Dr-Fix-It Explains a Common Pneumatic Comfort Control Circuit.
The operation of a pneumatic thermostat / actuator circuit in everyday language with clear definitions of pneumatic concepts, diagrams and examples. A Comprehensive Resource for Owners, Managers, Builders and Operators of Commercial and Residential Buildings
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Lesson One: the Basics
Let’s start with the simplest pneumatic circuit: one thermostat and one controlled device. The controlled device could be an actuator like a water valve actuator or a damper actuator or else it could be a control device like a pneumatic-electric switch. The controlled device can itself be direct or reverse acting . In addition, controlled devices can be also be normally open or normally closed.
The main line pressure is the constant pressure coming from the air compressor. The main line pressure is always regulated. The pressure is usually set in the range of 15 to 25 PSI. For the purposes of this article, we will set the main line pressure at 20 PSI.
Direct Acting (DA) pneumatic thermostat increases branch line pressure as temperature increases.
Reverse Acting (RA) pneumatic thermostat decreases branch line pressure as temperature increases.
The amount of change in temperature that is required to boost the output pressure of the pneumatic thermostat branch line from 3 to 13 PSI (full closed to full open) is called the Proportional Band or the Throttling Range. Normally, a pneumatic thermostat throttling range is set from 2 to 6 degrees. Lets use an example of a 4 degree throttling range.
Example: a thermostat has a throttling range (Proportional Band) of 4 degrees. The occupant sets the pneumatic thermostat to 71 degrees. A direct acting (DA) pneumatic thermostat should increase the branch line pressure from 3 PSI at 69 degrees to 13 PSI at 73 degrees. 71 degrees should correspond to 8 PSI which, in pneumatics, is "top dead center". A reverse acting (RA) pneumatic thermostat would decrease the branch line pressure from 3 PSI at 73 degrees to 13 PSI at 69 degrees. But again, 71 degrees should correspond to 8 PSI "top dead center".
Now, lets define the controlling device as a water valve with a pneumatic actuator. An actuator is like a bellows or a piston. As the air pressure increases, the piston moves. When air pressure is removed, the piston returns to its minimum position. How does it get back to minimum position? A SPRING, of course! Springs are a critical component of pneumatic controlled devices. What is frequently misunderstood is that the springs are "tuned" to the actuation specifications.
Springs are color coded according to their spring pressure and size. (Sometimes, the color is only a little stamp. You may have to clean the entire spring with glass cleaner to locate the color code) Then, call your controls vendor and ask them , for instance, what pressure is a 3 inch diameter blue Honeywell spring ? The answer will be a "spring range". For example, 4 to 8 PSI)
Lets assign this actuator as a direct-acting water valve actuator with a spring range of 9 to 13 PSI and a Normally Closed Valve. So, the thermostat has a throttling range (Proportional Band) of 4 degrees. The occupant sets the pneumatic thermostat to 71 degrees. A direct acting (DA) pneumatic thermostat should increase the branch line pressure from 3 PSI at 69 degrees to 13 PSI at 73 degrees. 71 degrees should correspond to 8 PSI which in pneumatics is "top dead center". But remember; the spring range is 9 to 13 PSI.
What happens at the other end?
Room temperature of 69 degrees and below: thermostat sends a signal of 3 PSI but the spring’s lowest capability is 9 PSI. The spring’s pressure outweighs the air pressure so the actuator does not move and the valve remains in its normally closed position.
As the temperature rises, the branch line air pressure from the thermostat also rises. When the air pressure rises above 9 PSI, the air pressure outweighs the spring’s minimum pressure. The actuator moves to the balance point and the normally closed valve opens somewhat. As the air pressure continues to rise, the direct-acting actuator moves to proportionately higher balance points. The normally closed valve opens by the same proportion.
Pressures at or above 13 PSI outweigh the spring’s maximum pressure. The actuator is inflated to its fully extended position and the normally closed valve remains fully open. So, the flow of water starts as a trickle at a room temperature slightly more than 71 degrees and increases to full flow as the temperature rises to 73 degrees. Full flow would be maintained at any temperature above 73 degrees.
This sequence of operations is commonly used in chilled water air conditioning fan-coil units. When the room temperature exceeds the set-point, the chilled water is allowed to flow through the coil which serves to cool the room. A properly tuned pneumatic circuit will respond to the temperature by opening the water valve to the exact position necessary to provide the correct amount of chilled water to the coil.
Lesson 2: Lets add another actuator to our example circuit.
This actuator will be another direct-acting water valve actuator. But this time, the a spring range will be from 3 to 7 PSI and the valve will be a normally open type. Once again, the thermostat has a throttling range (Proportional Band) of 4 degrees and the occupant sets the pneumatic thermostat to 71 degrees. As before, the direct-acting (DA) pneumatic thermostat increases the branch line pressure from 3 PSI at 69 degrees to 13 PSI at 73 degrees with 71 degrees corresponding to 8 PSI.
Room temperature of 69 degrees and below: thermostat sends a signal of 3 PSI. The spring’s lowest capability is 3 PSI. The spring’s pressure balances with the air pressure so the actuator does not move and the valve remains in its normally open position.
As the temperature rises, the branch line air pressure from the thermostat also rises. When the air pressure rises above 3 PSI, the air pressure outweighs the spring’s minimum pressure and the actuator moves to the balance point. The normally open valve starts to close. As the air pressure continues to rise, the direct-acting actuator moves to proportionately higher balance points. The normally open valve closes by the same proportion.
That is, until the air pressure exceeds 7 PSI. At all pressures above 7 PSI, the air pressure outweighs the spring’s maximum pressure so the actuator is inflated to its fully extended position. The normally open valve is completely shut off. To recap: there is full flow of water at or below 69 degrees which comes to a stop when the room temperature reaches a little bit less than set-point of 71 degrees.
Also, note the system will go to full heating in the event of a loss of air pressure. This is a normal design called "fail to heat". In other words, in the event of a failure, the system will default to heating mode.
Finally, note that the system shuts off the water flow at BOTH actuators within the range of 7 to 9 PSI. This is the pneumatic "top dead center" of 8 PSI plus and minus 1 PSI. Within this range, the thermostat is said to be "satisfied" and no action is required. The temperature band that corresponds to the pressure range when no action is required is called the "dead band".
It is easy to calculate the relationship of any temperature to the corresponding air pressure. In our example, the throttling range was 4 degrees and the pressure range was 3 to 13 PSI for a difference of 10 PSI.
A change in 4 degrees corresponds to a change in 10 PSI
1 PSI= 4/10 degrees = 0.4 degrees.
Now, You can easily see that the dead band in our example would be 70.6 to 71.4 degrees.
(The Greek letter Delta, a symbol resembling a triangle, is frequently used as a short-cut for the phrases "the difference of…" or "the change in…" From above,we can substitute: "Delta 4 Degrees is Delta 10 PSI")
The circuit we have examined is probably the most common application of pneumatic comfort controls. It is used in "four-pipe" hot water / chilled water fan-coil units. When it is cold, the hot water flows. When it is hot, the chill water flows. When the thermostat is satisfied, no water flows and the fan simply ventilates the space. Safe, silent, precise and durable, pneumatic comfort control is a proven workhorse!
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