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ContentsIntroduction ............................................................................... 2Features and Benefits .............................................................. 3Typical MCC SDU Applications and Designs ........................... 4Application Guidelines .............................................................. 7Unit Construction and Operation ............................................ 10Designing and Selecting the MCC SDU ................................. 12Summary ................................................................................ 15

EngineeringBulletin

Split Dehumidification Unit for Modular Climate Changer® Air Handlers

2 CLCH-PRB005-EN©American Standard Inc. 1999

Introduction

Controlling building humidity in humid climates is extremely important in today’s market, with increased litigation over Indoor Air Quality (IAQ) problems. Indoor humidity levels measuring over 60 percent for long periods of time can promote microbial growth, resulting in poor IAQ and premature deterioration of the building furnishings and structure. Moisture problems can occur in buildings in most geographic locations, but they are more prevalent in humid climates.

ASHRAE 62-1999 recommends that relative humidity in the occupant space be maintained at less than 60 percent while at the same time meeting its standard ventilation requirements. HVAC systems usually have sufficient latent capacity to adequately remove moisture at design load; however, if not properly controlled, the system may not have sufficient latent capacity at part load conditions to maintain relative humidity below 60 percent.

There are many methods for controlling building moisture. One commonly used method is a single-path Air-Handling Unit (AHU) with new energy reheat. This simple system design is effective yet costly because new energy reheat is required to dehumidify the space when the mixed outdoor air (OA) and recirculated air (RA) is overcooled. Another method for controlling building moisture is a single, dedicated, 100 percent OA unit using a separate duct air path to existing air-handling units in the building. Other methods may include complex energy recovery systems that use air-to-air sensible energy recovery or total energy recovery with a desiccant type energy recovery wheel. These energy recovery systems are complex to design, involve high first cost due to higher equipment costs and often involve complex installations.

The Trane Modular Climate Changer® Split Dehumidification Unit (MCC SDU), a flexible, pre-engineered air handler that provides direct humidity control in the space, is a simple solution for dehumidification. It independently dehumidifies the outside air, uses sensible energy in the recirculated air for reheat, and reduces building operating costs. The MCC SDU provides good control of space humidity at peak and part loads. The dual path solution also provides low first cost compared to costly, more complex energy recovery dehumidification methods.

In addition, Trane’s flexible modular design and inherent structural integrity allows for ease of stacking, is great for retrofit, and reduces footprint in crowded mechanical rooms. A smaller OA unit can easily be stacked upon a larger RA unit without the need for external support or field modifications. Thus, the Trane MCC SDU provides a packaged unit — factory engineered with a reliable, tested design — while providing single-source reliability.

This engineering bulletin explains the features and benefits of the MCC SDU and its applications and suggests specific guidelines to follow to ensure proper system design. Review it carefully before beginning the design process. The MCC SDU can be designed using unit sizes 3–25 for the OA unit and unit sizes 3–30 for the RA unit. The factory needs to evaluate any designs outside of these size ranges.Note: The MCC SDU is intended for indoor applications only.

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Features and Benefits

The MCC SDU independently pre-treats the outside air, which is dehumidified and then mixed with conditioned recirculated air. Because of this design, the MCC SDU can provide more accurate humidity control at both peak and part load. It directly controls the space relative humidity because the outside air coil stays continuously wet, providing more stable space humidity control. Condensate from the OA cooling (dehumidification) coil does not evaporate back into the airstream, but is drained away from the unit, providing enhanced dehumidification OA coil module drainage.

Also, the MCC SDU, used in conjunction with an airflow measuring and control dampers system, can control space relative humidity below 60 percent while providing the required amount of ventilation air to meet ASHRAE 62-1999 standards.

Other features and benefits of the MCC SDU include:• Thermal enhanced casing for the OA coil and downstream modules —

provides improved water management as well as a thermal break between the panels and the framing

• Lower first cost because:– The supply air is delivered to the space via a single duct system not

using a separate, dedicated 100 percent OA unit, which usually requires a separate air path to the space

– The MCC SDU features a simple design compared to complex energy recovery systems with multiple fans and components

– Installation of equipment is simple and requires only one supplier• Energy efficient — uses free sensible energy from the recirculated air for

reheat, which can reduce operating costs and potentially even eliminate the need for new energy reheat.

• Single-piece, factory-packaged humidity control system — eliminates the need to coordinate multiple suppliers of various component installations.

