Engineering Bulletin Recirculation Energy Recovery Unit Controls
Introduction The following represents just one of many configurations that are possible with the Trane Climate Changer air handler with an energy wheel. Not all features shown will be used on every job, and some additional options or changes may be required, depending on the system requirements and the unit type. The purpose of this bulletin is to provide a sample control sequence for a Recirculation Energy Recovery Unit, which then can be modified to meet each particular job requirement. The UPCM code for the following unit has also been written and is available from the factory for field modifications and installation. American Standard Inc. 2000
Contents Introduction Features Architecture Sequence of Operation Component Operation Summary 2 4 5 7 9 12 3
Features The Recirculation Energy Recovery Unit is a VAV air handler that combines an energy recovery wheel and two bypass dampers to optimally condition the incoming outdoor air. By recovering energy that would otherwise have been wasted, the unit is able to maintain a constant discharge air temperature while meeting a minimum outside airflow requirement and reducing the energy demands placed on the heating and cooling coils. Outside airflow is monitored as it enters the unit through a Traq damper, filtered, and then passed over an array of electric preheat recovery elements. These elements help prevent frosting of the downstream energy wheel during extreme winter conditions. Outside air then enters the energy wheel module, which is the unique feature of this air handling unit. The integrated control sequence modulates two bypass dampers located above and below an energy recovery wheel to regulate both the outside and exhaust airflow passing through the wheel. This enables the same section to handle up to 100% outside air during economizing without exceeding the wheel airflow limits and significantly decreasing the wheel pressure drop. The control sequence also enables the unit to vary the amount of energy recovered from the airstream and in turn reduce, if not eliminate, the load on the preheat and cooling coils downstream. During the Heating Recovery mode, the wheel effectiveness will be controlled by the wheel exhaust bypass damper such that when full heat recovery is not needed, it will transfer just enough heat from the exhaust airflow to the outside airflow to maintain the discharge air temperature. In the Cooling Recovery mode, heat and moisture are transferred from the outside airflow to the exhaust airflow. During periods of operation when the unit is not in either recovery mode, both dampers are driven 100% open to allow both airflows to bypass the wheel, minimizing any pressure drop. After the energy wheel module, the outside airflow is then mixed with return air and passes over the preheat and cooling coils. When the desired discharge air temperature can not be maintained by economizing or use of the energy recovery wheel alone, the coils are activated to treat the air before being discharged into the space. Return air passing through the unit is first filtered, then directed by a return air damper to be recirculated and mixed with the outside airflow or exhausted from the building. The exhaust air flows through the energy recovery module (previously described) before exiting the unit. 4
Architecture Recirculation Energy Recovery Unit EARB EA EF EFVD FIL RA WHL RAD OA OAD OAF PHR MA PH CC SF SVFD SA OARB A universal programmable controller (UPCM) processes all the safeties, diagnostics and control functions of the Recirculation Energy Recovery Unit. The existing control routines are written in UPCM programmable control language (PCL). In addition to the UPCM, a Tracer Summit building control unit (BCU) may also be required to provide occupancy schedules, setpoints, on-line editing and alarm reporting. Note: For the BCU to communicate with the UPCM, the BCU must include a Comm 4 communications card. Component and Actuator Identification Airstream EA = Exhaust Air OA = Outside Air RA = Return Air SA = Supply Air MA = Mixed Air HVAC Controls and Actuators CC - Cooling Coil (Valves) EARB - Exhaust Air Recovery Damper (Actuator) EF - Exhaust Fan (Off, On) EVFD - Exhaust Fan VFD FIL - Return Air Filter OAD - Outside Air Damper (Traq Damper) (Actuator) OAF - Outside Air Filter OARB - Outside Air Recovery Damper (Actuator) PH - Preheat Coil (Valves) PHR - Preheat Recovery (Switches or SCR) RAD - Return Air Damper (Actuator) SF - Supply Fan (Off, On) SVFD - Supply Fan VFD WHL - Energy Recovery Wheel (Off, On) 5
