Digital Precise Air Control System

D-PAC

Total Control

Indoor Air Quality and Comfort

Indoor air quality (IAQ) and occupant comfort are two of the most important factors to consider with any HVAC system design. One of the leading causes of poor IAQ and occupant discomfort is too much moisture in the air, commonly referred to as high humidity. IAQ problems associated with high humidity include mold growth, condensation and increased sickness and allergic reactions. As for occupant comfort, the saying goes “It’s not the heat, it’s the humidity”. Improving indoor air quality and occupant comfort by controlling the humidity and the temperature will help with these problems, boost productivity, and even improve the general well-being of the occupants.

One way to improve IAQ and occupant comfort is with uniform humidity and temperature control. Ideally indoor conditions should remain consistently around 75°F dry bulb and 45% relative humidity. This will keep the occupants comfortable and decrease the likelihood of IAQ issues.

Energy Use

Controlling both temperature and humidity can be very energy intensive. This is because both the sensible (temperature) and latent (humidity) loads require energy from the HVAC equipment to be controlled. With a conventional rooftop unit extra energy is used to satisfy the sensible load during part load conditions because cooling is staged with only a few compressors which will not always match the load. To satisfy the latent load the system must either satisfy the latent load while satisfying the sensible load, include an energy recovery wheel to reduce the outside air load, or there must be some form of cooling and reheating to dehumidify the air and avoid overcooling the space. Satisfying the latent load while satisfying the sensible load and
including an energy recovery wheel will not control the humidity at all conditions. Cooling and reheating will control the humidity at all conditions, however, it uses extra energy.

The Solution

The AAON energy efficient rooftop unit solution to improving indoor air quality and occupant comfort by controlling both temperature and humidity is the patented digital precise air control system, D-PAC (Patent No. 6,792,767).

The Competition

Most of the HVAC industry assumes that as the dry bulb temperature is being controlled the humidity will be controlled as well. This, however, is not true at many ambient conditions and space loads with higher humidity. Humidity is also especially uncontrollable when in ventilation mode, when the mechanical cooling is off and outside air is being introduced into the system.

Previously there have been only a few solutions for controlling both temperature and humidity.

One method is to have a chiller and boiler system with air handling units. This method allows modulation of both cooling and reheating for tight control of temperature and humidity. The problems with this system are that it is large, expensive to implement, and energy is wasted controlling the humidity because both the chillers and boilers must be running.

The second solution is a conventional rooftop unit with on/off hot gas reheat. The problems with this system are there is poor control of the amount of reheat, there will be uncomfortable discharge air temperature swings during operation, especially in make up air applications, and finally the temperature is still only controlled by a few compressor stages.

The last solution is to use an energy recovery wheel to control humidity. This, however, is not a total solution because at higher latent loads humidity will still be an issue.

What is D-PAC?

The D-PAC control system consists of a Copeland Digital Scroll compressor, modulating hot gas reheat, an economizer with three independently controlled sections - outside air, return air, and return air bypass - and an AAON D-PAC controller.

The system uses the Digital Scroll compressor, with modulating capacity control, for energy efficient load matching temperature control. For humidity control the system uses the combination of the Digital Scroll compressor, return air bypass, and modulating hot gas reheat for energy efficient load matching humidity control. The AAON D-PAC controller optimizes the performance of the system. The D-PAC system provides an energy efficient, cost effective solution for temperature and humidity control.

The Digital Scroll Compressor varies the volume of refrigerant that flows through the cooling system. This allows the compressor to match the load needed by the unit. The compressor can modulate from 10-100% of its cooling capacity. This allows the unit to have tighter temperature control than a conventional unit. The compressor will also run for a longer period of time, dehumidifying the air more and cycling the compressor on and off less.

The compressor operates in two states, loaded and unloaded, to be able to modulate from 10-100%. The loaded state is the standard scroll compressor operation. During the unloaded state a solenoid valve opens and the top of the scroll moves up separating from the bottom of the scroll allowing refrigerant to circulate back to the suction line and keeping it from leaving out the discharge line. There is a power reduction during this unloaded state that allows the compressor and unit to save energy at part load conditions (Figure 3 and 4). By pulsing between the loaded and unloaded states the capacity of the compressor can be varied for energy saving load matching capability.

Return Air Bypass consists of an economizer with an independently controlled outside air damper for ventilation, an independently controlled return air damper to allow return air to pass through the evaporator coil, and an independently controlled return air bypass damper to allow return air to bypass around the evaporator coil (Figure 4). The economizer routes all of the outside air across the evaporator coil and the return air either through or around the evaporator coil. Up to 50% of the return air can be bypassed around the evaporator coil. This allows the mixed return and outside air to be dehumidified by the evaporator coil and then reheated by the return air bypassed around the coil. Return air bypass is an energy efficient solution to controlling light humidity loads.