• Single-source reliability and responsibility from an established HVAC system supplier — Trane

• Proven Trane technology — engineered and tested coil moisture carryover limits

• Reduced footprint — the vertical MCC SDU takes up less equipment room space because of its stacked unit design

• Great for retrofit — the flexible unit design and modularity of the MCC SDU can easily be installed as an upgrade to many existing systems and in sections to get through tight spaces

• Factory packaged controls — allow control of the MCC SDU as a stand-alone air handler or as part of an Integrated Comfort System (ICS)

• Integral airflow measuring and control dampers — control system outside airflow

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Typical MCC SDU Applications and Designs

Typical Applications

Typical applications of the MCC SDU include any project with significant latent outside air ventilation loads and significant space latent loads. Buildings, both new and existing, often use dual-path AHUs to manage building moisture due to heightened concerns about IAQ. A few of these applications include:• Buildings with significant variable occupancy resulting in periods of low

load operation. • Buildings that require compliance with ASHRAE 62-1999 in reference to

maintaining space relative humidity at less than 60 percent.• Retrofit applications where it is difficult and/or costly to add ductwork for a

remotely located, dedicated 100 percent OA unit.• Multiple groups of small buildings where it is impossible to run ductwork

from a single remotely located, dedicated OA unit to each building.• Constant volume systems in single-zone applications.• Variable air volume systems with a fixed amount of outdoor air. Note that

at part load, these systems are less likely to benefit from any sensible energy reheat in the recirculated air and typically require some type of reheat.

Typical MCC SDU application examples include:• Elementary, middle, high schools and universities• Hospitals, nursing homes and day care centers• Shopping malls and casinos• Hotels and motels• Indoor swimming pool areas and sports arenas• Offices, condominiums and apartment buildings• Convention centers, airports and prisons

MCC SDU Designs

In the MCC SDU, the OA unit is stacked on top of the RA unit to provide dehumidified outside air for mixing with conditioned recirculated air. Note that:• The OA unit is sized for the outside air volume.• The RA unit is sized for the recirculated air volume.• The OA cooling (dehumidification) coil is sized for the sensible and latent

load of the outside air.• The RA cooling coil is sized for the recirculated air sensible and latent

load. • The supply fan is sized for the total amount of cfm for recirculated air and

outside air, using the greater static pressure path.

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The MCC SDU can be designed in two ways: horizontal and vertical.

Horizontal MCC SDU Design

The horizontal MCC SDU is designed so that air goes through the OA cooling (dehumidification) coil and turns 90 degrees into the RA unit. The outside air is mixed with the conditioned, recirculated air just before the fan and after the RA cooling coil. Figure 1: Horizontal MCC SDU Typical Design shows a horizontal MCC SDU designed using a size 6 OA unit stacked on top of a size 17 RA unit.

Figure 1: Horizontal MCC SDU Typical Design

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Vertical MCC SDU Design

Vertical MCC SDUs can be used to reduce footprint. They mix the outside air with conditioned, recirculated air inside the fan module. Figure 2: Vertical MCC SDU Typical Design shows a vertical MCC SDU designed using a size 6 OA unit stacked on top of a size 17 RA unit.

Figure 2: Vertical MCC SDU Typical Design

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Application Guidelines

The MCC SDU is available in unit sizes 3–25 for the OA unit and sizes 3–30 for the RA unit. The factory needs to evaluate any MCC SDU design outside these size ranges.

Please follow these basic guidelines when designing an MCC SDU:• Design the OA unit for the percent of outside air required for the building.• Design the RA unit for the recirculated air cfm.• Design the MCC SDU for indoor applications in recirculated-air or supply-

air pressurized mechanical rooms (see Trane’s Managing Building Moisture SYS-AM-15).

• Size the supply fan for the OA unit cfm and the RA unit cfm, using the greater static pressure path.

• Ensure that the stacked OA unit length is smaller than or equal to the RA unit length beneath it.

• Center the OA unit width-wise on top of the RA unit. Do not stack the top OA unit so that it cantilevers the RA unit.

• Include drain pans on all coil modules and required blank modules.• Include 4- or 6-inch mounting legs or base rails (base rails are optional

unless the MCC SDU is suspended from the ceiling).

Trane recommends using coils with Trane’s enhanced fin types (Delta-Flo™ H and Prima-Flo™ H) because they have higher moisture carryover limits, which will allow higher velocities for optimizing the OA and RA unit sizes. Because our enhanced fin type is more efficient, Trane can provide fewer fins to meet the capacity, providing less first cost and lower fan operating cost, which results in reduced building operating costs.