Architecture Recirculation Energy Recovery Unit EARB EA EXATEMP EXAFLOW EXAFANST FIL RAT RA ROTFAIL OA CFM WHL RAD OA OAT PHRTEMP MAT HISTATIC STATPRESS SAT SA OAD FIL PHR PH CC SUPFANST OARB DIFFPRES ROOMSTAT, SPACETMP Sensor Identification Airstream EA = Exhaust Air OA = Outside Air RA = Return Air SA = Supply Air MA = Mixed Air Sensors OA CFM Outside Airflow (CFM) SUPFANST Supply Fan Status OAT Outside Air Temperature HISTATIC High Duct Static Pressure MAT Mixed Air Temperature STATPRES Duct Static Pressure (Optional) SAT Supply Air Temperature EXATEMP Exhaust Air Temperature (Optional) RAT Return Air Temperature EXAFLOW Exhaust Air Flow (Optional) ROTFAIL Wheel Rotation Failure EXFANST Exhaust Fan Status DIFFPRES Differential Building Pressure SPACETMP Space Temperature FREZSTAT Freezestat (Low Limit) 6
Sequence of Operation Based on user defined time of day (TOD) schedules, the BCU will communicate an operating mode of Occupied, Unoccupied or Optimal Start to the UPCM. Morning Warm-up (Optimal Start) When Tracer commands the UPCM to the Optimal Morning Warm-Up mode, Tracer Summit will also send a morning warm-up Discharge Air Temp Setpoint. During the unoccupied period of this mode: The supply fan will run to maintain duct static pressure. The outdoor air damper will remain closed. The return air damper will open to oppose the outdoor air damper position. The preheat coil (PHVALVE) will modulate to achieve and maintain Discharge Air Temperature (DISCHTMP) at the morning warm up setpoint. During the occupied period of this mode: The outdoor air damper will be controlled to maintain its minimum flow setpoint. The outdoor air damper and the return air damper are electronically linked for complementary operation, the Return Air Damper will open in inverse proportion to the outdoor air damper position. The energy recovery wheel will run. The outside air recovery bypass damper will be controlled closed by the UPCM. The exhaust air recovery bypass damper will be controlled closed by the UPCM. The preheat coil (PHVALVE) will modulate to achieve and maintain discharge air temperature (DISCHTMP) at the morning warm-up setpoint. Energy Recovery Modes The Recirculation Energy Recovery Unit has five occupied modes of operation: Cooling Recovery, Cooling No Recovery, Cooling Economizing, Heating Recovery, and Heating Recovery with Preheat Frost Prevention. Figure 1 shows how these modes are defined as a function of dry bulb temperature. Figure 1 Occupied Modes of Operation Dry Bulb Temperature A Cooling Recovery Mode B Cooling No Recovery Mode C Economizing Mode D Heating Recovery Mode E Heating Recovery Mode w/preheat Frost Prevention E * D OAT<ESP A Cooling Recovery Mode Outside air temperature is above the economizer enable setpoint and the return air temperature. B Cooling No Recovery Mode Outside air temperature is above the economizer enable setpoint but below the return air temperature. C Cooling Economizer Mode Outside air temperature is below the economizer enable setpoint and below the return air temperature. *Alternative Method of Control for Cooling Modes Enthalpy-based control can be used for enabling Modes A, B and C. This would require an additional outside air and return air humidity sensor; however, it may result in a more energy-efficient system. D Heating Recovery Mode Outside air temperature is below the economizer enable setpoint, the cooling valve is closed, and the OA flow is at the minimum flow setting. This mode divided into three stages of heat: 1. First-stage heat: The energy recovery wheel will be turned on and both the outdoor air recovery bypass and C * B OAT>ESP OAT<RAT A OAT>RAT exhaust air recovery bypass dampers are 100% open. 2. Second-stage heat: The energy recovery wheel continues to run, the two-position outdoor air recovery bypass damper is closed, and the exhaust air recovery bypass damper modulates to maintain discharge air Temperature (DISCHTMP). 