Example 8

Supply and Exhaust Air Handling Unit with Twin-coil Exchanger, Water Heaters and Water Cooler, with Humidification and Dehumidification Function

Inputs and outputs configured for supply and exhaust air handling unit with twin-coil exchanger, water heaters and water cooler, with humidification and dehumidification function

The air handling unit consists of:

2-position inlet and outlet damper;
air supply and exhaust filter
twin-coil exchanger with a valve and exchanger pump;
water preheater with a valve and a preheater pump;
water cooler with a valve;
water reheater with a valve and, optionally, a reheater pump;
supply and exhaust fans controlled by an inverter;
humidifier.

The AAC20 controller is sufficient if the dehumidification and humidification functions are not used. To enable humidity control, it is required to add an additional extension I/O module, iSMA-B-4U4A or -MIX18, to the AAC20 controller (depending on the demand and required sensors/signals).

The required sensors/signals are:

discharge air temperature sensor;
return air temperature sensor;
outside air temperature sensor;
twin-coil exchanger pressure switch;
discharge air humidity sensor (for humidification and dehumidification functions);
return air humidity sensor (for humidification and dehumidification functions);
anti-freeze thermostat;
humidifier failure status (if is required and installed, can be used in conjunction with a hygrostat for the MINI module);
hygrostat (if is required and installed);
fans operating status (pressure switch, signal from the inverter) – connected in series;
manual switch control (permission to operate the ventilation system).

Some of the useful sensors/signals, optional but not required:

preheat pump failure status;
reheat pump failure status (if a reheat pump is installed);
filters dirty status (for monitoring only) – connected in parallel;
manual reset (for MIX18 only, in case of manual fault reset).

In addition, the system can be equipped with extra sensors/signals, such as:

outside air humidity;
heater return water temperature;
exchanger medium temperature;
cooler limit air temperature;
discharge air pressure sensor (only when there is a need to maintain constant pressure on the supply duct);
return air pressure sensor (only when there is a need to maintain constant pressure on the exhausts duct).

The application allows for:

controlling the opening/closing of the 2-position inlet and outlet dampers;
the fans to start and controlling their speed (constant or pressure dependent)
controlling the modulating (analog 0-10 V DC) twin-coil valve actuator and the exchanger pump;
controlling the modulating (analog 0-10 V DC) preheater valve actuator and the preheater pump;
controlling the modulating (analog 0-10 V DC) reheater valve actuator (used for the dehumidification function) and, optionally, the reheater pump;
controlling the modulating (analog 0-10 V DC) cooler valve actuator;
the humidifier to start and controlling its humidification performance.

If the MIX18 module is used, it is also possible to signal alarm states for the fans (common alarm), heaters pump (common alarm), twin-coil pump and humidifier.

AHU configuration example:

heater mode – PreHeater & ReHeater [2]
energy recovery mode – Twin Coil & Return Pump or Twin Coil & Supply Pump [4 or 5] (depending on the need);
cooler mode – Cooler [1];
humidifier mode – Water Humidifier or Steam Humidifier [1 or 2] (depending on the need)
economizer mode – None, Eco Return Temp or Eco Enthalpy [0, 1 or 2] (depending on the need)
temperature control mode – Cascade Temp, Constant Disch Temp or Comfort Zone Temp [1, 2 or 3] (depending on the need)
humidity control mode – Comfort Hum, Return Hum, Disch Abs Hum or Constant Disch Abs Hum [1, 2, 3 or 4] (depending on the need)

Application Algorithm Description

Start-up

The first step after configuring the application for a particular type of the AHU is to make sure that the time schedule is correctly configured, which allows the AHU to run in a normal mode (Plant Mode as Occupied Period [4]) or in a night mode (with reduced efficiency - Plant Mode as Night Cycle [2]). If not, the time schedule must first be set correctly to allow a proper AHU operation.
The signal that allows the AHU to start is the signal connected to the I4 input of the AAC20 (Operating Switch). Activating this signal (physical short circuit at the input) and correctly setting the time schedule allows the AHU to start, while deactivating the signal automatically stops the AHU.

