Feature Notes
Air Handing Unit Static Pressure Submodule with 2 x 0-10V DC Outputs
SPC / DIN / AOP / ...
Main Features Controls the Static Pressure in VAV Air Handling Unit Plants Can be used to control Air Quality or CO2 levels in Public Areas Works with SeaChange AHU Controller to ensure correct interlocks Speed tracking of Supply and Extract fans Night Purge Interlock Facility

Summary of Features

General

This Static Pressure Controller is an AHU Submodule,and must be used in conjunction with a SeaChange AHU Controller. It can be used to control Static Pressure in a Variable Air Volume (VAV) Air Handling Unit (AHU) A Static Pressure Transmitter with 0-10V DC output measures the pressure in the Supply Duct, and is wired to the controller’s input via a dedicated transformer (see later section).

The Controller’s first 0-10VDC output channel is wired to the device which caries the volume to the Supply Duct; typically this could be a Variable Speed Drive (VSD) controlling the Supply Fan, variable Inlet Guide Vanes, or a Variable Pitch Fan.

A second 0-10V DC output channel is available for driving the Extract Fan; this can be run at the same speed as the Supply Fan, or if desired can be run at a speed offset from that of the Supply Fan.

Air Quality or CO2 Control

The Static Pressure Controller can also be used in schemes without VAV boxes to control Air Quality or CO2 levels; this is useful for public spaces, auditoria, function rooms etc. where there is variable occupancy rates (use CO2 control), or where the environment can be contaminated by smoking etc.(use Air Quality Control). Instead of a Static Pressure sensor mounted in the supply duct, this application needs a AQ or CO2 sensor mounted in the space or Return Air ductwork.

As with the the Static Pressure sensor, the AQ or CO2 sensor must have a 0-10V DC output, and must be powered from a separate supply, and have an isolated output (see later section).

Registration

Registration is the simple process by which logical connections are made between Controllers in a SeaChange system; it is done at time of commissioning and involves pressing buttons on the Controllers in a specific sequence (for further details of the registration process, see our publication “Commissioning Guide”).

The Static Pressure Controller must be registered to an AHU Controller; this is done by putting the AHU Controller into Configuration Mode, and registering the Heat Recovery Controller to it. This automatically enables all necessary demand signals, interlocks, temperatures etc. to be exchanged between the Controllers across the network.

The Static Pressure Submodule is Registered to the AHU Controller; note that you cannot use a Fan Changeover Module on the Supply Fan (for twin Supply Fans) if you have registered a Static Pressure Controller (because they both use the same address slot AnF1).

Description of Features

Static Pressure Sensor

A pressure sensor must be fitted into the ductwork to measure the Static Pressure; normally, sensors with suitable ranges will be of the Differential Pressure type, and the high pressure port will be connected to the ductwork, whilst the low pressure port will be vented to atmosphere. The sensor is normally mounted some distance from the AHU, often 2/3 of the total length of the index duct from the AHU. This is done in order to give a representative reading when the VAV boxes open and close.

Air Quality or CO2 sensor

Air Quality or CO2 sensors must be fitted in the space to be controlled, or in the Return Air / Extract ductwork.

Sensor Power Supply

The Sensor must be fed from its own separate 24V AC supply, because the 0-10V DC input on the Static Pressure Controller is non-isolated; the 24V supply for the sensor cannot therefore be taken from the general panel supply, as the sensor reading will be upset by earthing connections of other devices. Use a small separate transformer; the SeaChange TRN/DIN/4.5/002 is likely to be suitable; check that the power consumption of your sensor does not exceed 4.5VA if you want to use this transformer. If the sensor is mains-powered, ensure that its output is isolated.

Scaling the sensor reading

The 0-10V DC signal from the sensor may be scaled into engineering units using parameters MNPV (sensor reading at 0V DC) and MXPV (sensor reading at 10V DC); the number of decimal places of the reading can be set on DPPV ; the 3 parameters allow ranges in the sensor reading to be set from 0 to 9.999 to 0 to 9999 units, so any common unit of measure can be used (Pa, kPa, ppm CO2 etc.)

Commissioning - Pressure Control

The Static Pressure setpoint needs to be determined first; the Fan Speed (or guide vanes etc.) need to be driven manually to achieve system balance with all VAV boxes delivering their design volumes. Normally the Mechanical Commissioning specialist should do this part of the work. When stability has been reached, the current reading of Static Pressure is noted using the Monitoring Parameter PVKP. This value is now used for the operational setpoint by setting parameter SPOC to this value. This will ensure that there is sufficient pressure in the duct to ensure all VAV boxes are adequately fed with air.

The Controller can now be connected to the VSD and automatic operation will commence. The inbuilt Fuzzy Logic Control Loops should automatically tune themselves to achieve stable operation; this may take some tens of minutes to settle. If however the pressure reading continues to oscillate, some manual intervention may be necessary.