3. Third-stage heat: The energy recovery wheel continues to run, the outdoor air recovery bypass damper remains closed, the exhaust air recovery bypass damper has modulated fully closed, and the preheat valve now modulates to maintain discharge air temperature (DISCHTMP). E - Heating Recovery Mode with Preheat Outside air temperature is below the economizer enable setpoint, the cooling valve is closed, the OA flow is at the minimum flow setting, and the outside air temperature is below the preheat setpoint. This mode is divided into the same three stages of heat as the Heating Recovery mode. The difference between the two stages is that the electric preheat is energized. 7
Sequence of Operation Sequence Summary of Occupied Modes Recirculation Recovery Unit Preheat Frost Control Exhaust Air Ourside Air OAD Outside Air Damper EARB Exhaust Air Recovery Bypass Damper WHL Total Energy Wheel, for Partial OA RAD Return Air Damper RAT Temperature Sensor Return Air SAT Discharge Temperature Sensor EA Secondary Path 1E. Economizer box with Angle Filter 2E. Exhaust Fan Module OA Primary Path 1. TRAQ Damper Module with Flat Filter 2. Electric Preheat 3. Energy Wheel Module 4. Mixing Box 5. Medium-Large Cooling Coil Module 6. Heating Coil Module 7. Supply Fan Module Supply Air OAT Temperature Sensor (or Remote Mounted Sensor) OARB Outside Air Recovery Bypass Damper Summarized Sequence of Operation for Wheel (A) Cooling Recovery Mode IF OAT > ENABLSP AND OAT>RAT ENERGY WHEEL IS ON OAD@MIN, RAD@MAX OARB & EARB ARE CLOSED COOLING COIL IS ON (B) Cooling No Recovery Mode IF OAT > ENABLSP AND OAT<RAT ENERGY WHEEL IS OFF OARB AND EARB ARE OPEN COOLING COIL IS ON (C) Cooling Economizing Mode IF OAT<ENABLSP AND OAT<RAT ENERGY WHEEL IS OFF OAD & RAD MODULATE TO MAINTAIN SAT OARB & EARB ARE OPEN COOLING & HEATING COILS ARE OFF GIVEN SET POINTS ENABLSP = Economizing SetPoint Temperature, System Determined PHRTMPSP = Outside Air Frost Control Setpoint, System Dependent* DISCHSP= Discharge Set Point Temperature,System Determined *From TraneEW Selection Software MEASURED DRY BULB TEMPERATURES OAT = Outside Air Temperature RAT = Return Air Temperature SAT = Discharge Temperature (D) Heating Recovery Part Load Economizing "Free Heat" (Heating Stage1) IF OAT<ENABLSP AND OAT<RAT AND SAT<DISCHSP ENERGY WHEEL IS ON OAD@MIN EARB & OARB ARE OPEN HEATING COIL IS OFF Part Load Economizing "Free Heat" (Heating Stage2) IF OAT<ENABLSP AND OAT<RAT AND SAT<DISCHSP ENERGY WHEEL IS ON OADMPR@MIN EARB MODULATES TO MAINTAIN SAT, OARB IS CLOSED HEATING COIL IS OFF Heating Recovery Mode (Heating Stage 3) IF OAT<ENABLSP AND OAT<RAT ENERGY WHEEL IS ON OAD@MIN, RAD@MAX OARB & EARB ARE CLOSED HEATING COIL IS ON (E) Heating Recovery, Frost Protection Mode Preheat IF OAT<OAFPSP PREHEAT COIL IS ACTIVATED TO MAINTAIN OATEMP=OAFPSP HEATING STAGES 1,2,3 SAME AS ABOVE 8
Component Operation Supply Fan Control (SUPFANSS) The supply fan will operate continuously whenever Tracer commands the mode of the UPCM to be either occupied or optimal start. The supply fan will be off whenever: Tracer Summit commands the mode of the UPCM to be unoccupied. The exhaust fan has failed. The freezestat has tripped. A high duct static condition exists. The discharge air temperature sensor fails. The supply fan status indicates a failure. Supply Fan Speed (SFANVFD) The speed of the supply fan will be modulated by the UPCM to maintain duct static pressure (STATPRES) if the supply fan status is ON. If the duct static pressure (STATPRES) sensor fails, the UPCM will alarm and the fan will be shut down. Two Stages of Electric Preheat (PREHEAT1, PREHEAT2) The electric heat will be staged on by the UPCM to help prevent freezing of the energy recovery wheel and/or the cooling coil whenever the preheat recovery temperature (PHRTEMP) falls below the preheat coil setpoint. Outside Air Damper (OADAMPER) When the UPCM is unoccupied, or in the unoccupied portion of morning warmup, the