Preheater

The next step before starting the AHU is for the controller to verify that the heater has no active alarm conditions associated with the antifreeze protection procedure.
The antifreeze protection of the heater is used in the winter period (when the outside temperature is lower than the limit set by OAT_FrostProtection (by default, 6°C) with a central hysteresis of 1K).
The basic element is the antifreeze thermostat whose physical setting should be in accordance with or slightly lower than the above limit (e.g., by 1°C). The antifreeze thermostat is connected to the I1 input on the AAC20 controller (Freeze Thermostat Status).
Activation of the antifreeze thermostat (physical short circuit at the input) triggers the antifreeze protection procedure consisting of stopping the AHU (turning off the fans and closing the dampers), opening the heater valve at output A1 of the AAC20 controller (Heating Valve) to the level specified in HTV_FrostProt_ValveUp (default 100%), and switching on the heater pump at output O3 (Heating Pump Start Command) if configured for use by HPU_Enable (active by default - true).
An additional element of antifreeze protection is the use of a return water temperature sensor connected to the U4 input in the AAC20 (Heating Coil Return Water Temperature). If the antifreeze thermostat is triggered, the setpoint for the return water temperature is raised additionally by the level specified in FEZ_RWT_SptUp (by default, 15K).
When the temperature of the heater rises and the antifreeze thermostat returns to the normal state, In such case, if there is a sensor for the water temperature at the return from the heater, its setpoint will begin to fall at the rate of FEZ_SetupRamp (default 1K/min) to the normal setpoint (calculated on the basis of the outside temperature); if there is no sensor, the valve closes at the rate of HTV_Ramp (by default, 10%/min) to the normal opening state resulting from the current system control.
The heater return water temperature sensor also serves as a high limit for the water temperature in the heater by closing the valve if the temperature specified in RWT_MaxSetpoint is too high (85°C by default).
If the pump is activated in HPU_Enable (active by default - true) and connected to the O3 output in the AAC20 (Heating Pump Start Command), it is switched on with the delay HPU_OnDelay (by default, 5 s) when the heater valve connected to the A1 output in the AAC20 (Heating Valve) opens to 3% or more, and switched off with the delay specified in HPU_OffDelay (by default, 300 s) when the valve is opened 1% or less.
In winter, the pump is permanently activated if the outside temperature falls below the OAT_FrostProtection limit (by default, 6°C) with a central hysteresis of 1K and if the antifreeze thermostat is activated.
When using the pump failure signal, which is connected to the I2 input on the AAC20 (Heating Pump Fault), and when the value in HPU_StatusAlarmDelay (by default, 1 s) is set to default, then in the winter period the AHU can be turned off if:
1. DME_EmergencyStop (deactivated by default - false) is active (set to true),
2. an alarm is generated on the O2 output of the MIX18 module (Heat Pumps Failure Alarm), and
3. the pump control output is turned off.
  The alarm is automatically reset if the signal at the input is deactivated and the pump and AHU system resume operation (if stopped).
An additional feature to ensure a proper and long-lasting operation of the devices operating the heater is exercising of the pump and valve. First, the pump is exercised, then the valve, which, by default, takes place every Sunday at 1am in case they are not in use. The exercise consists of turning on the pump for the time specified in HPU_Pmp_Vlv_Exercise (120 s by default) and then opening the valve to 100% for the same time as above.

Dampers

In winter, when the outside temperature is lower than the limit specified by OAT_FrostProtection (by default, 6°C) with a central hysteresis of 1K and the return water temperature sensor is used, then, it must reach the setpoint (calculated from the outside temperature) to allow the dampers to open after the time specified in DME_OnDelay (by default, 300 s).
If the return water temperature sensor from the heater is not used, the dampers are opened only after the time specified in DME_OnDelay (by default, 300 s), during which the valve is opened according to the HTV_FrostProt_ValveUp parameter (by default, 100%) and drops at the rate specified by HTV_Ramp (by default, 10%/min) to the normal opening state resulting from the current system control.
When the outside temperature is above the limit specified in OAT_FrostProtection (by default, 6°C) with a central hysteresis of 1K (summer period), the dampers are opened without the delay specified in DME_OnDelay (by default, 300 s) and the heater return water temperature is not controlled at that time.
The dampers connected to the O1 output in the AAC20 (Dampers Start Command) open after the time specified in DME_OnDelay (by default, 300 s) only after the permission from the normally operating heater, and then the signal from the dampers are transmitted as a start permission for the fans with a delay of MAD_RunTime (by default, 150 s)).

