Engineers Newsletter - Trane

1 Providing insights for today's hvac system designer Engineers Newsletter volume 46 4. Condensation in Air-Handling Systems Understanding the different sources of Common sources of moisture in an air- In HVAC systems, undesired moisture undesirable moisture in HVAC systems handling system include: on surfaces can cause damage to can help prevent or solve moisture-related building materials or mechanical outdoor air that enters through intake problems. equipment. hoods, dampers, or louvers Moisture carryover from dehumidifying leaks from hydronic or steam coils, This Engineers Newsletter will focus on how to properly design and operate an coils, condensation on cold surfaces and or their associated piping and valves air-handling system to minimize fogging often have different sources and undesired condensation within HVAC prevention strategies.

2 Because they are moisture carryover, fogging, or systems, and how to properly identify often mistaken for each other, it's condensation related to causes and return the system to dehumidifying coils important to clearly define them. normal operation. Moisture carryover is defined as humidifiers, evaporative air coolers water droplets from a dehumidifying or air washers coil that are not captured in the drain condensation that is a byproduct of pan. gas-fired burners Condensation is caused when the infiltration of moist air from outside, temperature of a surface is below the or leakage of liquid water from dew point of the air contacting the outside surface. Moisture condensing on a condensate drain pans cold surface often results in the accumulation of water droplets. saturated or supersaturated air from building processes Fog is visible water vapor in air that can form when moist air mixes with air The specific location within the system at a colder temperature.

3 Where leaks, drips, or external water occur can often be a clue to mitigation and can often be traced to their source with some careful investigation. This EN will focus on condensation and fogging with guidance for prevention and problem resolution. 2017 Trane , a business of Ingersoll Rand. All rights reserved. 1. Figure 1. Typical air handler layout. Condensation: Where, Why and How Condensation requires a cold surface and air at a dew point above the temperature of that surface. Most often a dehumidifying coil is involved, but 1. there are other contributing factors 5 3 2. including: 4 mixing box with angled filters heating access cooling infiltration coil coil panel direct-drive bypass supply fan coil control uneven air temperatures fogging contractor-assembled joints when the Specify air-handling equipment with a AHU is shipped in separate pieces low leakage rate.

4 Condensation due to infiltration. Air drain pan condensate trapping Minimize and properly seal all pressure inside an air- handling unit penetrations. penetrations for electrical and control (AHU) cabinet can be both positive and wiring Use factory-installed controls to negative with respect to the eliminate variability during field surrounding environment, depending on access doors or panels installation and avoid additional the location of the fans (supply, return, exterior joints or associated duct cabinet penetrations due to electrical or relief/exhaust). When the pressure connections or control wiring. inside the AHU is negative with respect to the exterior there is an opportunity Figure 1 shows a typical layout for an air Ensure access doors are closing and for air to infiltrate into the AHU. handler. The air pressure in sections 1 sealing correctly.

5 Thru 4 would be at a negative pressure Check that drain pan traps are properly Infiltration can lead to interior relative to the ambient due to the location sized and primed. This issue is often condensation if the dew point of the air of the supply fan in section 5. Infiltration overlooked as a potential infiltration leaking in is higher than the temperature that happens prior to the cooling coil path. of the air inside the AHU. For example, (section 3) would not necessarily cause when an AHU is installed outside on the Use thermal imaging during unit any condensation issues, but infiltration roof, the ambient dew point could easily operation to identify leakage paths. into sections 3 or 4 could result in be above 60 F during the humid condensation. summer months, while the cooling coil might be producing air temperature Figure 2 shows an operating AHU system Figure 2.

6 Unsealed coil drain and vent holes. around 55 F. The more humid the where the supply- and return-water piping ambient air, and the colder the supply connections were sealed, but the coil air, the greater the potential for drain and vent holes were not. The AHU. condensation to occur due to infiltration. fan configuration results in this section being under negative pressure with Where it happens. Any penetration or respect to the exterior, resulting in a joint in the air- handling system, direct path for infiltration directly whether created in the factory or in the downstream of the cooling coil. field, is a potential path for air leakage. Common culprits include: Guidance. Identifying possible paths of factory and/or field provided infiltration and preventing it can be accessories and penetrations challenging. The following coil piping penetrations (supply and recommendations could help minimize return connections, drain and vent infiltration during design, installation, and connections) operation of the equipment.

7 Humidifier supply and drain connections 2 Trane Engineers Newsletter volume 46-4 providing insights for today's HVAC system designer Condensation due to bypass air Condensation due to coil control. Guidance. If cycling control of the valve (dehumidifying coils). Untreated, Control sequences are not often reviewed is necessary, the valve should be closed humid air that is allowed to bypass when looking for causes of condensation. gradually rather than abruptly. If possible, around (or through) a dehumidifying coil But close analysis of control valve stopping airflow in conjunction with and mix with cold air leaving the coil operation shows how this might be an closing the water flow would also help may result in condensation downstream issue. When a system is at steady state minimize potential issues. of the coil. with typical leaving-air conditions, the interior surface temperature of the AHU Acquiring trend data from control Properly mounting and sealing a downstream of the cooling coil will be devices over an extended period of time dehumidifying coil in an AHU typically close to the leaving-air temperature.

8 If the can help diagnose this potential cause. requires block-off (blank-off) plates that control valve is rapidly closed, while Figure 3 shows trend data for discharge must be properly fitted to prevent air airflow remains relatively constant, this air temperature, entering air temperature from bypassing around the coil. Also, air could result in condensation. and humidity, entering and leaving water must pass through the coil without any temperature, and valve position in an large inactive sections or gaps that may For several minutes after the control valve AHU. In this particular system the allow varying leaving air conditions. is rapidly closed, the air continues to be discharge air temperature setpoint is cooled (and humidified) as the residual 52 F and the fan speed is held constant, Where it happens. Common locations condensate on the fins of the while the control valve modulates.

9 In for air bypass are: dehumidifying coil evaporates into the certain intervals, the control valve is passing air. This cool, humid air then closed while the fan speed is maintained seams where the block-offs mount passes over the cold interior AHU which could result in condensation on to the coil or walls of the AHU, surfaces, possibly resulting in the cold interior panels downstream of gaps between the coil casing and condensation. Condensation is more likely the data must be taken rapidly fins, if this valve cycling happens often, and the enough, such as every minute. Figure 3. under or above the coil where the dew point of the air entering the coil is shows a 30-minute interval. Trend data mounting and retaining structures well above the leaving-air dry-bulb taken slower than this could mask the are installed, temperature.

10 Cause. inactive portions of the coil (either by design or due to coil capacity control), Figure 3. Coil trend data. internal wiring chases (control or power) where wiring passes through the coil section. relative humidity Guidance. Factory mounting of dehumidifying coils typically results in discharge air temperature closer attention to block-off design, installation and sealing. Use gaskets fan speed and sealants as needed to minimize or eliminate air gaps. Coil design also plays a role in air bypass. Minimizing gaps and bypass valve position paths is an important design criteria. Using multiple rows with dehumidifying coils tends to minimize risks of downstream condensation compared to one- or two-row coils, since the leaving- air conditions tend to vary less. providing insights for today's HVAC system designer Trane Engineers Newsletter volume 46 4 3.