This document discusses evaporative cooling air conditioning systems. It describes direct evaporative cooling systems which cool and humidify air by passing it through wet pads or sprayed water. Indirect evaporative cooling systems cool air through tubes surrounded by evaporating water without adding moisture to the air. An indirect/direct evaporative cooling system combines both for sensible cooling and dehumidification. Direct systems provide more cooling but add more humidity while indirect offer slightly less cooling but much less humidity rise. Evaporative cooling provides energy savings but risks higher humidity and noise levels compared to vapor compression systems.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Mechanical Engineering
Subject:- Heating, Ventilation and Air
Conditioning (ME 411)
B.Tech.Mechanical
Purushottam W. Ingle
Assistant Professor

2. 1
Evaporative-Cooling Air Conditioning Systems
• This system is based on the principle that when moist but
unsaturated air comes in contact with a wetted surface whose
temperature is higher than the dew point temperature of air, some
water from the wetted surface evaporates into air.
• The latent heat of evaporation is taken from water, air or both of
them. In this process, the air loses sensible heat but gains latent heat
due to transfer of water vapor. Thus the air gets cooled and
humidified. The cooled and humidified air can be used for providing
thermal comfort.
P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
9/4/2023

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4. Direct evaporative cooling system
• In a direct evaporative cooling system shown by process 1-2, when the outdoor
air passes over the wetted pads or through sprayed water as shown, it gains the
moisture and at the same time it losses its heat, process 1-2 is also called
adiabatic humidification.
• In this process, total enthalpy remains constant.
• At the inlet outdoor air has higher amount of sensible heat and lower amount
of latent heat, now when it flows through water spray or wetted pads, its
sensible heat reduced and latent heat is increased.
• So the total enthalpy remains constant.
• If outlet temperature is less than WBT of entering air, enthalpy rise happens,
and if temperature is more than WBT of entering air, then enthalpy drop
happens.
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6. Indirect evaporative cooling system
• As shown in figure, the water is sprayed over the bunch of tubes, and
through the tubes the primary air is flowing.
• In indirect evaporative cooling system, there is no direct contact
between primary and water.
• Now with the help of any external agency like fan the secondary air is
made to flow as shown in order to enhance rate of evaporation.
• The water get evaporated, and air takes away that heat and some
water also.
• Here no water is added in water, only sensible cooling of air takes
place.
• If only cooling is required, then indirect system is used.
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8. • In indirect direct evaporative cooling system, both systems are used
in combination connect in series, so in indirect cooling system only
sensible cooling is occurred, whereas in direct cooling system cooling
and humidification occurs.
• As shown on slide next, the primary air flows through the channel
through evaporative heat exchanger, liquid is spray by recirculation
pump, so sensible cooling happens here.
• Now when same air is made to flow over evaporative pad, both
cooling and humidification occurs.
• Two psychometric processes are shown on chart, sensible cooling
and Cooling and humidification.
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10. • Comparing between, direct evaporative and indirect evaporative
system, 5.1 percent more cooling is taking place in IDEC than DEC.
• Up to 60 % less moisture added in IDEC.
• Better comfort is found in IDEC.
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23. • Advantages and disadvantages of evaporative cooling systems :
• Lower equipment and installation costs.
• Substantially lower operating and power costs. Energy savings can be
as high as 75 %.
• Ease of fabrication and installation.
• Lower maintenance costs.
• Ensures a very good ventilation due to the large air flow rates
involved, hence, are very good especially in 100 % outdoor air
applications.
• Better air distribution in the conditioned space due to higher flow
rates.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

24. • The fans/blowers create positive pressures in the conditioned space, so that
infiltration of outside air is prevented.
• Very environment friendly as no harmful chemicals are used.
• Disadvantages:
• The moisture level in the conditioned space could be higher, hence, direct
evaporative coolers are not good when low humidity levels in the conditioned
space is required. However, the indirect evaporative cooler can be used
without increasing humidity.
• Since the required air flow rates are much larger this may create draft and/or
high noise levels in the conditioned space.
• Precise control of temperature and humidity in the conditioned space is not
possible.
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25. • May lead to health problems due to micro-organisms if the water
used is not clean or the wetted surfaces are not maintained properly
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon