Designing a Dedicated Outdoor Air System (DOAS) - Part 2

This is a continuing series in topics relating to DOAS. If you missed "Part 1", because I didn't title the previous post as such please check it out: Part 1 - DOAS design. As I mentioned previously there are 7 steps to follow in designing a DOAS, and step four requires an understanding of ASHRAE 62.1.

What is ASHRAE 62.1 anyway?

No, no ASHRAE is not where you put cigarette butts in. For HVAC people (and I assume you are), ASHRAE 62.1 is the standard for Ventilation for Acceptable Indoor Air Quality. As of this posting, I am using the 2010 edition. Hence for "step four" of our design, to determine the amount of outside air ventilation we have:

V_bz = (R_p * P_z + R_a * A_z ) / E_z

where:
A_z = zone floor area: the net occupiable floor area of the ventilation zone ft² (m²)
P_z = zone population: the number of people in the ventilation zone during typical usage
R_p = outdoor airflow rate required per person as determined from Table 6-1 of Ashrae 62.1
R_a = outdoor airflow rate required per unit area as determined from Table 6-1 
E_z = zone air distribution effectiveness factor from Table 6-2

 Important Note: The Equation above accounts for people-related sources and area-related sources independently in the determination of the outdoor air rate required at the breathing zone. 

The ventilation effectiveness factor E_z is typically 1.0 for cooling systems that use ceiling diffusers and 0.8 for heating systems that use ceiling diffusers. For a complete list of the factors used for varying applications refer to Table 6-2 of Ashrae 62.1.

The rate factors for calculating the amount of outside ventilation air can be found in Table 6-1 of Ashrae 62.1-2010 but some of the more common factors are shown below:

The most important aspect, and should not confuse you, for determining the outside air requirement is to consider the two most important factors, the people and the area they would occupy in the breathing zone.

In the next post, I shall discuss further the other steps in the design process for DOAS. And in the subsequent posts illustrative examples shall be shown to get a better grasps of the principles. Don't forget to subscribe.

7 Basic Steps in DOAS Design

Design Steps

In our continuing series of DOAS topics, I'll share some approaches on designing a dedicated outdoor air system.
In general, three factors will determine the DOAS unit cooling coil load, and these are: (a)Ventilation airflow; (b) Outside air enthalpy condition; and (c) Leaving air enthalpy condition.
Normally the ventilation airflow is a constant. Enthalpy difference determines load as given by the formula as

4.5 * ventilation cfm * enthalpy difference between outside air and coil leaving air.

Determining leaving air design state is the key. In determining the required coil leaving air temperature for the DOAS:

• Design for coil leaving ventilation air that is drier than the space humidity target.
• Design to handle the ventilation air sensible and latent load plus the space latent load.

Look for the worst humidity case for the outside air:

• This will likely not be at the peak DB temperature for the location.
• The engineer must look at the peak dew-point and the mean coincident peak WB for the highest enthalpy condition.
  

With that said, there are seven basic steps to determining the DOAS coil load and ventilation air requirement: 

1. Determine outside air enthalpy conditions based on the project location.
2. Determine the maximum allowable space humidity by asking the end-user or using ASHRAE recommended levels.
3. Determine the space latent loads by determining the number of occupants and applying the ASHRAE latent load factors based upon their activity levels. Add any unusual latent machine contributions, if any.
4. Determine the required ventilation cfm by using the formulas in ASHRAE Standard 62.1.
5. Determine the required dewpoint and enthalpy of the supply air.
6. Calculate the coil load as described previously.
7. Determine the required leaving air temperature by deciding if this will be a “neutral” or “cold air” system.

In our next post, I shall discuss these steps in more detail and provide an illustrated example to show how to calculate the required load for the DOAS.

DOAS Options

Dedicated Outdoor Air System (DOAS)

As promised in my previous post about DOAS, more information shall be shared in this post. As some sort of review, a simple schematic diagram is given here which will aid us in the foregoing discussions.

Design Options

When designing a dedicated outdoor air system an engineer has to consider two basic options. He can opt for the outdoor air unit to supply air at the same temperature as the inside design temperature or at the temperature required to assure dehumidification of the air.

As a simplified guide the matrix below illustrates how to best design a DOAS:

Neutral air to the space	Cold air to the space
Neutral air to the parallel system	Cold air to the parallel system

Neutral Air

This option requires that the outside air be reheated because the outside air temperature will be reduced below the outside air dewpoint to dry the air. Another challenge is to make use of available sources of heat that should be utilized to avoid having an impact on the cost of the system. As an alternative the design engineer could require that the DOAS include hot gas reheat coils/controls that essentially capture rejected heat from the condenser side of the DOAS unit. The advantage of a "neutral air" concept is that it is easy to mix it with the occupied zone air. The outside air can then be ducted independently from a parallel sensible cooling system. Unfortunately neutral air does not provide any cooling assistance to the parallel system. Although the neutral air DOAS uses energy to lower the ventilation outside air temperature to a point that also provides sensible cooling, the neutral air concept “gives that energy back” by reheating the air to a temperature that does nothing to offset any building sensible cooling load. The parallel system cooling capacity will remain unchanged as will the parallel system air delivery requirements.