Modulating Hot Gas Reheat consists of a reheat coil downstream of the evaporator coil, a modulating reheat hot gas valve, a modulating condenser hot gas valve, and a reheat controller. To minimize energy usage, reheat begins after the return air bypass damper is fully open. The evaporator coil can then cool and dehumidify the mixed air even more and then reheat the air with the reheat coil. The modulating valves allow only the needed amount of reheat to be used, creating consistent supply air temperature. Modulating hot gas reheat is an energy efficient solution to controlling high humidity loads.

The AAON D-PAC Controller controls the fans, outside air, return air, and return air bypass actuators, modulating hot gas reheat, compressors, heating, and optional AAONAIRE® energy recovery wheel. Using these components, the controller controls the temperature and humidity of the space under all conditions in the most energy efficient manner. The controller is factory installed and tested to ensure proper operation. The WattMaster VCM-X controller and the Tridium Niagara/JACE-2 controller are available for the D-PAC system to meet any controls application. With a choice of these factory installed controllers a D-PAC unit can used as a stand alone unit or integrated into an existing building automation system. The factory installed and tested D-PAC unit controller optimizes performance of the complete D-PAC system.

Sequence of Operation

As the space temperature increases or decreases, the controller modulates the compressor’s capacity to maintain the space temperature setpoint.

As the space humidity rises, the controller modulates the compressors capacity to maintain a low evaporator coil temperature to maximize dehumidification and meet the space latent load. The controller then modulates the return air damper closed and the return air bypass damper open, diverting return air around the evaporator coil to maintain the space temperature. After the return air bypass damper is fully open, the controller uses modulating hot gas reheat to increase the dehumidification capacity of the unit while still maintaining the space temperature. Thus, the humidity setpoint and temperature setpoint will be maintained with minimum energy usage.

There four common ways to modulate the refrigerant capacity of a cooling system: hot gas bypass, multiple compressors, an inverter driven compressor and a Digital Scroll compressor.

A hot gas bypass system mixes hot refrigerant gas from the compressor with cool refrigerant liquid at the evaporator to control the cooling capacity. Hot gas bypass is an inefficient modulation technique because it is adding a false load that the system must satisfy.

A multiple compressor system stages the compressors on and off to control the cooling capacity. The problem with this system is that it has a finite number of capacity steps for modulation and will have inefficient operation at many part load conditions. Another issue is at smaller tonnages multiple compressors are often not available.

An inverter driven compressor system varies the speed of the compressor motor to control the cooling capacity. This system, however, has oil return issues and the modulation range is limited by the motor speed range.

A Digital Scroll compressor system modulates the volume of refrigerant that flows through the cooling system to control the cooling capacity. It is a simple, reliable, energy efficient system with wide modulation capability.

Example 1: The first comparison is the space full load conditions of 40 MBtu/h sensible load and 10 MBtu/h latent load. An example of this would be a large well lit conference room with occupants, laptops, projectors and other sensible heat sources.

Assume the ambient conditions of 95°F DB and 75°F WB with 2,800 cfm of supply air, 700 cfm of which is outside air. The unit is attempting to control to 75°F DB space temperature and 45% space relative humidity. The space full load conditions are 40 MBtu/h sensible load and 10 MBtu/h latent load for a sensible heat ratio of 0.8. Supply fan motor heat is neglected. Calculations can be recreated using your specific ambient and loading conditions within the AAONEcat32 software.

Conventional Rooftop Unit

With no humidity control the conventional rooftop unit can control to 75°F DB space temperature with an uncontrolled 65% space relative humidity. The load on the compressor is 6.0 tons, thus a 6 ton rooftop unit is required.

Rooftop Unit with Return Air Bypass

With the addition of return air bypass the unit can control to the conditions of 75°F DB space temperature and 53% space relative humidity. The load on the compressor is 6.7 tons, thus a return air bypass 7 ton rooftop unit is required. Controlling the humidity with return air bypass alone required an extra 0.7 tons of load. Return air bypass alone, however, cannot control to 45% relative humidity because only up to 50% of the return air can be bypassed around the evaporator coil.

Rooftop Unit with Modulating Hot Gas Reheat

With the addition of modulating hot gas reheat the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The load on the compressor is 9.8 tons, thus a modulating hot gas reheat, 10 ton rooftop unit is required. Controlling the humidity with modulating hot gas reheat alone required an extra 3.8 tons of load.

