Energy saving through evaporating cooling in comfort and industrial applications

EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016”

Luigi Nalini, Speaker [email protected]

Energy saving through evapora7ng cooling in comfort and industrial

applica7ons


Everybody has certainly experienced the cooling effect caused by a current of air on the swea1ng skin or on wet clothes, as well as the perceived lower temperature in the vicinity of waterfalls where microscopic water droplets are suspended in the air.

Based on empirical observa1ons, even without knowing its basic physical principle, humankind has

Evapora7ve cooling has been used by humankind since 50 centuries ago!

used since from the third millennium B.C. the evapora1ve cooling to mi1gate the temperature of spaces, par1cularly in areas with a hot and dry climate.

Only over the last two centuries, scien1sts have studied the basics of thermodynamics and processes related to the exchange of sensible and latent heat and found the theore1cal principles of cooling by evapora1on which, however, has played a marginal role in the recent past due to the extensive use of mechanical refrigera1on systems.


According to the molecular kine1c theory, as any element water assumes the solid, liquid, gaseous state in func1on of the internal energy of molecules, that occurs as vibra1onal, rota1onal, transla1onal mo1on and reciprocal collisions.

Temperature is a measure of the average internal energy and therefore the higher the temperature, the greater the internal energy of the molecules. Upon an energy input, liquid water molecules increase their internal energy.

Part of them reaches an energy level sufficient to enter in the evapora7on process, overcoming the aWrac1ve forces of the bulk of the liquid, passing to the gaseous state (vapor) and spreading in the available space around.

The Water Evapora7on Process


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temperature -‐ °C

Vapo

r pressure -‐ P

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SATURATION PRESSURE VPS OF WATER vs TEMPERATURE The diagram shows the pressure exerted by the water vapor molecules vs temperature just above the surface of liquid water. In this condi1ons water vapor is in equilibrium with its condensed state and therefore that pressure is said Satura7on Pressure PVS.

Leaving the liquid water and entering into the atmosphere the vapor molecules must «compete» with the pressure exerted by the other gases.

The vapor molecules, due to their kine1c energy, exert over the con1guous bodies a macroscopic pressure propor1onal to the number and to the force of the collisions. As well as the internal energy, also vapor pressure depends on temperature.

Water Vapor Pressure


The atmosphere, a mixture of dry air (i.e.: permanent gases -‐ such as N2, O2, Ar -‐ without vapor) and vapor, has a pressure PATM (around 1,033 bar at sea level) that is equal to the sum of the the individual p re s su re o f t he seve ra l ga seous components.

According to the gas laws, the individual pressure of any gas (called also par7al pressure) in the mixture is propor7onal to

THE ATMOSPHERE IS A MIXTURE OF GASES OVERALL PRESSURE = 101.325 Pa @ SEA LEVEL

NITROGEN OXYGEN

ARGON WATER VAPOR

Of course the maximum quan1ty is got when the vapor par1al pressure equals the satura1on pressure PVS ; in this condi1on, the air is said Saturated.

However, differently from permanent gases (whose rela1ve percentage is stable) water vapor concentra7on varies with 1me, loca1on and weather.

its volumetric frac7on; therefore the number of molecules of water contained in the air is propor1onal to the vapor par1al pressure.

Vapor in the Atmosphere


The ra1o between the actual pressure PV and the satura1on pressure PVS at the same temperature is defined rela7ve humidity RH:

RH = PV / PVS [%] (1)

The Vapour Pressure Deficit, or VPD, is the difference between the actual water vapor pressure and the satura1on pressure: it indicates the maximum capability by the air to absorb addi1onal vapor at that temperature.

The formula closely appoxima1ng the

VAPOR PRESSURE OF WATER vs TEMPERATURE AND RH

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PVS = 3170

PV = 1270

VAPOR PRESSURE DEFICIT

ATMOSPHERIC PRESSURE (101,325 Pa)

When the content of vapor in the atmosphere is not enough for satura7on, also the vapor pressure PV is lower than the saturated pressure PVS.

saturated vapor pressure Pvs vs. temperature T [°C] between 0°C and 80°C is:

PVS = exp (23,5771 -‐ 4042,9/(235,57 + T)) [Pa] (2)

Psychrometric Expressions 1/2


The vapor content in the atmosphere is called absolute humidity x, expressed as mass of water vapor per unit mass of dry air; it can be calculated knowing the par1al pressure Pv -‐ func1on of temperature -‐ and rela1ve humidity:

x = 0,622 * PV/(PATM – PV) [kgv/kga] (3)

The expression (3) shows that, at a certain atmospheric pressure, absolute humidity x is func7on exclusively of the vapor pressure PV.