Fans

If pressure sensors is connected at the U5 and U6 input in the AAC20 (Discharge Air Pressure, Return Air Pressure), each fan in is controlled smoothly connected at the A3 or A4 output in the AAC20 (Discharge Fan Speed Control, Return Fan Speed Control) in the range from FSC_MinSignal (by default, 30%) to FCS_MaxSignal (by default, 100%) so as to maintain the set pressures, or is controlled from the control signal resulting from humidity or manually with a set speed in the above ranges in the absence of connected pressure sensors (the higher control value is selected).
Each fan has its preset DischPressSpt pressure (500 Pa by default) and ReturnPressSpt pressure (500 Pa by default), which it is supposed to maintain by adjusting its speed accordingly.
Fans connected to the O2 output in the AAC20 (Fans Start Command) start with the delay specified in FSC_OffOnDelay (by default, 5 s) after receiving a permission from the dampers.
After receiving the startup permission, the fan speed control signal slowly ramps up at the rate of FSC_Ramp (by default, 1%/min) to the desired value resulting from the corresponding control as indicated above. The rate of rise or fall of the control signal is determined by the same speed.
The pressure setpoint behaves similarly, which during the AHU startup ramps up from zero to the desired setpoint at the speed of DSP_PressSptRamp (by default, 50 Pa/min). If it is changed during a normal operation, it will correspondingly start falling or rising at the same speed specified in DSP_PresSptRamp (by default, 50 Pa/min).
For a night mode, the set pressure is reduced by multiplying it by the DSP_NightReduce factor (by default, 50%).
To protect the fans operation, a fans confirmation signal is connected to the I3 input in the AAC20 (Fans Status/Pressure Switch), which if not activated by the time FDP_BeltAlarmDelay (default 120s), then, an alarm is generated on the O1 output of MIX18 (Fans Failure Alarm) and this causes the AHU to stop. This function can be disabled by setting the zero value in FDP_BeltAlarmDelay (by default, 120 s).
In the automatic reset mode specified in FSC_ManualReset (by default, false), after an alarm from the fan, the alarm will be reset and the AHU will resume operation, while after three unsuccessful attempts to start, the system stops and in the automatic mode it can be reset by removing and reactivating the I4 input in AAC20 (Operating Switch), while in the manual reset mode specified in FSC_ManualReset (deactivated - false by default) set to true, after an alarm from the fan the AHU will be stopped and the controller will wait for a reset signal activated by the input connected to the I5 input in MIX18 (Reset) and only then will it attempt to restart the AHU.

Control

In winter, during the AHU startup, the temperature control system temporarily raises the setpoint for the discharge temperature by an additional value specified in DAT_SptRamp (by default, 1K) which drops in time specified in DAT_SptDecTime (by default, 600 s).
The application allows to configure the TempCtrlMode temperature control function in 3 modes described below.
The cascade control function consists of regulating the discharge temperature connected to the U1 input of the AAC20 (Discharge Air Temperature), whose setpoint is determined by the control loop of the return temperature connected to the U2 input of the AAC20 (Return Air Temperature) and the setpoint of the return temperature TempSpt (by default, 22°C) in the heating or cooling function (with the exchanger as the first stage in both cases or without), as well as the fan speed allowing the fans to run above the value derived from RT_DATSpt_shift (by default, 60%), as long as pressure control is not used and the manual setting is lower than that derived from the temperature control function.
The fixed-value temperature control function consists of adjusting the discharge air temperature connected at the U1 input of the AAC20 controller (Discharge Air Temperature) based on the set TempSpt temperature (by default, 22°C), and based on the control loop the control level for the heater or cooler and exchanger is calculated.
The cascade control function with comfort zone consists of adjusting the discharge air temperature connected at the U1 input of the AAC20 (Discharge Air Temperature) to achieve a set discharge air temperature separate for the heating and cooling functions, taking into account the RT_ComfortZone comfort zone between them (the optimal zone in which heating and cooling are not adjusted – by default, 2°C). The setpoint of supply discharge is the output from the return temperature control system connected to the U2 input of the AAC20 controller (Return Air Temperature) to achieve the set temperature defined by the TempSpt variable (default 22°C). In addition, the discharge air temperature control system can affect the speed of the fans by allowing them to run above the value derived from RT_DATSpt_shift (by default, 60%), unless pressure control is used or the manual speed setting is lower than that derived from the discharge air temperature control function.
For the cascade function with comfort zone, the heat or cooling recovery system works in the comfort zone by controlling the exchanger accordingly. When using the economizer function, the control signal can be reversed.
The setpoint of discharge temperature is calculated from DAT_LowSetpoint (by default, 14°C) to DAT_HighSetpoint (by default, 26°C).
If the discharge air temperature falls below the low limit, the cooler is gradually closed, the heat exchanger begins to work on heat recovery, and the heater begins to open.
If the discharge air temperature rises above the high limit, the heater is gradually closed, the heat exchanger begins to work on the recovery of cooling, and the cooler begins to open.
The TempSpt set temperature (by default, 22°C) can take into account the winter compensation specified by the WC_DischSptRaise parameter (by default, 4K) or the summer compensation specified by the SC_DischSptRaise parameter (by default, 4K).
In the case of a night temperature drop, the RT_ComfortZone comfort zone is expanded four times.
In the case of active humidification function, the fan speed can gradually increase with the humidifier control signal, as long as pressure control is not used and the manual setting is lower than the humidification control function.
In the case of active dehumidification function using an economizer in mode 2, the fan speed is gradually increased (with attention to the above), and the cooler is opened in turn.
When the absolute humidity outside is higher than that of the return, the sequence changes, causing the cooler to open first, and then increase the speed of the fan (with a note as above).
In summer, if the outside temperature rises above the OAT_HeaterLimit (by default, 20°C), heater control is blocked, and in winter, if the outside temperature falls below the OAT_CoolerLimit (by default, 14°C), cooler control is blocked.
When the antifreeze protection is triggered, all control signals are reset except for the signal for the heater which is set to the level specified in HTV_FrostProt_ValveUp (by default, 100%). When the risk of freezing ceases, all signals return to normal operation.
The heater valve is opened or closed by the valve actuator according to the control signal coming from the temperature control system according to the configured TempCtrlMode.

Reheater

When HeaterMode (mode 2) is selected as operation mode for both primary and secondary heaters, then when the humidification function is active, only the primary heater works, and when the dehumidification function is in operation, only the secondary heater is used for temperature control.
If no humidity control is used (or no active humidification or dehumidification function), both heaters work simultaneously in the temperature control function, but can be configured as needed for gradual operation (the primary heater is the first stage, and the secondary is the second).
If the secondary heater pump is activated in HPU_Enable (active by default - true) and connected to the A4 output of the module 4U4A-H or MIX18 (ReHeating Pump Start Command), it is switched on with the delay HPU_OnDelay (by default, 5 s) when the secondary heater valve connected to the A2 output in the module 4U4A-H or MIX18 (ReHeating Valve) opens to 3% or more, and is switched off with the delay specified in HPU_OffDelay (by default, 300 s) when the valve is opened 1% or less.
In winter, when the outdoor temperature drops below the OAT_FrostProtection value (by default, 6°C) at the start of the AHU system, the secondary heater valve opens according to the HTV_FrostProt_ValveUp parameter (by default, 100%) and drops at the rate specified by HTV_Ramp (by default, 10%/min) to the normal opening state resulting from the current system control.
In winter, the secondary heater pump is permanently activated if the outdoor temperature falls below the OAT_FrostProtection limit (by default, 6°C) with a central hysteresis of 1K.
When using the secondary heater pump failure signal which is connected to the U4 input of the module 4U4A-H or the I2 input of the module MIX18 (ReHeating Pump Fault) and when the value in HPU_StatusAlarmDelay (by default, 1 s) is set to default, an alarm is generated on the O2 output of the module MIX18 (Heat Pumps Failure Alarm) and the pump control output is disabled. The alarm is automatically reset if the input signal is deactivated and the pump resumes operation.
An additional feature to ensure a proper and long-lasting operation of the devices operating the secondary heater is exercising the pump and valve. First, the pump is exercised, then the valve, which, by default, takes place every Sunday at 1am in case they are not in use. The exercise consists of turning on the pump for the time specified in HPU_Pmp_Vlv_Exercise (120 s by default) and then opening the valve to 100% for the same time as above.

