Dewpoint temperature has a specific meaning in the engineering world: The temperature at which air with a given moisture content and at a given pressure will become saturated. The name comes from the fact that this is the temperature at which "dew" will form. (Think a cold glass of iced tea on a hot, humid day. If the glass is colder than the dewpoint temperature of the air, water condenses on the outside of the glass.)

When air is saturated, the drybulb temperature, wetbulb temperature, and dewpoint of air are all equal and the relative humidity is 100%.

While that little fact about air may be interesting, one way we use it in the HVAC is a sort of shorthand for moisture content in the air. Remember we said that dewpoint is the temperature at which air with a specific moisture content will condense. Moisture content is typically measured in terms such as pounds of moisture per pound of dry air - air at 75 degF and 50% relative humidity has a moisture content of 0.092 pounds of moisture per pound of dry air. Not an easy number to remember. But that moisture content corresponds to a dewpoint of 55F. Much easier to remember.

When air is saturated, the drybulb temperature, wetbulb temperature, and dewpoint of air are all equal and the relative humidity is 100%. So, in practical use in HVAC, when we use a cooling coil to cool warm, moist air down to 55 degF, we also know that the dewpoint is 55, therefore we know the moisture content. For those who like to think in terms of psychrometric charts, moisture - and therefore dewpoint - corresponds to the horizontal lines.

How is dewpoint different from relative humidity? It's right there in the name - "relative." Relative humidity is a measure of how much moisture is in the air relative to how much moisture it can hold at that temperature. If all you know about the air in a room is the relative humidity, you don't really know how much moisture it has. 50% RH at 60 degF has much less moisture than 50% RH at 90 degF - you have to know the temperature as well as RH to know the moisture content. Dewpoint, though, indicates moisture content independent of temperature, which makes it much more useful in describing HVAC processes.

Based on climate zone, supply air temperature setpoint reset (SATR) may be limited by the need for dehumidification. Dedicated Outdoor Air Systems (DOAS) dehumidify outdoor air prior to either mixing with return or supplying directly to the occupied space, which improves the opportunity for SATR. Additional benefit is achieved when DOAS incorporates exhaust air energy recovery. In hospitals, DOAS is especially important for spaces requiring both low temperature and controlled humidity, such as operating/procedure/C-section rooms, but can be beneficial in any space where code-required air change rates often exceed cooling load: intensive care units (ICUs), post-anesthesia care units (PACUs), exam rooms, pharmacy, and even standard patient rooms.

Technical Description

Why humidity control?

ASHRAE 170 sets minimum requirements for outdoor air delivery, overall supply air flow, and maximum relative humidity for many hospital spaces. The 60% upper limit of relative humidity is intended to prevent uncontrolled growth of mold spores on surfaces and building materials, and to potentially reduce the spread of infectious microorganisms.

Occupants typically want indoor temperatures in the low- to mid-70s (Fahrenheit). That means maintaining an indoor dewpoint of about 55F for general patient spaces. In operating rooms and other specialty areas where lower space temperatures are the norm, indoor dewpoint may need to be as low as 46F. Whenever the outdoor dewpoint is above those temperatures, the introduction of outdoor air for ventilation brings unwanted moisture. Removing that moisture is one of the principal functions of a hospital HVAC system.

Moisture removal can be a very large fraction of the total load that an HVAC system deals with - 75% or even more, depending on outdoor conditions. Figure 1 below illustrates this for the ASHRAE 0.4% dehumidification design condition in Atlanta, Georgia.

Figure 1: As shown on this psychrometric chart, Dew Point is the primary design metric that determines the amount of moisture in the air which can then be removed using a DOAS.

High moisture content is expected in a place known for its hot and humid climate, but most climate zones in the US have at least some days with elevated dewpoint conditions. Figure 2 shows a map with the ASHRAE design dewpoint for selected cities around the US. - all above 55F and well above the OR design dewpoint of 46F. Figure 3 shows the number of hours with dewpoint above 46 for those same cities. Clearly, for operating rooms at least, most parts of the country need to dehumidify.

Figure 2: ASHRAE design dew point temperatures for selected cities across the US.

Figure 3: Hours with design dewpoint 46°F or higher for selected cities across the US.

Traditional v. DOAS with Heat Recovery

In a traditional HVAC system, outside air is mixed with return air, the mixed air is cooled, typically to about 52-55 degF, and that air is delivered to the space through VAV reheat terminals. This is a very effective system, but there are several points of waste:

1. Exhaust air energy was typically wasted. IECC and ASHRAE 90.1 both now require exhaust air energy recovery in many cases, but ASHRAE 170 restricts the type of energy recovery device for some airstreams and bans energy recovery altogether for others - based on the potential for contamination of outdoor air.
2. Air-change ventilation requirements in hospitals mean that reheat is required full-time in some spaces. See the chapter on reheat for more discussion of this topic. Supply Air Temperature Reset (SATR) can reduce reheat requirements, but the need for dehumidification of outdoor air restricts the use of SATR. Dehumidification and cooling need to be decoupled so that SATR can be more effective.
3. Air returned from the space is already dehumidified, so it doesn't necessarily need to be cooled down to the design dewpoint. But, when return air and outdoor air are mixed, the entire mixed-air stream has to be cooled down - just to dehumidify the outdoor air. There is waste in re-cooling return air, especially when it has to then be reheated.

