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Free cooling and its implication in DOAS design

DOAS, which is based on the principle of marshalling fresh air directly to cool an occupied space, has gained relevance and immediacy in the light of escalating energy costs. Deepak Pahwa and Rahul Aeron walk us through the concept and its practical application.

| | Feb 15, 2013 | 10:58 pm
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DOAS, which is based on the principle of marshalling fresh air directly to cool an occupied space, has gained relevance and immediacy in the light of escalating energy costs. Deepak Pahwa and Rahul Aeron walk us through the concept and its practical application.


The concept of free cooling in HVAC is an old one. However, its relevance has now come into the limelight due to skyrocketing energy price and the advent of green buildings.

There is no single definition of free cooling, and therefore, it is understood and applied differently in different cases. This paper aims to unveil the concept of free cooling by applying it to a Dedicated Outdoor Air System (DOAS)and understanding its dependency on the ambient air conditions with relevance to different geographical climates.


Free cooling is perceived differently by different people. To many, it is the natural cooling energy available in the environment. However, it has to be noted that it requires a datum to ascertain what the free cooling energy is, before one can begin to analyse the concept of free cooling.

The main sources of natural cooling are:

  • Deep seawater
  • High altitude coldness
  • Nighttime coldness
  • Subterranean geothermal energy

The definition of free cooling is very subjective, even from an HVAC perspective. Nighttime coldness and the differential between design conditions and ambient conditions form the basis of free cooling.


Free cooling takes place when the external ambient air enthalpy is less than the indoor air enthalpy and the cool external air is transferred to the building envelope.

Free cooling is used in conjunction with air systems. Air conditioning systems either provide or supply air using one of the following:

  • A mixture of outside air and re-circulated air
  • 100% outside air system or in other terms, dedicated outdoor air system

Free cooling may be used with mixed outside air and re-circulation systems by the use of modulating dampers. Dampers are provided on the outside air intake duct work and the recirculation duct work. In the event of cool outside air, the quantity of outside air is increased and the quantity of recirculated air is reduced to provide the required supply air temperature. In this way, cooling by means of refrigeration equipment is avoided altogether at certain times of the year and often at night.

DOAS is an independent unit for bringing in all the fresh air directly into an occupied space. It is based on the divide and conquer approach, wherein all the external latent and sensible load is taken care of by the DOAS unit.

In addition, special DOAS units also condition fresh air to a dew point where it is lower than the room design dew point, enabling the dehumidified fresh air to also take care of internal latent load.


Figure 1 (see bottom gallery) simplifies the concept of free cooling and how it is applied:


  • The room air and return air are maintained at 75°F/50% RH.
  • The red line depicts RA enthalpy line.
  • The triangle shown in blue depicts free cooling on account of enthalpy, which can be calculated using the enthalpy difference of outside air and return air.

The area with supply air temperature less than the room return air temperature will give sensible free cooling calculated by difference in DBT between outside air and return air. The area when the air condition is less than the dew point temperature of a room is the area with free latent load. This area has constant load not based on the enthalpy difference. There is no free sensible cooling in this area.


R Mark Nunnely and several other authors have explained in various papers the advantages of the “Divide and Conquer (DAQ)” approach, which separates the fresh air unit from the parallel terminal unit.

To explain further (Figure 2see bottom gallery), in a DAQ approach, the fresh air unit takes care of the entire fresh air load and deliver the ventilation/supply air at design temperature neutral with a dew point sufficiently lower than the design dew point, to take care of the internal latent load.

Thus, the parallel unit, be it FCU or chilled beams or AHUs, has to cater only to the building envelop and the internal sensible load, as both can be handled and managed independently. This design provides a simple and economical approach to the building air conditioning design.


As explained above, in the DAQ approach, the importance shifts to the configuration and type of fresh air unit, often referred to as DOAS. Depending on different geographical reasons and several other considerations, various DOAS have been configured, designed and developed. The authors have been continuously studying and developing different configurations for a DOAS by using one or more of the following:

  • Enthalpy wheel
  • Cooling coil
  • Active dehumidification wheel
  • Passive dehumidification wheel
  • Sensible wheel
  • Evaporative cooling pads

The authors in this and another other paper, ‘Applied DOAS – a tropical climate retrospect’, demonstrate that an intelligent combination of a total enthalpy wheel, cooling coil and passive desiccant dehumidification wheel (Figure 3 see bottom gallery) offer the best solution in terms of substantial energy saving, offsetting internal latent load and effective RH management by supplying air at a dew point lower than room design to take care of internal latent load, in addition to removing fresh air latent and fresh air sensible load.


Geographical weather profiling is an understanding and study of various measurable parameters which govern the natural climatic conditioning of the various geographical regions.

Profiling plays a major role in assisting and applying HVAC concepts and designs with a weather profile backdrop.

From this viewpoint, the globe can be divided into three main regions:

  • America
  • Europe
  • Asia

From Figure 4 (see bottom gallery), it is apparent that the HVAC designs are built around considering the America region as cooling-centric; the Europe region as heating-centric; and the Asia region as moisture-centric for a good and viable HVAC design.

The weather profile of a city is the key driving parameter in assessing free cooling value that the climate offers to any HVAC design.


The authors now intend to apply the free cooling concept for the most energy efficient DOAS design (enthalpy wheel, cooling coil and passive desiccant dehumidification wheel). The idea is to get a holistic picture taking into account a global perspective. To further demonstrate the importance of hourly data, the study is further categorised into a 24 x 7 (Table 1 – see bottom gallery) scenario and a normal working hours – 9am to 5pm (Table 2 – see bottom gallery) scenario.


As it is evident from the above data (Table 3 – see bottom gallery), the free cooling hourly basis from one city to another, the DOAS should be intelligently designed to decide its control logic on the basis of the free cooling available.

This can be done by controlling various parameters, like supply air fan speed and bypass, different component speed and cooling coil temperature. It needs to be pointed out that an intelligent DOAS will offer a logical solution to ably utilise the advantage of free cooling.


Through this paper, the authors attempt to elaborate the concept of free cooling from an HVAC perspective and are inclined to conclude that the most important criterion that governs free cooling is the ambient air condition. The HVAC equipment system design will govern the way free cooling is applied in the most effective manner.

Image Gallery

Click on any image thumbnail below for a larger view: –



  1. A Practical Guide to Free Cooling, Alternative Cooling, Night Cooling and Low Energy Systems for Air Conditioning Systems: Mike Hardy
  2. Dehumidification and Cooling Loads from Ventilation Air: Lewis Harriman, Plager D, Kosar D
  3. 30% Surplus OA. Does it Use More Energy?: Stanely A Mumma
  4. Designing Dedicated Outdoor Air Systems: Stanley A Mumma
  5. Designing for Absolute Moisture Control: R Mark Nunnelly and J Patrick
  6. ABCs of DOAS: Wayne Morris
  7. Overview of Integrating Dedicated Outdoor Air System with Parallel Terminal Systems: Stanley A Mumma
  8. Applied DOAS: A Tropical Climate Retrospect: Deepak Pahwa and Milind Mate

About the Authors:

  • Deepak Pahwa is the Managing Director of Bry-Air (Asia) Pvt Ltd, and Fellow ASHRAE. He serves on TC-3.5, TC-5.5 and TC-5.7.
  • Rahul Aeron is the National Sales Manager of Desiccant Rotors International Pvt Ltd

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