Control Logic for Dedicated Outdoor Air, Dual-Wheel Air Handling Unit Project Orientation A hypothetical dedicated outdoor air (DOA) commercial air handling unit (AHU) was considered for a control logic map. The AHU is to be configured for use during summer and winter conditions. The control logic scheme of this particular system can manipulate various components in the AHU in order to achieve the desired temperature and humidity conditions within the space (e.g. a class room, restaurant, etc.). Utilization of a psychrometric chart, such as in Figure 2, was necessary to aid in the mapping of the control logic scheme, and will be demonstrated for one case. It should be noted that the AHU is a constant-air volume system, meaning that the flow rate of air through the unit does not vary over time in order to provide a suitable climate within the space; instead, the system adjusts the temperature and humidity of the supply air. Climates within buildings and outdoors vary continuously; annual and diurnal cycles in some areas of the world may swing from one extreme to the other, and building loads may be introduced when a group of people enter a room, and then subside when the people leave. Thus, it is necessary that the AHU be capable of quickly responding to the conditions by adjusting the operations of certain components, such as heat exchangers and humidifiers. Sensors are fixed at three locations within the system to monitor climate conditions: (1) the outdoor air intake to the AHU, (2) the inlet to the cooling coil, and (3) the return air duct. Each of these three locations has two sensors; one to measure dry-bulb temperature, and one to measure relative humidity, as shown in Figure 1. 1
Transcript
Control Logic for Dedicated Outdoor Air, Dual-Wheel Air Handling Unit
Project Orientation
A hypothetical dedicated outdoor air (DOA) commercial air handling unit (AHU) was considered for a control logic map. The AHU is to be configured for use during summer and winter conditions. The control logic scheme of this particular system can manipulate various components in the AHU in order to achieve the desired temperature and humidity conditions within the space (e.g. a class room, restaurant, etc.). Utilization of a psychrometric chart, such as in Figure 2, was necessary to aid in the mapping of the control logic scheme, and will be demonstrated for one case. It should be noted that the AHU is a constant-air volume system, meaning that the flow rate of air through the unit does not vary over time in order to provide a suitable climate within the space; instead, the system adjusts the temperature and humidity of the supply air.
Climates within buildings and outdoors vary continuously; annual and diurnal cycles in some areas of the world may swing from one extreme to the other, and building loads may be introduced when a group of people enter a room, and then subside when the people leave. Thus, it is necessary that the AHU be capable of quickly responding to the conditions by adjusting the operations of certain components, such as heat exchangers and humidifiers. Sensors are fixed at three locations within the system to monitor climate conditions: (1) the outdoor air intake to the AHU, (2) the inlet to the cooling coil, and (3) the return air duct. Each of these three locations has two sensors; one to measure dry-bulb temperature, and one to measure relative humidity, as shown in Figure 1.
Project Outcome
The control logic presented herein is based on an established set point (SP), and monitors of outdoor air (OA), return air leaving the space (RA), and the air leaving the heat wheel before it enters the cooling coil (3). A controller for an AHU configured such as the one in consideration can be programmed to apply the logic scheme developed in this study.
System
A schematic of the AHU is provided in Figure 1. The procession of the air follows as such: OA enters the AHU, and passes through the desiccant wheel (1-2), followed by the heat wheel (2-3). After leaving the heat wheel, air passes through a cooling coil (3-4), a heating coil (4-5), and finally through a steam humidifier (5-6) before it is supplied to the space. The air in the space gains or loses heat and moisture, depending on the load, and after a time it is returned to the AHU, where it passes back through the heat wheel (7-8), after which the air is heated in a
1
heating coil (8-9) before entering the desiccant wheel and finally being exhausted outdoors (9-10).