The psychrometric chart in Figure 4 shows the typical mixed-air cooling process for a 10,000 cfm air handling system with 30% outdoor air in Atlanta. In this example, latent heat removal is almost entirely due to moisture in the outdoor air. There is a small contribution of moisture from building occupants, about 4% of the total cooling coil load. This approach depends heavily on reheat because supply air temperature reset is limited by the need to dehumidify outdoor air, because as supply air temperature moves upward, less moisture is removed. The example assumes that the supply air temperature actually needed by the spaces served is about 60F, so terminal reheat coils will reheat to that temperature. The system will not always need this much reheat, but this illustrates the burden of overcooling and reheat.

Cooling Coil Outdoor air dehumidification needs to be decoupled from space cooling to pass tWhen a system is reheatin Almost 40% of the energy consumed in a typical HVAC system is used to dehumidify the air, according to an American Society of Mechanical Engineers (ASME) study. In older systems, outdoor air, which may be the primary source of moisture, is mixed with return air, then the mixed airstream flows through the cooling coil, which dehumidifies the mixed air by cooling it to the desired dewpoint, typically 52°F for noncritical spaces and lower for operating rooms and some other critical environments. This need for dehumidification often drives the supply air temperature down lower than is needed to satisfy space conditions. This leads to the air having to be reheated by passing through a reheat coil before it enters the space it will be supplying. In healthcare spaces where airflow cannot be reduced due to ventilation requirements, reheat is the only way to maintain comfort conditions. SATR is a great solution for reducing reheat, but is limited by the need for dehumidification.

Figure 4: System and process diagrams for Mixed-Air Cooling System with Reheat

A dedicated outdoor air system (DOAS) decouple the dehumidification of outside air from space conditioning. Conditioned outside air can be mixed with return air, as in a traditional VAV system, or can be delivered directly to occupied spaces. Direct delivery is especially applicable in a system with distributed cooling units, such as chilled beams, fan coil units, or variable refrigerant flow. Even when mixed air is delivered through a VAV system, though, there is substantial savings.

In the diagrams below, the same 10,000 cfm system is modeled with a DOAS. Note the 20% reduction in cooling load and potential elimination of reheat load.

Figure 5: System and process diagrams for a system using DOAS for outside-air preconditioning.

The psych chart is a little hard to read because the AHU is doing only sensible cooling, making for a very flat diagram, but it shows the return air being cooled separately prior to mixing with the dehumidified outside air. Note that reheat is not required in this illustration.

DOAS enables more-aggressive SATR by treating outdoor air separately from return air. This decoupling of the necessary outdoor air dehumidification from the sensible cooling needs of the space enables higher supply air temperature, simultaneously reducing cooling loads and reheat.

Energy Recovery Opportunity

Additional energy reduction can be achieved by adding exhaust air energy recovery into a DOAS, using exhaust air and/or relief air to pre-cool or pre-heat entering outdoor air. Energy recovery is specifically required by ASHRAE 90.1, depending climate zone, hours of operation, and percent of outside air. ASHRAE 170 limits the use of exhaust air energy recovery based on the spaces being exhausted, but exhaust air energy recovery is a viable option for the majority of hospital airflow. ASHRAE 170 classifies energy recovery systems as those having "leakage potential," generally considered to be rotating wheels, and those that do not "allow for any amount of cross-contamination of exhaust air back to the supply airstream via purge, leakage, carryover, or transfer..." Energy recovery wheels permit transfer of moisture and thus provide for more energy recovery. Consult ASHRAE 170 for specific limitations.

Figure 6: System and process diagrams for a system using DOAS with energy recovery.

Energy recovery in combination reduces cooling load by 28% compared to DOAS without energy recovery. The net effect of a DOAS with energy recovery compared to a mixed-air cooling system is 42% reduction in cooling load and eliminating 82 kBtu/h of full-time reheat for this 10,000 cfm system.

Design Considerations

Especially in hot and humid climates, it is quite energy intensive to dehumidify outdoor air to meet ventilation requirements. Traditional systems mix outside air with return air before cooling to dehumidify, then rely on reheat to maintain space temperature. Decoupling ventilation air dehumidification from space cooling reduces both cooling energy and heating energy.

Factors affecting the benefit of these systems include climate - both design dehumidification requirement and hours of operation at elevated dewpoint, as well as the percentage of outside air required. Quick analysis of the design conditions is easily accomplished with psychrometric analysis software. Determining annual savings requires weather data. A simple analysis with bin weather data yields a good estimate. To get a more accurate estimate of energy savings requires energy modeling to account for supply air temperature reset strategies and the ability to more aggressively raise the supply air temperature.