Rooftop Unit with a Digital Scroll Compressor, Return Air Bypass and Modulating Hot Gas Reheat (D-PAC)

With the addition of a Digital Scroll compressor, return air bypass and modulating hot gas reheat the unit cancontrol to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The load on the compressor is 7.8 tons, thus a D-PAC, 8 ton rooftop unit is required.
Controlling the humidity with the D-PAC unit required an extra 1.8 tons of load. Therefore, the D-PAC unit requires 2.0 tons load less than the modulating hot gas reheat only unit and unlike the return air bypass only unit can control to 45% relative humidity.

D-PAC with an AAONAIRE Sensible Energy Recovery Wheel

With the addition of an AAONAIRE sensible energy recovery wheel the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The addition of the sensible energy recovery wheel reduced the entering outside air conditions to 79°F DB and 70°F WB. The load on the compressor is 6.7 tons, thus a sensible AAONAIRE, D-PAC, 7 ton rooftop unit is required. Therefore, the D-PAC unit with an AAONAIRE sensible energy recovery wheel requires 1.1. tons less than a D-PAC unit alone.

D-PAC with an AAONAIRE Total (Enthalpy) Energy Recovery Wheel

With the addition of an AAONAIRE total energy recovery wheel the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The addition of the total energy recovery wheel reduced the entering outside air conditions to 79°F DB and 65°F WB. The load on the compressor is 5.6 tons, thus a total AAONAIRE, D-PAC, 6 ton rooftop unit is required. Therefore, the D-PAC unit with an AAONAIRE total energy recovery wheel requires 2.2 tons less than a D-PAC unit alone. It also requires the same tonnage as the conventional unit and controls both temperature and humidity.

Psychrometric Chart Comparisons

To see how each of these systems is controlling the temperature and humidity, psychrometric charts for each system are shown on the next page.

OA Outside Air introduced into the unit or for energy recovery wheel units outside air after going through the wheel.

RA Return Air from the space.

MA Mixed Air (Return and outside air)

CCLA Cooling Coil Leaving Air before mixing with bypassed return air or before reheat coil.

SA Supply Air

Example 2: The second comparison is the space part load conditions of 10 MBtu/h sensible load and 10 MBtu/h latent load. An example of this would be the same conference room with the lights and electronics turned off while the occupants are watching a video on a projection screen.

Assume the ambient conditions of 95°F DB and 75°F WB with 2,800 cfm of supply air, 700 cfm of which is outside air. The unit is attempting to control to 75°F DB space temperature and if the unit includes humidity control options 45% space relative humidity. The space part load conditions are 10 MBtu/h sensible load and 10 MBtu/h latent load for a sensible heat ratio of 0.5. Supply fan motor heat is neglected. Calculations can be recreated using your specific ambient and loading conditions within the AAONEcat32™ software.

Conventional Rooftop Unit

With no humidity control the conventional rooftop unit can control to 75°F DB space temperature with an uncontrolled 89% space relative humidity. The load on the compressor is 2.2 tons.

Note: By adding a Digital Scroll compressor to a conventional rooftop unit the compressor work will be greatly reduced at part load conditions because the compressor can match the required load. The Digital Scroll compressor will also cycle on and off less resulting in less compressor wear, tighter temperature control, more dehumidification, and longer compressor life.

Rooftop Unit with Return Air Bypass

With the addition of return air bypass the unit can control to the conditions of 75°F DB space temperature and 86% space relative humidity. The load on the compressor is 2.4 tons. Controlling the humidity with return air bypass alone required an extra 0.2 tons of load. Return air bypass alone, however, cannot control to 45% relative humidity because only up to 50% of the return air can be bypassed around the evaporator coil.

Rooftop Unit with Modulating Hot Gas Reheat

With the addition of modulating hot gas reheat the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The load on the compressor is 9.8 tons. Controlling the humidity with modulating hot gas reheat alone required an extra 7.6 tons of load.

Rooftop Unit with a Digital Scroll Compressor, Return Air Bypass and Modulating Hot Gas Reheat (D-PAC)

With the addition of a Digital Scroll compressor, return air bypass and modulating hot gas reheat the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The load on the compressor is 7.8 tons. Controlling the humidity with the D-PAC unit required an extra 5.6 tons of load. Therefore, the D-PAC unit requires 2.0 tons load less than the modulating hot gas reheat only unit and unlike the return air bypass only unit can control to 45% relative humidity.

D-PAC with an AAONAIRE™ Sensible Energy Recovery Wheel

With the addition of an AAONAIRE sensible energy recovery wheel the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The addition of the sensible energy recovery wheel reduced the entering outside air conditions to 79°F DB and 70°F WB. The load on the compressor is 6.7 tons. Therefore, the D-PAC unit with an AAONAIRE sensible energy recovery wheel requires 1.1. tons less than a D-PAC unit alone.