Figure 1 – Configuration of dedicated outdoor air AHU & monitor points
Table 1 – Notation and description of controllable outputs and monitor points in AHU
Controllable Outputs
DW HW HC1 HC2 CC SHROTATION OF
DESICCANT WHEEL
ROTATION OF HEAT WHEEL
FLOW RATE THRU HEATING COIL 1
FLOW RATE THRU
HEATING COIL 2
FLOW RATE THRU
COOLING COIL
FLOW RATE THRU STEAM HUMIDIFIER
Monitor Points
TRA DPTRA TOA DPTOA TSP DPTSP T3 DPT3TEMP OF
ROOM AIRDEW
POINT TEMP
OF ROOM
AIR
TEMP OF OUTDOOR
AIR
DEW POINT TEMP OF
OUTDOOR AIR
TEMP OF
ROOM SET
POINT
DEW POINT
TEMP OF ROOM SET
POINT
TEMP OF AIR AT
POINT 3
DEW POINT TEMP AT POINT 3
Assumptions/simplifications
The system is constantly in operation. The source of heat energy is cheap, such as the waste heat from the Rankine Cycle at a local power plant. The primary method for dehumidification is the desiccant wheel.
Methodology
2
T3DPT3
TRADPTRA
TOADPTOA
2 3 4 5 6
78910
1
EA
OA
RA
SA
SH
CC
HWHC2
HC1DW
A space set point (i.e. the desired dry bulb temperature and relative humidity of space air) was chosen to be 23°C and 50% RH. Four OA cases were considered around this set point; two summer conditions, and two winter conditions. On the psychrometric chart in Figure 2, region A is the winter condition for which the OA dew point temperature (DPT) is less than the DPT of the set point. Similarly, region D is the winter condition for which the OA DPT is greater than that of the set point; region B is the summer condition for which the OA DPT is less than that of the set point; and region C is the summer condition for which the OA DPT is greater than that of the set point. These four regions also apply to the return air. In order to illustrate the general methodology, a walkthrough for one case is provided below.
Figure 2 – The four outdoor air (and return air) cases around the space set point
Case C occurs during summer conditions when the outdoor air is more humid than the space set point (i.e. the dew-point temperature is greater outdoors). As such, the outdoor air needs to be cooled and dehumidified; ultimately, the supply air should be in region A on the psychometric chart in Figure 2. The return air condition must also be considered because it determines whether the room or space requires heating or cooling and humidification or dehumidification. If the return air has a lower dry-bulb temperature and a lower DPT than the set point (region A), then no adjustments should be made; the current operation is sufficient. A diagram of this process is shown in Figure 3.
3
SP
B
CD
A
Figure 3 – Process for Case C-A (OA-RA)
If the return air has a greater dry-bulb temperature and a lower DPT than the set point (region B), then the rotation speeds of the desiccant and heat wheels should be increased; the flow rate of refrigerant through the cooling coil should be increased; and the flow rates of water through both heating coils and through the stream humidifier should be increased. A diagram of this process is shown in Figure 4.
Figure 4 – Process for Case C-B (OA-RA)
4
If the return air has a greater dry-bulb temperature and DPT than the set point (region C), then the rotation speeds of the desiccant and heat wheels, and the flow rate of water through heating coil 1 should be increased; and the flow rate of water through heating coil 2, the cooling coil, and the steam humidifier should be increased. However, three loops must be inserted in this case to ensure that the supply air is cool and dry enough. Here, the temperature and humidity of air at point 3 become important. If the air at 3 has a lower dry-bulb temperature and DPT than the set point (region A), then no additional adjustments are required. If the air at 3 has a higher dry-bulb temperature, and a lower DPT than the set point (region B), then the flow rate of refrigerant through the cooling coil should be increased. Finally, if the air at 3 has a greater DPT, and either a higher or lower dry-bulb temperature than the set point (region C or D), then the flow rates of refrigerant through the cooling coil, and of water through heating coil 2 should be increased. Process diagrams for these three loops are illustrated in Figure 5.
The following logic statements are for all possible outdoor and indoor conditions, including the case explained in detail above. Figures 6, 7, and 8 accompany cases A, B, and D, respectively. Refer to Figures 3, 4, and 5 for case C. The processes for the condition when room air is in region C or D in Figure 8 illustrate the simplest logic when no further action is needed after point 3.