Cooler

When cooling is needed, the cooler valve connected to the A2 output (Cooling Valve) opens based on a stronger signal coming from the temperature control system or dehumidification function.
The control of the cooler valve is blocked below the OAT_CoolerLowLimit temperature (by default, 14°C) with a central hysteresis of 1K.
It is possible to block the control of the cooler valve in the humidification function with the CCV_CoolOnHumDisable parameter (active by default – true).
During the AHU startup and each time the valve is opened from zero, the control signal gradually increases at the rate specified in CCV_VlvRamp (by default, 5%/min).
To protect against too low air temperature behind the cooler, it is possible to use a temperature sensor connected to the U3 input on the MIX18 module (Cooling Limit Air Temperature) causing the cooler valve to gradually close when the temperature drops to the LIM_Setpoint value (by default, 14°C).
An additional feature to ensure a proper and long-lasting operation of the cooler is exercising the valve. It is carried out by default every Sunday at 1am in case it is not being used. Exercise consists of opening the valve to 100% for the time specified in CCV_Vlv_Exercise (120 s by default).

Exchanger

In order to increase energy efficiency, the AHU system is additionally equipped with an exchanger to recover thermal energy (heat or cold) from either return or outside air in order to use this energy to heat or cool the incoming outside air.
One of five types of exchanger can be selected in EnergyRecovMode. Type 4 indicates a twin-coil exchanger with a pump on a return, and type 5 means a twin-coil exchanger with a pump on a supply.
When the AHU system is started, the permission to switch on an exchanger pump connected to the O4 output in the AAC20 controller (TwinCoil Pump Start Command) starts with the delay specified in ERP_OnDelay (by default, 5 s).
The exchanger is controlled at the A5 output of the AAC20 (TwinCoil Exchanger Valve) controller primarily from the temperature control system, but its control is also affected by the economizer function, humidity control.
To protect the exchanger from frosting, a signal connected to the U7 input of the AAC20 (TwinCoil Exchanger Pressure Switch) is used to disable the exchanger’s pump control and the control signal is set to zero.
In addition, in the case of using a temperature sensor connected at the U4 input of the MIX18 module (Medium Air Temperature) located on the piping installation of the exchanger, the opening of the exchanger valve can be controlled in type 4 by decreasing its control when the outside temperature decreases, and in type 5 by increasing its control when the outside temperature decreases, which serves to protect the exchanger’s fluid against frosting.
If the exchanger control signal is equal to or greater than 3% the exchanger pump will be switched on.
If the exchanger control signal is equal to or less than 1% after the ERP_OffDelay (by default, 300 s), the exchanger pump will be turned off.
The exchanger pump can be switched on permanently during normal operation of the AHU system if the outside temperature falls below OAT_FrostProtection (by default, 6°C).
When using the exchanger pump failure signal which is connected to the I4 input on the MIX18 module (TwinCoil Pump Fault) and when the value in ERP_StatusAlarmDelay (by default, 1 s) is set to default, an alarm is generated on the O3 output of the MIX18 module (TwinCoil Pump Failure Alarm), the exchanger control is reset, and the exchanger pump is deactivated. The alarm is automatically reset if the input signal is deactivated and the exchanger resumes operation.
An additional feature to ensure a proper and long-lasting operation of the devices operating the exchanger is exercising of the pump and valve. First, the pump is exercised, then the valve, which, by default, takes place every Sunday at 1am in case they are not in use. The exercise consists of turning on the pump for the time specified in ERP_Pmp_Vlv_Exercise (120 s by default) and then opening the valve to 100% for the same time as above.

Economizer

In addition, an economizer function is used to support energy efficiency, the mode of which can be set in EconomizerMode.
Economizer in operation mode 1 compares only the outdoor and return temperatures to determine the optimal signal for energy recovery by the exchanger.
In mode 2, the economizer determines the energy recovery function based on measurements of outdoor and return temperatures and humidity, deciding in the summer using the calculated enthalpy and absolute humidity which air flow to use for cooling (return or fresh).
Calculations based on absolute humidity apply only when using the dehumidification function.
Both functions, enthalpy and absolute humidity, are triggered when the appropriate conditions are met taking into account the central hysteresis of EnthalpyDiff (by default, 5kJ/kg) and AbsHumDiff (by default, 1g/kg) respectively, with a delay specified by SwitchoverDel (by default, 300s).