D-PAC with an AAONAIRE™ Total (Enthalpy) Energy Recovery Wheel

With the addition of an AAONAIRE total energy recovery wheel the unit can control to the desired conditions of 75°F DB space temperature and 45% space relative humidity. The addition of the total energy recovery wheel reduced the entering outside air conditions to 79°F DB and 65°F WB. The load on the compressor is 5.6 tons. Therefore, the D-PAC unit with an AAONAIRE total energy recovery wheel requires 2.2 tons less than a D-PAC unit alone.

Psychrometric Chart Comparisons

To see how each of these systems is controlling the temperature and humidity psychrometic charts for each system are shown on the next page.

OA Outside Air introduced into the unit or for energy recovery wheel units outside air after going through the wheel.

RA Return Air from the space.

MA Mixed Air (Return and outside air)

CCLA Cooling Coil Leaving Air before mixing with bypassed return air or before reheat coil.

SA Supply Air

The D-PAC System

Digital Scroll

Compressor

Full Load Steady State Conditions
	Space DB	Space RH	Supply Air DB	Compressor Load	Reheat Amount	Return Air Bypass Amount
Controlling Temperature Only
Conventional System	75°F	65%	62°F	6.0 tons	NA	NA
Controlling Temperature and Humidity
With Only Return Air Bypass (RAB)	75°F	53%	62°F	6.7 tons	NA	1,050 cfm
With Only Modulating Hot Gas Reheat (MHGR)		45%		9.8 tons	33,100 Btu/h	NA
With a Digital Scroll, RAB and MHGR (D-PAC)				7.8 tons	8,800 Btu/h	1,050 cfm
D-PAC with Sensible Energy Recovery Wheel				6.7 tons
D-PAC with Total (Enthalpy) Energy Recovery Wheel				5.6 tons

System Comparison

Full Load Conventional	Full Load with Return Air Bypass
Full Load with Modulating Hot Gas Reheat	Full Load D-PAC with Sensible Energy Recovery Wheel
Full Load D-PAC with Sensible Energy Recovery Wheel	Full Load D-PAC with Total Energy Recovery Wheel

Psychrometric Chart Comparisons

Part Load Steady State Conditions
	Space DB	Space RH	Supply Air DB	Compressor Load	Reheat Amount	Return Air Bypass Amount
Controlling Temperature Only
Conventional System	75°F	89%	72°F	2.2 tons	NA	NA
Controlling Temperature and Humidity
With Only Return Air Bypass (RAB)	75°F	84%	72°F	2.4 tons	NA	1,050 cfm
With Only Modulating Hot Gas Reheat (MHGR)		45%		9.8 tons	63,100 Btu/h	NA
With a Digital Scroll, RAB and MHGR (D-PAC)				7.8 tons	38,800 Btu/h	1,050 cfm
D-PAC with Sensible Energy Recovery Wheel				6.7 tons
D-PAC with Total (Enthalpy) Energy Recovery Wheel				5.6 tons

Part Load Conventional	Part Load with Return Air Bypass
Part Load with Modulating Hot Gas Reheat	Part Load D-PAC with Sensible Energy Recovery Wheel
Part Load D-PAC with Sensible Energy Recovery Wheel	Part Load D-PAC with Total Energy Recovery Wheel

Psychrometric Chart Comparisons

System Comparison Summary

Conclusion

Return Air Bypass alone can control humidity only at high sensible heat ratio loads.

Modulating Hot Gas Reheat alone can control humidity at a majority of load conditions, however, a larger unit with more capacity may be needed.

D-PAC (Digital Scroll Compressor, Return Air Bypass and Modulating Hot Gas Reheat) can control humidity and temperature at all load conditions. It can also control humidity at those conditions with less compressor work than modulating hot gas reheat alone.

AAONAIRE Energy Recovery Wheel can greatly reduce the compressor work at all load conditions. Thus, the overall size of the unit can be reduced resulting is less initial and running cost.

The D-PAC control system uses a Digital Scroll compressor, return air bypass, and modulating hot gas reheat to control both temperature and humidity. With the combination of these components and an AAON D-PAC controller to optimize them the system can control temperature and humidity as efficiently as possible. Thus, the D-PAC control system is an energy efficient system providing total temperature and humidity control under all conditions.

Contact your local AAON Sales representative to learn more about the AAON D-PAC features and many more ways AAON can provide HVAC solutions to your applications.

Functionality

Factory Testing

Ease of Installation

Ease of Maintenance

Energy Efficiency

AAON • 2425 South Yukon Avenue • Tulsa, Oklahoma 74107 • (918) 583-2266 • Fax (918) 583-6094 • www.aaon.com

For More Information and Other Controls and Dehumidification Options, contact your local AAON sales representative.

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D-Pac • R58460 • 100122