Heat Exchangers

A sample Calculation on how a DOAS System can be applied to – DOAS Technology Options: Various technologies are  available in market today and each one of them carries a few advantages unique to them. No technology is panacea and each one of them prove superior in a particular application for a particular internal and external load profile. Let us introduce you to these technologies, however it is outside preview of this paper to analyse each technology in detail from an application point of view. To judge the right application one would need a detailed analysis including simulation techniques to compare them with each other and conventional HVAC system from first cost and operating cost perspective. However we have provided the performance parameters of all the options on the peak DB with MCWB and peak WB with MCDB conditions for India. Delhi summer and Mumbai monsoon have been chosen as the outside conditions for the same.The various DOAS technologies evaluated in the paper are:

  • OPTION I: Baseline system with dehumidification coil only (CC)
  • OPTION II: Rotary passive desiccant air-to-air heat exchanger coupled with dehumidification coil (EW+CC)
  • OPTION III: Rotary passive desiccant air-to-air heat exchanger
  • Ccoupled with dehumidification coil and sensible air to air heat exchanger (EW+CC+SW)
  • OPTION IV: Active desiccant dehumidification wheel (with condenser heat reactivation) coupled with DX Cooling coil (CC+ADESW)
  • OPTION V: Rotary passive desiccant air-to-air heat exchanger coupled with dehumidification coil and passive desiccant dehumidification wheel (EW+CC+PDHC).
Doas Approach
Figure 6: DOAS approach
Doas Flow Diagram
Figure 7: DOAS flow diagram

OPTION I: Baseline system with dehumidification coil only (CC)

Schedule Of Doas System With Cooling Coil
OPTION I: Schedule of DOAS System with Cooling Coil

Table 1

OPTION II: Rotary Passive Desiccant Heat Exchanger With Coil (EW+CC)

This option gives high efficiency heat recovery and reduces the total installed tonnage of the HVAC system. Since the recovery reduces the both latent and sensible load of the outside air (the recoveries can be as high as 85%) and the dehumidification coil reduces the dew point to almost 45OF, this approach is most widely used and is highly cost effective and paybacks are often negative.

Schedule Of Doas With Enthalpy Wheel And Cooling Coil
OPTION II: Schedule of DOAS with Enthalpy Wheel and Cooling Coil

Table 2

The benefits of this option are:

  • Installed tonnage reduction
  • Lower power consumption of the installed HVAC system
  • Higher ADP’s of sensible cooling devices hence lower Row deeps (lower pressure drop) and higher CHW temp of main Chiller
  • Better performance of the chiller in terms of IKW/TR due to higher CHW temperature.

OPTION III: Rotary Passive Desiccant Heat Exchanger With Coil And Rotary Sensible Heat Exchanger (EW+CC+SW)

This option is designed keeping in mind the following objectives:

  • Always provide conditioned air that is drier than the air in the space Deliver cold conditioned air whenever possible, and use recovered energy to reheat during mild weather.
  • Select equipment to limit indoor relative humidity to 55% in
    all seasons.

This option has all the features of OPTION II  and in addition supplies air at almost room temperature. The advantage here lies in the fact that during moderate weather i.e. when outside ambient temperature is low but latent loads are high (typically monsoons), the OPTION II too can have difficulty in handling the RH.

This option supplies air at almost room temperature with lower dew point than space, hence, it can continue to control the moisture without the risk of lowering the temperature.

Schedule Of Doas With Enthalpy Wheel Cooling Coil And Sensible Wheel
OPTION III: Schedule of DOAS with Enthalpy Wheel, Cooling Coil and Sensible Wheel

Table 3

The benefits of this option are:

  • Possibility of obtaining substantial LEED points
  • Good RH control in all the seasons
  • Reduced installed tonnage and lower power consumption of HVAC system
  • Internal sensible cooling devices have higher CHW, lower ADP’s resulting in reduced pressure drops and better IKW/TR for chiller.

OPTION IV: Active Desiccant Dehumidification Wheel (With Condenser Heat Reactivation) Coupled With DX Cooling Coil (CC+ADESW)

This option combines the benefits of desiccant dehumidification with cooling of the DX air conditioners. Contrary to the first two options, this approach uses desiccant wheel to remove moisture and lower the dew point of the supply air instead of using a cooling coil.

