Sensible wheel
Sensible-energy rotary heat exchangers (sensible-energy wheels) are similar to total-energy wheels in both price and appearance.
The main difference between the two types of energy wheels is a coating that is applied to the total energy wheel that allows it to transfer moisture in addition to sensible heat. Sensible energy wheels are typically applied in scenarios where the addition of moisture would be detrimental to the system.
Figure 4–12, and Figure 4–13 on page 4–35, show the typical arrangements for sensible-heat recovery in mixed-air and dedicated ventilation systems. In Figure 4–12, which shows mixed-air systems, sensible-heat recovery can be used to reheat the dehumidified supply air (Insets A and C) or to precondition the outdoor air (Inset B).
In Figure 4–13 on page 4–35, which shows dedicated ventilation systems, sensible-heat recovery is often used to reheat the dehumidified outdoor air (Insets A and C), or to precondition the outdoor air (Insets B and C). Insets A and C are typically used with systems that deliver dehumidified outdoor air at a neutral dry-bulb temperature.
Figure 4–12 Mixed-air arrangements
Inset A • parallel recovery Inset B • parallel recovery
Inset C • series transfer (upstream to downstream) (stream to stream)
(stream to stream)
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Each of the arrangements in Figure 4–12 and Figure 4–13 can be modeled using TRACE 700. However, only the arrangement shown in Figure 4–13, Inset A, will be covered in this discussion of sensible-wheel energy recovery. For arrangements other than this one, select the appropriate energy-recovery configuration (similar to step 1 in this example) and refer to the following:
Figure 4–12:
■ for Inset A, refer to steps 1 through 6 in “Fixed-plate heat exchanger” on page 4–21
■ for Inset B, refer to steps 1 through 6 in “Total-energy (enthalpy) wheel” on page 4–15
■ for Inset C, refer to steps 1 through 6 in “Heat pipe” on page 4–
27
Figure 4–13:
■ for Inset B, refer to steps 1 through 6 in “Total-energy (enthalpy) wheel” on page 4–15
■ for Inset C, refer to steps 2 through 7 in “Coil loop for exhaust-air energy recovery” on page 4–8
Related reading
■ “Air-to-Air Energy Recovery,” Engineers Newsletter (volume 29, number 5)
■ Air-to-Air Energy Recovery in HVAC Systems Applications Engineering Manual (Trane literature number SYS-APM003-EN) Figure 4–13 Dedicated ventilation arrangements
Inset A • parallel recovery Inset B • parallel recovery Inset C • series transfer (upstream to downstream) (stream to stream)
(stream to stream)
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Sample scenario
To input the type of heat recovery in Figure 4–13, Inset A, we will model an office building with a dedicated ventilation unit (make-up air unit) that dehumidifies the outdoor air and then reheats the air to a room neutral dry-bulb temperature, before the air is delivered directly to the space. Water-source heat-pump (WSHP) units are installed in the ceiling plenum above each room and handle the cooling and heating needs for the rooms. To reduce the amount of reheat energy needed, a sensible-energy wheel will be used to remove heat from the exhaust air stream and reheat the dehumidified outdoor air.
1 After defining the WSHP system and the dedicated ventilation system, select the desired air-to-air energy recovery / transfer type, which determines the type of energy recovery / transfer and the basic configuration. In this case, Sensible wheel (parallel SA tempering).
2 A message will appear that asks whether the user wants to account for energy recovery / transfer during the design and energy analysis simulations or only during the energy analysis simulation. Select Yes for this example.
3 Select the appropriate supply-side deck for the device. For this example, Ventilation downstream will be selected as the supply-side deck. The sensible-energy wheel will be used to reheat the ventilation air downstream of the optional ventilation cooling coil.
Click Yes to activate energy recovery / transfer for design and energy analysis. Click No to ignore energy recovery / transfer during the design calculation and include it only during the energy-analysis simulation.
Note: This message will only appear whenever an energy recovery / transfer device is added and Allow energy recovery / transfer during design calculations is turned off.
■ Select Ventilation downstream if the device is located in the outdoor air stream and downstream of the optional ventilation cooling coil.
■ Select Return / outdoor air downstream if the device is located after the mixed air stream, downstream of the main cooling coil.
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deck for the device. In this case, the energy from the system exhaust air stream will be used to reheat the ventilation air.
As a result, System Exhaust will be selected.
Note: If the proper airstreams are not selected for supply-side and exhaust-supply-side decks of the energy recovery/transfer device, then the unit will not function correctly or may not function at all. The Airflows section of the System Checksums report can be used to verify that a specific airflow is available for energy recovery. On the System Checksums reports, Room Exhaust = Rm Exh, System Exhaust = Exhaust, and Return air = Return Air.
5 Choose the schedule that describes when the sensible-energy wheel is permitted to operate. The schedule Available 100%
will allow the device to operate year-round.
■Select System Exhaust if main system exhaust is used exclusively as the exhaust side.
■Select Room Exhaust if the room exhaust airflow is used exclusively as the exhaust side. Room exhaust airflows are input on the Airflows tab of Create Rooms. If no room exhaust is input, then all exhaust air will be at system level.
■Select Return air if airflow returning back through the plenum or return duct (pre system exhaust and post room exhaust) is used as the exhaust side.
Note: If two energy recovery/transfer devices are input, and both stage 1 and stage 2 devices are given the same exhaust-side deck, then stage 1 will be upstream of stage 2.
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Additional Items
1 When adding an energy wheel to an airside system, it is recommended that the minimum and maximum cooling supply-air dry bulbs be set equal to each other to fix the value for the cooling supply-air dry bulb. This is suggested because
TRACE 700 cannot psychrometrically solve for the cooling supply-air dry bulb when an energy-recovery device is attached to the Return/Outdoor deck. For the ramifications of fixing a cooling supply-air dry bulb, please refer to “Frequently asked questions”
on page 6–20 and “Oversizing and undersizing” on page 6–51.
2 Energy recovery / transfer devices are generally applied at higher airflows (higher face velocities) than what the manufacturer submits for ARI certification. For example, if a wheel is rated at 400 fpm, a typical sensible-heating effectiveness is around 74 percent. However, it is likely that this wheel will be applied at a much higher airflow (600-700 fpm), which will decrease its effectiveness. The user should work with the manufacturer's representative to determine the effectiveness of the device at the desired airflow (and at 75 percent of the desired airflow), and 6 Input the effectiveness of
the sensible-energy wheel (essentially how efficiently the wheel recovers energy) by clicking Effectiveness and entering the sensible effectiveness
percentages at different airflows. Click OK and then click Apply when finished.
Note: To further refine the model of the energy-recovery device, the Options button can be used to define such items as parasitic energy consumers, static pressure drop, bypass dampers, part load controls, economizer lockout, and frost prevention.
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airflow points for accurate modeling.
3 Refer to Table 4–4 for recommendations on energy-recovery applications in different scenarios.
Table 4–4 Applications for Air-to-Air Energy recovery Ventilation System Energy-Recovery Technology
Source Supply-air
Temperature Supply-air
Tempering Outdoor-air Preconditioning Dedicated
outdoor air Cold Not applicable Sensible or total
Neutral Sensible
(parallel or series)
Sensible or total
Mixed air (outdoor air plus recirculated return air)
Modulated (constant volume)
Sensible (parallel or series)
Sensible or total
Cold
(variable volume)
Not applicable Sensible or total
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