Refer to the Installation and Maintenance manual for shipping limitations, outdoor storage, and installation. When the SDU design falls outside of the standard product offering size range (3–25 OA, 3–30 RA), the maximum single piece shipment may be exceeded, and the unit may require field assembly. Contact your Regional Marketing Engineer for the SDU Assembly Guide.

Horizontal MCC SDU Application Guidelines

As part of the horizontal MCC SDU design, air goes through the OA cooling (dehumidification) coil and turns 90 degrees into the RA unit. Due to the velocity profile at the lower half of the coil, there is potential for water condensate to be swept out of the drain pan and through the turning module. On sizes 12–25, include a small blank module with drain pan after the cooling coil of the OA unit. This module allows a more uniform velocity profile leaving the cooling coil. Its length has been pre-designed around a maximum velocity limit of 1250 ft/min through the turn.Note: See “Selecting the OA Cooling (Dehumidification) Coil for the

Horizontal MCC SDU” on page 12 for special coil velocity requirements.

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Other guidelines are as follows:• Include a blank (turning) module for the OA unit. Do not include a drain

pan with this module because the bottom of this module must be fully open so that air can turn 90 degrees into the RA unit.– For unit sizes 3–12, use a medium 15.5” module.– For unit sizes 14–25, use a medium large 24.5” module.

• Include a blank (turning) module with drain pan for the recirculated air unit. This module is required because the dehumidified outside air turns 90 degrees into this module.– For unit sizes 3–12, use a medium 15.5” module with drain pan.– For unit sizes 14–30, use a medium large 24.5” module with drain pan.

• Stack the OA unit on top of the RA unit so that the blank (turning) module of the OA unit lines up evenly length-wise with the blank (turning) module of the RA unit beneath it (see Figure 1: Horizontal MCC SDU Typical Design).

Vertical MCC SDU Application Guidelines• For unit sizes 3–30, include a small blank module with drain pan before

the fan module of the RA unit. Note that this module must be the same unit size as the RA unit.

• Stack the OA unit on top of the RA unit so that it is centered length-wise on top of the RA unit and provides air into the small blank module and fan module (see Figure 2: Vertical MCC SDU Typical Design).

Special Water Management Considerations

The standard drain pans have been designed for a maximum amount of water to be removed before the drain pan overflows. For design conditions outside the range of this engineering bulletin, use the following guide to determine the maximum condensate flow rate through the drain pan connection:• For sizes 3–30: maximum flow rate = 1.81 GPM• For unit sizes 35–50: maximum flow rate = 3.60 GPM• For unit sizes 66–100: maximum flow rate = 5.29 GPM

When the condensate flow rate exceeds the maximum allowable flow rate through the drain pan connection, contact the factory for the availability of a specially designed larger connection or multiple drain connections.

To calculate the condensate flow rate for your selection, plot the outside air entering temperature drybulb / wetbulb (OA) and temperature of the outside air leaving the dehumidification coil (T OA). The difference in the humidity ratio of these two points is the grains of water removed from the outside air

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and can be used to convert to gallons per minute [GPM]. The following equation can be used:

Condensate Flow Rate [GPM] = (.075 lb dry air / cubic feet) X (face area of the coil in square feet) X (delta grains of water / lb. dry air) X (air velocity in ft. / min. through the dehumidification coil) X (1 lb. water / 7000 grains of water) X (1 lb. water•HRS / 500 GPM) X (60 min. / 1 HR) Example: Size 30 with an 8 row UW coil:

OA: 95 db / 78 wb = 118 grains of water / lb. dry airT OA: 49 db / 48.5 wb = 50 grains of water / lb. dry airDelta grains = 118 – 50 = 68 grains of water / lb. dry air

Condensate Flow Rate = (.075 lbs. dry air / cubic feet) X (29 square feet) X (68 grains of water / lb. dry air) X (517 ft. / min.) X (1 lb. water / 7000 grains of water) X (1 lb. water•HRS / 500 GPM) X (60 min. / 1 HR) = 1.31 GPM

Therefore, for this example, the condensate flow rate of 1.31 GPM is less than the maximum condensate flow rate for a size 30 (1.81 GPM). The standard drain connection is sufficient and a special larger drain connection is not required.

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Unit Construction and Operation

MCC SDU Construction

The MCC SDU is constructed with double-wall solid panels, with a double-sloped, non-corrosive drain pan in all coil modules and required blank modules as noted earlier. Two-inch support channels are used to evenly distribute the weight of the OA unit on top of the RA unit. The two-inch channels also provide sufficient height for the condensate pipe from the OA coil to be pitched properly for condensate drainage. The OA cooling (dehumidification) coil and downstream modules contain special water management features, such as caulk tape gasketing on all the panels and a 4” airflow diverter mounted at the leaving airside of the drain pan, to help prevent condensate from being swept into the airstream.