A DOAS with energy recovery separates ventilation from cooling by only treating around 20-30% of the total air that is required for ventilation (the outdoor air) while the return air from the space just needs to be heated or cooled. The percentage of outdoor air needing to be dehumidified is dependent on the types of spaces being served and their respective required outdoor air changes per hour (ACH). Since humidity is removed from the outdoor air, the remaining cooling components operate solely based on dry bulb temperature and can eliminate the need for reheat. In this way, humidity control is improved and energy consumption is reduced since the minimum outdoor air is being treated separately on particularly hot and muggy days.

As noted by a technical feature [2] in the July 2014 issue of the ASHRAE Journal titled, “Do All DOAS Configurations Provide the Same Benefits?”, a dedicated outdoor air system can be configured in numerous ways with the HVAC system. The conditioned outside air can be delivered directly to the occupied space, the supply-side of the local HVAC units, the intakes of the local HVAC units, the return air plenums near local HVAC units, or the intakes of centralized, multiple-zone HVAC units. Other energy efficient, sensible cooling approaches can also be taken such as implementing radiant ceiling cooling or chilled beams. The conditioned air flow rate can also be varied to meet load or air change requirements of many individual zones.

DOAS systems are well-suited for healthcare applications because they can help to improve indoor air quality and reduce the risk of airborne infections.

Control strategies

DOAS systems are used both to provide exhaust air energy recovery and dehumidification of outdoor air. It is important to recover exhaust air energy only when it is beneficial, as with any energy recovery system. Control sequences should compare exhaust air and outdoor air against the reset supply air temperature setpoint.

Dehumidification at the DOAS should focus on maintaining the required indoor conditions, using either indoor (or return air) relative humidity or supply air dew point. In either case, the setpoint should be dynamically adjusted based on operating schedules. When supply air dewpoint is used, careful selection of sensors is required, because many humidity instruments have reduced accuracy in saturated or near-saturated conditions.

A DOAS coupled with demand-controlled ventilation strategies can provide more energy savings such that the space is not conditioned unnecessarily when the space is unoccupied. Because the DOAS can be sized separately from the main air-handling unit, adequate ventilation can be applied at each zone for its zone population.

DOAS is also advantageous for handling part-load conditions. When space loads change, there may be a loss of indoor humidity control because less air may need to be delivered to the zone. With a DOAS, the local HVAC unit can deliver less air to handle the sensible load while the DOAS supplies air at a low enough dew point to maintain humidity levels. In this way, there is also potential for energy savings since the local HVAC unit can be reduced or turned off while the DOAS handles the ventilation.

How does this decarbonize?

Using a DOAS system can reduce energy use in a number of ways. First, by treating the ventilation (outside) air and return air separately, the amount of air being dehumidified or reheated is reduced, resulting in reduced energy use. This also presents more opportunity for SAT), which allows the system to reset the supply air temperature based on polling data from the zones so that minimal air reheating is needed. Secondly, by using an exhaust air energy recovery wheel, outside air can be heated using the recovered heat from the exhaust air leaving the building. This saves energy that would otherwise be needed to reheat the outside air, typically using a hot water heating coil where natural gas would power the boiler heating the water.

Implementation

Barriers: Cost

A primary barrier is cost - the cost to design and build what amounts to two air handling units is certainly higher than the cost of a single unit. Where energy codes require exhaust air energy recovery, though, the cost delta is based on upgrading the energy recovery unit to include cooling capability. There are savings associated with reduced cooling loads with energy recovery - the savings on cooling plant equipment helps pay for the expense of additional air handling equipment, but it still costs more to install a DOAS.

Barriers: Codes

ASHRAE Standard 62.1-2016 Section 5.16.3.2.5 states that, “Class 2 air shall not be recirculated or transferred to Class 1 spaces.” However, it gives the exception that recirculation from leakage, carryover, or transfer from the exhaust side for energy recovery is permitted as long as the recirculated Class 2 air does not exceed 10% of the outdoor air intake flow. ASHRAE sets the limit at 5%.

Barriers: Culture

As the use of a DOAS is increasing and its energy savings, especially in hot and humid climates, are becoming more attractive, many designers want to use them wherever possible.

Strategy

Energy recovery is required by ASHRAE 90.1 in many circumstances. So, once an exhaust air energy recovery unit is implemented, it’s more than halfway to a DOAS - just add cooling.

There are significant energy savings when using a DOAS, and they are better at achieving humidity levels that are so important to healthcare settings, especially in a humid climate. When coupled with enthalpy/energy recovery devices, there are energy savings as well as through reducing the size of the DOAS.

Financial analysis and business case

We are still developing the financial case for this technology. Do you have a project with applicable energy or financial data that uses this technology? Please reply in the comments section, so that we might connect further!

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