outside air damper (OADAMPER) will be closed. During the occupied portion of morning warm-up, the outside air damper (OADAMPER) will modulate to maintain its minimum flow setpoint. The economizer function is enabled when the outdoor air temperature is less than the economizer enable setpoint (ECONENAB) and less than return air temperature. When the economizer function is enabled, the outside air damper will modulate between its minimum flow setpoint and its maximum flow setpoint to maintain the discharge air temperature (DISCHTMP). If the outside air temperature is below the economizer enable setpoint but the unit is in the heating recovery mode, the outside air damper will be controlled to maintain its minimum flow setpoint. If the outdoor air temperature (OATEMP) is above the economizer enable setpoint (ECONENAB), the economizer function is disabled and the outside air damper (OADAMPER) is controlled to maintain its minimum flow setpoint. The outside air damper will also be controlled to maintain its minimum flow setpoint if the outside air temperature sensor fails. If the outside airflow sensor (OAFLOW) fails, the UPCM will alarm and the outside air damper (OADAMPER) will close to 15% open. Return Air Damper (RADAMPER) The position of the return air damper (RADAMPER) will be equal to 100 minus the position of the outside air damper. Chilled Water Valve (CWVALVE) The first stage of cooling will be economizing only. The chilled water valve (CWVALVE) will be enabled and modulated by the UPCM if economizing alone does not provide enough cooling to maintain the discharge air temperature (DISCHTMP) at the discharge setpoint (DISCHSP). When the outside air temperature (OATEMP) is above the economizer enable setpoint (ECONENAB), the chilled water valve will continue to modulate in order to maintain discharge air temperature (DISCHTMP). If the outside air temperature sensor (OATEMP) fails, the chilled water valve (CWVALVE) will close. Energy Recovery Wheel (EWHEEL) The energy recovery wheel (EWHEEL) will be controlled on when the unit is in either the Cooling or Heating Recovery mode. In the event of a wheel failure (ROTFAIL), the UPCM will alarm, the wheel will be turned off and both the outside air recovery bypass and exhaust air recovery bypass dampers will be controlled 100% open by the UPCM. If the outside air temperature sensor fails, the wheel will run, the UPCM will alarm, and both the outside air recovery bypass and exhaust air recovery bypass dampers will be controlled closed by the UPCM. During the unoccupied portion of morning warm-up, the energy recovery wheel (EWHEEL) will be off. During the occupied portion of morning warm-up, the energy recovery wheel (EWHEEL) will be on. Outside Air Recovery Bypass Damper (OARBYPAS) The outside air recovery bypass damper will be controlled to the following positions by the UPCM: In the Cooling Recovery mode, the damper will be closed. In the Cooling No Recovery and Cooling Economizer modes, the damper position is 100% open. In the first stage of the Heating Recovery mode, the damper will remain 100% open. In the second and third stages of the Heating Recovery mode, the damper will be closed. During the occupied portion of morning warm-up, the outside air recovery bypass damper (OARBYPAS) will be closed. 9
Component Operation Exhaust Air Recovery Bypass Damper (OARBYPAS) The exhaust air recovery bypass damper will be controlled to the following positions by the UPCM: In the Cooling Recovery mode, the damper will be closed. In the Cooling No Recovery and Cooling Economizer modes, the damper position is 100% open. In the first stage of the Heating Recovery mode, the damper will be 100% open. In the second stage of the Heating Recovery mode, the exhaust air recovery bypass damper will modulate to maintain discharge air temperature (DISCHTMP) at the discharge setpoint. In the third stage of the Heating Recovery mode, the exhaust air recovery bypass damper will be fully closed. During the occupied portion of morning warm-up, the exhaust air recovery bypass damper (OARBYPAS) will be closed. Exhaust Fan Control (EXFANSS) The exhaust fan will operate continuously whenever Tracer commands the UPCM to either the occupied mode or the optimal start mode. The exhaust fan will be off under any of the following conditions: The UPCM is in the Unoccupied mode. The supply fan has failed. The freezestat has tripped. A high duct static condition exists. The discharge air temperature sensor has failed. The building differential pressure sensor fails. The exhaust fan status indicates a failure. Exhaust Fan Speed (EXFANVFD) The speed of the exhaust fan will be modulated by the UPCM to maintain the differential building pressure (DIFFPRES). Safeties The UPCM will shut down the unit when: The UPCM is in the Unoccupied mode. The supply fan has failed. The discharge air temperature sensor has failed. The exhaust fan has failed. The freezestat (FREZSTAT) is tripped. The high duct static (HISTATIC) binary input point (BIP) is on. Alarms The UPCM will generate an alarm if: The supply fan has failed. The discharge air temperature sensor has failed. The duct static pressure sensor has failed. The outside air temperature sensor has failed. The mixed air temperature sensor has failed. The outside airflow sensor has failed. The exhaust fan has failed. The exhaust airflow sensor has failed (EXAFLOW). The freezestat (FREZSTAT) has tripped. The high duct static (HISTATIC) bip is on. The energy wheel is not rotating when controlled on. Points List The following list of points describe the binary outputs, binary inputs, analog outputs, and analog inputs used in the UPCM: Binary Outputs SUPFANSS - Supply fan start/stop EWHEEL - Energy recovery wheel PREHEAT1 - First stage of preheat PREHEAT2 - Second stage of preheat EXFANSS - Exhaust fan start/stop Binary Inputs SUPFANST - Supply fan status HISTATIC - High duct static pressure FREZSTAT - Freezestat (low limit) ROTFAIL - Wheel rotation failure EXFANST - Exhaust fan status Analog Outputs SFANVFD - Supply fan Speed PHVALVE - Preheat valve CWVALVE - Chilled water supply valve OARBYPAS - Outside air recovery bypass damper EARBYPAS - Exhaust air recovery bypass damper OADAMPER - Outside air damper RADAMPER - Return air damper EXFANVFD - Exhaust fan speed Analog Inputs STATPRES - Duct static pressure DISCHTMP - Discharge air temperature MIXEDTMP - Mixed air temperature PHRTEMP - Preheat recovery temperature OAFLOW - Outside airflow RATEMP - Return air temperature EXATEMP - Exhaust air temperature EXAFLOW DIFFPRES - Exhaust airflow - Differential building pressure 10
Component Operation Required Entries In addition to the hard-wired analog and binary inputs, the following setpoints must be set up in Tracer Summit and communicated to the UPCM. ENABLSP The temperature above which economizing is enabled. OATEMP The outside air temperature. SPACETEMP The space temperature. OAMINFLW Minimum outside air flow requirement in CFM. STATICSP The duct static setpoint. This must be a value between 0.0 and 5.0 inches of water column. DELTAPSP The differential building pressure. This must be a value between 1.0 and 1.0 inches of water column. (Typically -0.05 to +0.05). MWUSP The temperature to be used as the morning warm-up setpoint. DISCHSP The discharge air temperature setpoint. MIXEDSP Mixed air temperature setpoint. FLFAILSP This setpoint defines an outdoor air damper position to be used in the event of a flow sensor failure. PHRTMPSP The outside air temperature below which the electric preheat elements will be turned on. 11
Summary Conclusion The use of this sample sequence and control strategy should help ensure proper control of a recirculation energy recovery unit and help optimize the amount of energy recovered by the energy wheel. For more information on energy recovery units with energy wheels see the engineering bulletins CLCH-PRB007-EN and CLCH-PRB008-EN. The Trane Company An American Standard Company www.trane.com Literature Order Number File Number Supersedes Stocking Location PL-AH-CLCH-000-PRB009-EN-0800 New La Crosse For more information contact your local district office or e-mail us at comfort@trane.com Since The Trane Company has a policy of continuous product improvement, it reserves the right to change design and specifications without notice.