Tuning the Control Loop - Pressure control

VAV plants are difficult to control because of the combination of fast response times (the measured variable can change very rapidly) and the fact that the VAV boxes are introducing a Positive-Feedback element into the overall control loop which is inherently destabilising. Pressure independent VAV boxes work by maintaining their current desired volume when the pressure in the duct fluctuates; if the pressure rises, the box will tend to close its damper (thus making the pressure rise still further). If the boxes are controlled by non-SeaChange controllers, instability is more likely; often it is impossible to alter tuning parameters in the VAV box controllers, so it may be necessary to adjust the Static Pressure Controller’s parameters manually.

Two sets of parameters may be adjusted; firstly the filtering rate of the ADC (analogue to digital converter) may be adjusted. Normally, this is set to take the average of 128 readings over a 6.4 second period in order to generate a new pressure reading for the control loop. The filtering is there to exclude rogue values from the reading, either as a result of electrical noise, or resonance in the sensor’s mechanical parts. If the true time constant of the system is less than 6.4 seconds (i.e. real variations in duct pressure are occurring more frequently than every 6 seconds) then the filtering can be done across a smaller number of samples. The parameter ADCB is normally set to 8 (8 blocks of 16 values,each block takes 0.8 secs to process). Reducing this number will reduce the number of samples, but will increase the suseptibilty to noise. Similarly, if the perceived problem is that the sensor is seeing invalid pressure readings, the sampled number can be increased, but make sure that the new sampling time will not exceed the time constant of the system.

The second set of parameters are settings of the Fuzzy Logic Loop, found at parameters C70-C74. These parameters should only be adjusted with advice from SeaChange.

Commissioning - Air Quality/CO2 control

When controlling a variable like AQ or CO2, the sense of the control loop needs to be reversed. When controlling pressure, a rising pressure signal will result in a falling control output to the fan; with AQ or CO2, a rising AQ/CO2 signal (i.e. increasingly “dirty” air) needs to result in a rising control output to the fan. To achieve this, the parameter INVL (invert loop) must be set to 1. The desired AQ or CO2 setpoint can now be set on parameter SPOC; the supply fan should now adjust its speed until this setpoint is achieved.

Supply and Extract Fan Tracking

Usually a VAV scheme will have a Supply and Extract Fan; in order to maintain system balance and avoid over or under-pressurising the space it is often advantageous to track the speeds of Supply and Extract fans; as the Supply Fan’s speed increases, so does the speed of the Extract Fan.
It is possible to offset the outputs to the Supply and Extract Fan speed controllers by a fixed amount using parameter ELAG; setting this to a positive % value will cause the Extract Fan to lead the Supply Fan by.that percentage, setting it to a negative value will cause the Extract Fan to lag.

Night Purge

If required the fan speed can be set to a fixed value when Night Purge is in operation, this value is set on Knob 6 NPSD Night Purge Speed. When this is set to a non zero value then the Pressure control will be disabled and the fans will run at the set value during Night Purge.

Manual Override

The Static Pressure Controller may be put into a Manual Override mode for testing and commissioning purposes. The Controller may be put into Manual Override by using the Override pushbutton on the front of the module, or by using Doorway Supervisor.

Using the Override pushbutton:
The button is held down until the Status lamp starts to flash rapidly; the button may now be released. The Controller is now in an “Occupied” condition (Occupancy Override mode) and will control the fan speed to its normal setpoint (set on SPOC). A second press on the button selects Manual Hand mode; if no potentiometer is fitted to the circuit board, the controller will go to maximum output (see below). If a potentiometer is fitted, the speed can be adjusted by rotating the pot.

A third press of the button exits from Manual Override.

Using Doorway Supervisor:
The Controller may be set into Occupancy Override by setting software switches W1 and W2 together using the following syntax:
[AnF1]W1(S)/auto/override/10/12/W2(S)
(where An is the address of the AHU controller)

This will control the fan speed to its normal setpoint (set on SPOC).

The Controller may be set into Manual Hand mode by setting software switch W5 and Config parameter C83 together using the following syntax:
[AnF1]W5(S)/hand/auto/12/10/C83(V)
(where An is the address of the AHU controller).

This will allow the fan output level to be set using the special dialog box displayed by Doorway.

Maximum and Minimum Setting for Fan Speed

It is possible to limit the Maximum and Minimum speed of the fans by setting configuration parameters; normally, Inverter VSDs have a facility for setting these values, but the Static Pressure Controller can perform this function if required, and will ensure complete scaling of the control signal so may result in better overall control.

Parameters MXVA and MNVA are used to set the Maximum and Minimum output levels (in DC Volts) from Channel ‘A’ (the Supply Fan). MXVB and MNVB set the same limits for Channel ‘B’ (the Extract Fan). The 0-100% demand signal from the control loop will be automatically scaled between the minimum and maximum settings.

Options and Product Codes

Air Handling Unit Static Pressure Submodule

SPC / DIN / AOP / [driver option]

Driver options

SC Controls Ltd

PO Box313
Wadhurst
East Sussex
TN5 6JL

phone 08707 606040
fax 08707 606041
e-mail seachange@sccontrols.co.uk
http:// www.seachange.co.uk