General Operation

Figure 3: Dual Path MCC SDU Psychrometric Chart shows a psychrometric analysis for a typical MCC SDU. For the OA unit, the outside air passes through the cooling (dehumidification) coil, removing both sensible and latent energy (T OA). For the RA unit, the recirculated air is cooled, removing primarily sensible energy and some latent energy (T RA). The two airstreams (T OA and T RA) mix (T mix) just before the fan. Because of the fundamental advantage of the MCC SDU, the mixed air may not require any new energy reheat, depending on the sensible heat ratio of the space. This can lower operating costs up to 30 percent.

Figure 3: Dual Path MCC SDU Psychrometric Chart

Figure 4: Single Path AHU Psychrometric Chart shows the psychrometric analysis for a typical single path Air-Handling Unit (AHU). The outside air mixes with recirculated air to combine the two airstreams (T mix). The mixed

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air passes through the cooling (dehumidification) coil, removing both sensible and latent energy (T cc). Similar to the dual-path MCC SDU, the cooled, dehumidified, mixed airstreams may not need new energy reheat, depending on the sensible heat ratio of the space.

Figure 4: Single Path AHU Psychrometric Chart

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Designing and Selecting the MCC SDU

The MCC Selection Program or the new MCC TOPSS selection program can be used to design and select all of the modules required for the MCC SDU. Select the SDU as two units. Refer to the Lexington Custom Toolbox for detailed selection and ordering procedure instructions. Submittals for complete SDUs are available from the Lexington Business Unit. For more information on MCC SDUs, contact the Lexington Marketing Department at 1-800-228-1666 ext. 2615.

Selecting the OA Cooling (Dehumidification) Coil for the Horizontal MCC SDU

As part of the horizontal MCC SDU design, air goes through the OA cooling (dehumidification) coil and turns 90 degrees into the RA unit. Due to the velocity profile at the lower half of the coil, the potential for water condensate to be swept out of the drain pan and through the turning module does exist. The Trane MCC SDU includes water management techniques and requires slightly reduced velocities that minimize this potential. Trane has developed a maximum velocity limit through the dehumidification coil. This maximum velocity for the horizontal MCC SDU outside air coil is slightly lower than the standard moisture carryover limit set in the MCC Selection Program or the new MCC TOPSS selection program. See Figures 5–7 for the maximum moisture carryover limit for the horizontal MCC SDU OA coil. • Delta-Flo™ H, Prima-Flo™ H (enhanced fin), Delta-Flo™ E and Prima-

Flo™ E (non-enhanced fin), and coils with Baked Phenolic coating, should have a velocity 25 ft/min below the standard moisture carryover limit in the MCC Selection Program.

• Copper Sigma-Flo™ fin type selection velocities should have a velocity 75 ft/min below the standard moisture carryover limit in the MCC Selection Program. See Figures 5–7 for horizontal MCC SDU maximum OA coil velocities.

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Figure 5: Reduced Moisture Carryover Limits for the Horizontal MCC SDU Outside Air Cooling (Dehumidification) Coil with Delta-Flo Fins

Figure 6: Reduced Moisture Carryover Limits for the Horizontal MCC SDU Outside Air Cooling (Dehumidification) Coil with Prima-Flo Fins and Copper Sigma-Flo Fins

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Figure 7: Reduced Moisture Carryover Limits for the Horizontal MCC SDU Outside Air Cooling (Dehumidification) Coil with Baked Phenolic Coating

Selecting the OA Cooling (Dehumidification) Coil for the Vertical MCC SDU

The outside air coil should be selected at the standard moisture carryover limit in the MCC Selection Program or the new MCC TOPSS selection program.

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Summary

Designing and applying the Trane Modular Climate Changer Split Dehumidification Unit can be done successfully if the guidelines in this engineering bulletin are followed properly. Applying the MCC SDU is an easy, effective, and proven method of controlling space relative humidity that has been popular and successful since 1995.

Note that the factory must evaluate any MCC SDU designs that differ from the guidelines in this engineering bulletin.

Since The Trane Company has a policy of continuous product and product data improvement, it reserves the right to change design and specifications without notice.

Literature Order Number CLCH-PRB005-EN

File Number PL-AH-CLCH-000-PRB-005-EN-1100

Supersedes New

Stocking Location La CrosseThe Trane Company North American Commercial Group3600 Pammel Creek RoadLa Crosse, WI 54601-7599www.trane.comAn American Standard Company