Schedule Of Doas With Cooling Coil And Active Desiccant Wheel React With Condenser Heat 120
OPTION IV: Schedule of DOAS with Cooling Coil and Active Desiccant Wheel (React with Condenser Heat at 120°F)

Table 4

 

The reactivation of the desiccant wheel is undertaken by recycled heat from the DX condenser air.

The benefits of this option are:

  • Meets ASHRAE standard 90.1 requirements
  • CoP 65% more than a conventional DX system with reheat
  • Uses recycled heat from DX system for reactivation
  • No active reheat required
  • Maintains RH control in all the seasons (including for areas having high internal latent loads).

OPTION V: Rotary Passive desiccant heat exchanger with cooling coil and Passive desiccant dehumidification wheel (EW+CC+PDHC)

This approach utilizes the strengths of passive total energy recovery, conventional cooling and a new type of desiccant rotor, the passive dehumidification wheel. The ability of this system lies in the fact that it optimizes the moisture removal between the cooling coil and the desiccant wheel without the need of active re – activation. The passive desiccant wheel removes moisture from saturated air stream in an highly energy efficient manner.

Schedule Of Doas With Enthalpy Wheel Cooling Coil And Passive Desiccant Wheel
OPTION V: Schedule of DOAS with Enthalpy Wheel, Cooling Coil and Passive Desiccant Wheel

Table 5

The benefits of this option are:

  • Extremely good RH control in all seasons
  • High energy efficiency
  • Lower dew point of supply than the other three options
  • Versatile and adjusts well to varying climate
  • Installed tonnage reduction for the HVAC system
  • Substantial LEED points.

Analysis of Various Options in Indian Conditions

When we compared the performance of all the above five options for Delhi summer and Mumbai  Monsoon, it become apparent that baseline cooling coil coupled with Energy wheel had a clear advantage. In addition to the Energy wheel, if one added a passive desiccant dehumidification wheel then the need for deep dehumidification by cooling coil was transferred to passive desiccant Wheel, thereby lowering the IKW/TR of the cooling unit. Hence from the overall advantage on the ability to manage energy, IAQ and RH, the combination of cooling coil with passive desiccant heat exchanger and passive desiccant dehumidification wheel comes out as a winner. This reduces the energy consumed from baseline system by whopping 55%. On the same lines, the combination of cooling coil with passive desiccant heat exchanger and sensible heat exchanger provided the same advantages with higher energy consumption of approx. 20% then former.

Air Changing
Cooling Coil with active Desiccant wheel

The combination of cooling coil with passive desiccant heat exchanger is another winner with energy reduction from baseline cooling coil but there are limitations of RH control during  moderate weather conditions.

The options I & IV come into the focus where no relief air is available. In the absence of relief, air energy recovery is not possible and hence alternatives, like cooling coil only or cooling coil with active desiccant wheel needs to be looked at. Here too, the option IV i.e the combination of cooling coil with Active desiccant dehumidification wheel reduces the energy consumption by almost 10-15% for the same duty conditions. Also, it provides  the supply air  at elevated  dry bulb temperature helping managing the RH inside.

Thus, when relief air available there, the option V comes out to be the winner and not available option IV shows promise.

The Indoor air quality requirement of higher ventilation rates along with proper RH control and energy management will push the HVAC designer to the use of “Dedicated outside air system” in near future. DOAS holds a lot of promise with its ability to maintain the right humidity in all weather conditions. The “Divide and conquer” approach definitely allows the designer to have better management of the two key elements of air conditioning i.e., temperature and moisture. The original definition of Air conditioning can now be met in an energy efficient manner.

Extra Thermal Load

Extra Thermal Load 2

The DOAS has already proved itself with high potential of energy savings in nearly all applications and weather profiles. However, benefit of option-II, III and V over the first option in terms of energy saving is seen. Otherwise from IAQ, RH and energy point of view, option-V has turned out to be the most cost-effective.

However, in RH control, the last three options discussed have performed better for a wide variety of applications and weather profiles. To clearly state, that a particular option is the best solution would be a meaningless statement, as proper simulation exercises need to be conducted on a few key applications for a few weather profiles and “first cost + operating cost” matrix tabulated, giving due consideration to the importance of RH control. Some studies have indicated that the installed first cost of DOAS approach is lower than a conventional system for certain applications. A paradigm shift is taking place and designers are now actively considering these options to weed out the “Traditional problems”. In the words of Albert Einstein, “The whole Science is nothing more than a refinement of everyday thinking.”


Author Details:
Rahul Aeron

Rahul Aeron,
Assistant Vice President, Sales,
DRI, Pahwa Group.