Cold Air

A cold air system will not require a reheat load since the outside air temperature will be reduced below the outside air dewpoint to dry the air and then will be distributed in that condition. The result is that the load of the parallel sensible cooling system will be reduced.

But the big challenge is to prevent a draft effect when this cold air is introduced to the warmer zone air. (More on this to be discussed in succeeding posts).

Conclusion

For both the neutral air and cold air system, the economics should be evaluated properly to justify which system is viable to use for certain applications. But in all, the requirements of ASHRAE 62.1 should always be taken as prime consideration.

Next of this series on DOAS is the actual design calculations. Please be sure to subscribe.

How to measure an Air Conditioning equipment's efficiency?

Common Efficiency Standards

As there are various types of air conditioning equipment, there are a number of ways to measure efficiency of each. As they "diff'rent strokes for diff'rent folks". This is due to the fact that each type is used specifically for certain demands or limits of the designed application. 

air conditioning equipment

As defined by ASHRAE

• Coefficient of Performance (COP) - cooling: the ratio of the rate of heat removal to the rate of energy input, in consistent units, for a complete refrigerating system or some specific portion of that system under designated operating conditions. Higher COP means more efficient system.
• Energy Efficiency Ration (EER): the ratio of net cooling capacity in Btu/h to total rate of electricity input in watts under designated operating conditions. Higher EER means more efficient system.
• kW/ton: The term kW/ton is commonly used for larger commercial and industrial air-conditioning, heat pump and refrigeration systems. The term is defined as the ratio of energy consumption in kW to the rate of heat removal in tons at the rated condition. The lower thekW/ton the more efficient the system.

How to convert between kW/ton, COP and EER

• KW/ton = 12 / EER
• KW/ton = 12 / (COP x 3.412)
• COP = EER / 3.412
• COP = 12 / (KW/ton) / 3.412
• EER = 12 / KW/ton
• EER = COP x 3.412

If a chillers efficiency is rated at 1 KW/ton,

• COP = 3.5
• EER = 12

The major benefit of knowing the efficiency of air conditioning equipment in the market is the cost per consumption of power throughout the life cycle of the equipment. This greatly aids designers, installers and maintenance people in deciding what equipment is best suited for a certain application.

4 Important Benefits of a DOAS

What's a DOAS?

Simple put a DOAS is a "Dedicated Outdoor Air System". What it doas, i mean does, is it manages outdoor air quality. Outdoor air is a necessary component of any well-designed HVAC system. In many climates however the air outside is either so full of moisture or so contaminated that bringing it into a building can create problems for a building’s occupants and contents. Many engineers today have concluded that the best way to handle these issues is to use an HVAC unit that specifically solves these problems.

Benefits:

1. Prevents "sick-building illnesses"

Sick-building illnesses can most often be traced to a lack of effective ventilation air in the occupied spaces. Did you know that US companies lose as much as $48 Billion annually to cover medical expenses and $160 Billion annually in lost productivity as a result of sick-building illnesses? Wow! For these reasons ASHRAE has developed the ventilation requirements in Standard 62.1. Those gases caused by construction materials, fumes from office equipment, and normal build-ups of CO2 from human occupants, all need to be exhausted outside.

2. Better Humidity Control

Human occupants and certain machines in a building will generate moisture that must be controlled, but the largest element of moisture in a building comes from the outside. If this is unabated, mold and mildew could build-up in the conditioned space. Bear in mind that a good comfort index is determined by a combination of the dry bulb temperature, local air velocity, and moisture content of the air (even if you neglect the issue of molds). All three factors must be addressed in a well-designed building.

3. Limitations of conventional equipment

Let us assume that most commercial buildings use rooftop packaged equipment. Some of their limitations are as follows:

• Conventional rooftop units are designed to provide 350 to 450 cfm/ton, but effective dehumidification typically requires only 200 to 300 cfm/ton. ASHRAE 62.1 (section 5.10) limits RH to 65% or less at design dewpoint.
• Dehumidification and drain pans. Coils in conventional rooftop units are typically only 3 to 4 rows deep with a drain pan that extends just past the end of the coil. Effective dehumidification often requires coils up to 8 rows deep. It is also important to provide additional drain pan depth and IAQ-style designs in order to properly handle the amount of moisture that will be removed. ASHRAE 62.1 (section 5.11) requires that drain pan lengths be at least ½ of coil height or designed to limit moisture carryover to .0044 oz.per coil sq.ft. per hour at peak dewpoint condition. On most units this means that the drain pan must be at least 18” long and could approach 30” in larger tonnage units. This will almost never be found in a conventional packaged rooftop unit.
• You can have better filtration with a DOAS.
• Better operational flexibility especially when the outside conditions are beyond stable.

4. LEED Certification

One added bonus of employing the use of DOAS is LEED-NC2.2. But in order to comply with requirement, the criteria for indoor air quality must be met in accordance with ASHRAE 62.1. The application of a DOAS system generates up to 80% of the points required for a LEED certification.

Well there you have it folks. I will discuss some points further on the DOAS in my succeeding blogs. Be sure to subscribe. Thanks for reading.