Pneumatic controlled univent

Smith19

Hello all,

I have been working on servicing 1952 Nesbitt Syncritizer unit ventilators, which are part of a two pipe steam system of the same vintage. The system uses a pneumatic DA control system, which is in great disrepair. All units have pneumatically operated damper actuators which modulate between outside air intake and inside air intake. However, I have come across many which have been disabled, and the associated pneumatic circuit for them has been cut off. Does anyone have an idea of how to salvage this? I have some photos attached.

SWEI

If you can find a local pneumatic greybeard

you should be able to resurrect the original control system.  If this is not an option, I'd suggest removing one of the actuators to see what kind of shaft they drive.  Plenty of options for retrofit electric actuators (which work with modern DDC systems.)  Where is this job located?

Smith19

DDC

This job is in a Colorado school. Being a public school, the DDC money just isn't there. That can't happen. Right now, we need need to get fresh air moving through the heat exchanger, so a piping diagram for this pneumatic would be much appreciated if anyone has got one. The Powers pneumatically operated temp. sensor (large copper wand thing above blower) has been cut off, and I'm trying to find some diagrams that fit the old Nesbitt specs.

JUGHNE

Powers

Is this a single (air line) supply day/night control system with auto or manual change over?

Smith19

system type

What I can tell you is that it has no longer got a night setback. Down in the back of the boiler room we've got a new quincy compressor, which cycles in lead lag. The night set back controls have been taken off line, and there literally just direct air from the dryer to the t-stats. At one point, however, (probably until the 70's) the system was manual, when the school district could afford the staff to adjust the pneumatics and maintain things.

JUGHNE

1952 school

You must have some quite a few bodies in the classrooms if a 1952 building needs fresh air. 

I have had experience with this system, but only one.  The same Nesbit units with hot water.  Originally the night setting with higher pressure would close the outside air dampers and shut off the convector fans.  And with hot water the full length baseboard would heat the building.  The manual change over valve is simply a 3-way (plumbing type) valve.  You need 2 regulators, 1 set for day and 1 set for night.  I believe the disconnected device in the picture is a Powers TH 356 Limitem Rigid bulb T-stat. (See Siemens building Tech web site)  It was probably used to modulate the position of the outside air inlet damper.  I have had the old brass Powers day/nite T-stats rebuilt by NECC-controls.com.  Check their website for info. You might not to need the modulation of the air damper, just get it to open some for day operation and close on night mode. 

Dave in QCA

Pneumatic control is simple

Back in 1981 I was responsible for a building with about 32 uni-vents. One section of the building was on hot water and the 1960 part of the building ran on steam. Night set back consisted of turning off fans and outdoor air only. This was accomplished by a series of pneumatic switches located in the boiler room that applied control air (20 psi) to pressure switches located in the vicinity of the group of uni-vents being controlled. When pressure was applied, it activated a normally open switch, which in turn powered a normally closed contactor, to disconnect the power supply to the uni-vent fans.



Individual Uni-vent control was consistent to the schematic I have attached. This is a very typical setup for steam uni-vents. One of the most complicated, yet usually very reliable components is the damper actuator, which usually is designed to operated in two stages. With control pressures less than 4 Psi, the dampers are closed. In the range of 4-8 psi, the damper is in the normal minimum position to deliver the required amount of fresh air to the space. In the range of 9-13 psi, the dampers will index open to provide cooling with up to 100% outdoor air. The steam valve operates in the control range of 4-8 psi, with 4 being open and 8 being closed.



So, in a condition where the room temperature is well below the set point, say during a warm-up period, the thermostat might be sending out a pressure less than 4 psi. At this point, the dampers would be closed and the steam valve 100% open. As the room temperature rises, the thermostat output will also rise. AS the thermostat reaches a 4 psi output, the dampers will open to the minimum position, and the uni-vent will continue to heat, as the steam valve is 100% open. As the room temperature continues to approach the set point, the thermostat output will decrease, causing the steam valve to modulate toward the closed position. The thermostat and uni-vent will modulate to a point where the output temperature from the uni-vent is sufficient to maintain the room temperature at the set point of the thermostat. Now, if room conditions change, say the room has been empty and now 30 hot sweaty kids return from recess, this will have an effect on the room. As the room warms up, the thermostat will increase it's output signal causing the steam valve to index closed. If the room temperature continues to increase, the increase in thermostat output signal will cause the fresh air dampers to index open. If it is cold outside, the low limit thermostat, located in the output air stream of the uni-vent, will cause the steam valve to index open to a position to maintain an output temperature of no less than 60F.

It should also be noted that there is always an interlock device, usually a 2 position solenoid valve wired into the fan circuit. When there is no power being supplied to the fan, air connection to the air damper is cut off, causing he Outdoor air dampers to remain closed. As is always the case in pneumatic controls, lost of control air will cause the system and all components to go into full heat mode.

jumper

surprised

I'm surprised that the dampers themselves aren't stuck. In my day PLCs were not expensive. Nowadays control systems are oversold.