet of the air pump on the upper right of the air pump housing. The relative scale of the steam piston (right photo) and the condenser (this photo) can best be seen by the sketch below. The cylinder on the right edge of the photo is the water pump.

Below is the boiler for the engine. The boiler sits behind the engine; the brick column to the left of the boiler is the main support column; the wall on the right of the photo is the wall in the piston photo (above left) with the hanging tools.

This 1796 Watt pump is very similar to the sketch below. The primary difference between the engine at the museum and the sketch is the location of the condenser. In the museum, the condenser is on the pump side of the main support column rather than on the steam piston side. However, the principles of operation of the steam piston are the same.

Note that the sketch to the right is reversed relative to the photo above. Like the Newcomen engine, this cylinder does work on the downward stroke only of the steam piston; the engine relies on the weight of the pump side to tip the beam so that the steam piston rises. Note that chains connect the piston rod and the beam. It is not possible for the steam to push up the beam. Rather, the rising beam pulls up the piston.

The steam piston/cylinder are notably more complicated than Newcomen's. First, notice that the top of the steam cylinder is sealed. Steam will exist above the piston at atmospheric pressure during both the upward and downward strokes. Steam will exist below the piston in the upward stroke and a vacuum on the downward stroke. These steps are explained in more detail below.

The piston has a "jacket". This is a second shell around the main cylinder. Steam could be kept in this jacket to always keep the cylinder hot.  

Note the condenser water pump delivers cold water (probably from the mine) into the large well to keep the condenser cold. The air pump (as it is called by Watt) serves to pump the condensate and any non-condensable gas from the condenser h. The exit of the air pump i is into a smaller chamber separate from the cold water well. This chamber is filled with hot condensate from the air pump at k. This hot water is fed back to the boiler using the boiler feed water pump.

Step 1 - upward stroke of steam piston.

The weight of pump side pulls beam down on the left. (On some engines extra weight is added to the pump side of the beam to assure it is heavier than the steam side). This action pulls up the steam piston. Valves c, e, f are in the steam pipe to the left of the steam cylinder, top to bottom respectively. In step 1, c and f are closed, and valve e is open. Steam from above the piston travels through pipe o on the left and ends up on the bottom of the piston.

Note that the condenser water pump and feed water pumps appear to be of style B on the pump description page (except with the inlet on the bottom) and they would fill during this step as the pump rods drop. The air pump rod would rise, pushing out its contents during this step and simultaneously creating a vacuum in the condenser and removing any liquid. Note that the air pump is clearly of style B on the pump description page. There is a check valve between the air pump and the condenser, and the piston is drilled through with check valves on top. At the top of the steam piston stroke, the pins n on the air pump rod trip the three levers m to change valve positions.

Step 2 - downward stroke of steam piston

Valve e is closed, valves c and f are opened. In this valve position, the upper piston chamber is opened to the boiler, and the lower piston chamber is open to the condenser. This is the work stroke. The boiler is at about atmospheric pressure, the condenser is under a vacuum. Steam from under the piston rushes into the condenser . The steam condenses, maintaining the vacuum. Additional condensing water is admitted into the condenser using a jet through an opening in the side of condenser (the control rod for the jet can be seen starting between valves e and f, and running downward to the unlabeled white box on the side of the condenser). The piston is drawn downward by the vacuum, filling the space above the piston with steam drawn from the boiler. The piston remains hot during this stoke.

During this step the feed water pump and condenser water pump rods are rising, forcing out their pump contents and sucking in more liquid into the pump body simultaneously. The air pump rod is falling, allowing the piston to fall through the condensate and air that flows into the cylinder body during the upward motion. (Note that normally water has some air dissolved in it, and this air enters the piston, and must be removed). At the end of the downward stroke, some different pins n on the air pump rod trip the levers m and the valves c, e, f switch again, permitting the engine to repeat the upward stroke. The condenser jet is also closed.


Watt Pumping Engine 1811
Manufacturer: Boulton and Watt
30 hp (estimated)
10 strokes per minute
36 " bore
84" stroke

Note that this engine incorporates a steel beam instead of the wooden beam. The steam piston and pump piston are connected to the beam with parallel motion mechanisms. The parallel motion mechanism is the invention of Watt's that permits the pistons to move straight up and down while the end of the beam traces an arc.

There are two shafts coming down from the beam in this photo. The shaft on the left is for the air pump, which is located in the water well in the lower center of the photo (see water well photo below). This engine is not currently connected to a boiler. The hanging end of the steam inlet can be seen in front of the piston.

This photo is from the back side, and shows the steam channel connecting the upper and lower portions of the steam cylinder. The steam piston is on the left edge of the photo. The steam valve is in the lower center of the photo. The lower inlet to the piston is to the left of the valve. Note the levers on the right edge of the photo. These levers are tripped by the shaft to the air pump.

The photo at left is taken from the front right corner of the safety railing around the water well. Imagine the steam piston directly to your right as you down into the well; the main support column of the engine is in the rear of the photo. The condenser is the the lower left of the photo, and the air pump is in the upper center of the photo.

Updated 5/21/13 ,Copyright 2001-2013, Carl T. Lira, lira@egr.msu.edu All rights reserved.
Prepared as a supplement to Introductory Chemical Engineering Thermodynamics.