Humidifier

The AHU is also equipped with a humidification and dehumidification function. The humidification function is started with a delay of HUM_FanOnDelay (by default, 300 s) after confirming the operation of the fans connected to the I3 input in the AAC20 (Fans Status/Pressure Switch).
In case of lack of confirmation that the fans connected to the I3 input of the AAC20 (Fans Status/Pressure Switch) is operating, the humidification function is turned off immediately.
The humidification function is active when the outdoor temperature is lower than the OAT_HighHum parameter (by default, 14°C), and the dehumidification function is active when the outdoor temperature is higher than the OAT_LowDeh parameter (by default, 15°C).
To implement humidity control, a discharge air sensor connected to the U1 input of the 4U4A-H or MIX18 (Discharge Air Humidity) module and an return air sensor connected to the U2 input of the 4U4A-H or MIX 18 (Return Air Humidity ) module are required, depending on the selected humidity control function.
The application allows to configure the HumCtrlMode humidity control function in 4 modes described below.
The function of return relative humidity control with comfort zone (optimal zone in which humidification and dehumidification control does not take place) is to regulate humidity so as to achieve HumSpt as the setpoint for return (by default, 55%) taking into account the comfort zone HUMDEH_ComfZone (by default, 4%) using humidification or dehumidification function. The comfort zone is quadrupled in the case of night temperature drop.
The function of controlling the return relative humidity without comfort zone is to adjust the humidity to achieve the HumSpt as the setpoint for the return (by default, 55%) using the humidification or dehumidification function.
The function of the discharge absolute humidity control is to adjust the humidity so as to achieve the discharge setpoint determined by the value from the return humidity sensor. The discharge humidity setpoint is calculated by the control loop, which is then used to regulate the discharge humidity using humidification or dehumidification functions. An absolute (not relative) humidity sensor connected on the discharge air is required to perform this function.
The function of the fixed-value absolute humidity discharge control is to adjust the humidity so as to achieve the HumSpt as the discharge setpoint (by default, 55%) using the humidification or dehumidification function. An absolute (not relative) humidity sensor connected on the discharge air is required to perform this function.
The humidity control function has two independent control loops, one for humidification control directly controlling the humidifier connected to the A1 output of the AAC20 controller (Humidifier Control Signal), and the other for dehumidification control affecting the control of the cooler connected to the A2 output of the AAC20 controller (Cooling Valve). The maintenance of the previously reduced temperature to the set level according to the temperature control is done by controlling the secondary heater connected to the A2 output of the 4U4A-H or MIX18 module (ReHeating Valve).
Both humidification and dehumidification control functions ramp up gradually at the speed of HUM_RampUp (by default, 10%/min) and DEH_RampUp (by default, 10%/min), respectively.
The HumSpt set humidity (by default, 55%) can take into account either winter compensation determined by a linear function in relation to the outside temperature from RAH_MinHumSpt (by default, 30%Rh) to HumSpt (by default, 55%) or summer compensation determined by a linear function in relation to the outside temperature from HumSpt (by default, 55%) to RAH_MaxHumSpt (by default, 60%Rh).
Depending on the type of humidifier selected in HumidifierMode, the A3 output control in the 4U4A-H or MIX18 module (Humidifier Start Command) is activated as required: as a water humidifier pump (mode 1) or as switched on permanently (if there is no failure) during AHU operation as a steam humidifier enabler (mode 2).
In case of a humidifier failure activated at the U3 input of the 4U4A-H module or at the I1 input of the MIX18 module (Humidifier Fault), the humidifier control is turned off, the water humidifier pump/steam humidifier is shut down, and an alarm is generated at the O4 output of the MIX18 module (Humidifier Failure Alarm).
In addition to the hygrostat connected to the I3 input of the MIX 18 module (Hygrostat), which, when activated, turns off the humidification function (turns off the humidifier and pump / permit control), there is also a function to protect the occurrence of too much discharge humidity (to prevent the ducts from getting wet). If the discharge air humidity begins to increase towards the DAH_MaxSpt value (by default, 90%), then the humidifier control gradually decreases to zero.
Deactivation of the humidistat or humidifier failure automatically resets the alarm and returns to normal control mode.