5
CASE A: IF TOA < TSP and DPTOA < DPTSP
A. IF TRA < TSP and DPTRA < DPTSP heat & humidify
Increase: DW, HW, HC2, SH Decrease: CC, , HC1
B. IF TRA > TSP and DPTRA < DPTSA cool & humidify
Increase: SH Decrease: DW, HW, HC1, HC2, CC
C. IF TRA > TSP and DPTRA > DPTSP cool & dehumidify
Increase: nothing, supply OA Decrease: DW, HW, HC1, HC2, CC, SH
D. IF TRA < TSP and DPTRA > DPTSP heat & dehumidify Increase: DW, HW, HC2 Decrease: HC1, CC, SH
CASE B: IF TOA > TSP and DPTOA < DPTSP
A. IF TRA < TSP and DPTRA < DPTSP heat & humidify
Increase: SH Decrease: DW, HW, HC1, HC2, CC
B. IF TRA > TSP and DPTRA < DPTSA cool & humidify
Increase: DW, HW, CC, SH Decrease: HC1, HC2
C. IF TRA > TSP and DPTRA > DPTSP cool & dehumidify
Increase: CC Decrease: HC1, HC2, SH, DW, HW
D. IF TRA < TSP and DPTRA > DPTSP heat & dehumidify
Increase: nothing, supply OA Decrease: DW, HW, HC1, HC2, CC, SH
CASE C: IF TOA > TSP and DPTOA > DPTSP
6
A. IF TRA < TSP and DPTRA < DPTSP heat & humidify Increase: nothing, supply OA Decrease: DW, HW, HC1, HC2, CC, SH
B. IF TRA > TSP and DPTRA < DPTSA cool & humidify Increase: DW, HW, CC Decrease: HC1, HC2, SH
C. IF DPTRA > DPTSP cool & dehumidify Increase: DW, HW, HC1 Decrease: CC, HC2, SHa IF T3 < TSP and DPT3 < DPTSP
Do Nothing elseb IF T3 > TSP and DPT3 < DPTSA
Also Increase: CCc IF DPT3 > DPTSP
Also Increase: CC and HC2
CASE D: IF TOA < TSA and DPTOA > DPTSA
A. IF TRA < TSP and DPTRA < DPTSP heat & humidify Increase: DW, HW, HC1 Decrease: HC2, CC, SH
B. IF TRA > TSP and DPTRA < DPTSA cool & humidify
Increase: nothing, supply OA Decrease: DW, HW, HC1, HC2, CC, SH
C. IF TRA > TSP and DPTRA > DPTSP cool & dehumidify
Increase: DW, HW, HC1 Decrease: HC2, CC, SH
IF T3 < TSP and DPT3 < DPTSP Do Nothing else
IF T3 > TSP and DPT3 < DPTSA Also Increase: CC
IF DPT3 > DPTSP Also Increase: CC, HC2
D. IF TRA < TSP and DPTRA > DPTSP heat & dehumidify Increase: DW, HC1 Decrease: HW, HC2, CC, SH
IF T3 < TSP and DPT3 < DPTSP Also Increase: HC2
IF T3 > TSP and DPT3 < DPTSA Do Nothing Else
IF DPT3 > DPTSP Also Increase: CC, HC2
Conclusion
7
As the components in the analyzed air handling unit are numerous, different methods to achieve the same outcome in similar situations is possible. For example, dehumidification can be provided using the desiccant wheel or/and the cooling coil. Although not considered in the logic presented in this report, specific outdoor and room conditions may exists for which dehumidification using the cooling coil is more economical than using the desiccant wheel. More specific information such as the price of electricity, the price of heat energy, and the effectiveness of the desiccant wheel and heat wheel would be needed to provide an energy analysis of the system.
One possible future research endeavor is to determine the payback period of adding the first three components (DW, HC1, HW) to a typical DOAS. The exact behavior of the desiccant wheel and heat wheel would be required, as well as their relationship to other components in the system. In addition, the proposed dual-wheel system may operate well in a hot and humid climate, but not so well in a cold and dry climate.