Another important parameter of humid air is the enthalpy H, i.e. its energy thermal content, made of the heat contained in dry air and the internal energy of vapor molecules, that depends on temperature and on absolute humidity:

H = cpa * T + x * (r0 + cpv * T) = 1,005 * T + x * (2501 + 1,9 * T) [kJ/kga] (4)

where: •  T [°C] = temperature; •  cpa, cpv [kJ/kg°C]= specific heat of dry air and of water vapor; •  r0 [kJ/kg] = latent heat of water at 0°C.

The expressions from (1) to (4) are the bases of psychrometric chart, i.e. the graph of the thermodynamic parameters of moist air at a constant pressure.

Psychrometric Expressions 2/2


Water liquid molecules require an heat input to increase their internal energy in order to pass to vapor.

This heat can be given by an external source (i.e.: by the sun, by electricity or by burning a fuel) as it happens normally during winter humidifica1on.

Alterna1vely, the evapora1on heat can be supplied by the air itself with no external input: the molecules that evaporate absorb heat from the en1re air-‐liquid-‐vapor system which then undergoes a temperature decrease. This process is therefore defined adiaba7c (i.e. without transfer of heat) and isenthalpic because the heat content of air being humidified does not change. Just for the same reason this process is defined adiaba7c cooling. In an adiaba1c cooler an air stream is circulated over an extended water surface with which it comes into close contact. Within the cooler the air flow causes the evapora1on of water.

Adiaba7c Cooling 1/2

water

adiaba1c cooler

ADIABATIC COOLER SCHEME

cooled humid air

@ temp. T2 < T1

entering warm air

@ temp. T1


Isothermal Humidifica1on

Adiaba1c humidifica1on Air Hea1ng Air Cooling

Temperature

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Rela1ve Humidity

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LUTE HUMIDITY – g/kg

PATM = 101.325 Pa

10%

Process Trend

Psychrometric Chart and Basic Processes

ISENTHALPIC LINES


Due to the difference between the vapor pressure over the water surface and the par1al vapor pressure of unsaturated air, the evapora1on of water will take place.

The lowest temperature theore1cally aWainable corresponds to the intersec1on between the isoenthalpic line, followed along the process, and the satura1on curve: this is represented by the black arrow in the graphic. Along the process un1l the satura1on: •  the Vapor Pressure Deficit decreases

down to zero; •  the rela1ve humidity arrives to 100%; •  the cooling effect, due to evapora1on,

reaches the maximum value.

RH [%]

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ON GOING EVAPORATIVE COOLING PROCESS

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evapora1ve cooler

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Adiaba7c Cooling 2/2


However, a long the process, the temperature and the vapor pressure differen1als between humid air and water sharply decrease, making the satura1on of leaving air hardly achievable in prac1ce.

The capacity of an evapora1ve cooler to approach the satura1on, defined as Satura7on Effec7veness μe, is:

μe = (T1 – T2)/(T1 – TWB) [%]

DIRECT SATURATION EFFECTIVENESS

Dry bulb temp. -‐ °C T1 T2 TWB

The evapora1on of water involves a simultaneous transfer of heat and mass (evapora1ng molecules) between the air stream and the liquid surface. •  The heat exchange is propor1onal to the temperature difference. •  The mass exchange (evapora1ng water) is propor1onal to the vapor

pressure difference. •  Their rate depend linearly on the interface area between water and air.

In direct evapora1ve coolers μe ranges between 20-‐30% (typical of the tabletop equipment) up to 90% and more for large high performance ducted units.

Direct Satura7on Effec7veness


The adiaba1c evapora1on process is very efficient because, where prac1cable, produces a cooling effect with no energy consump7on.

An evapora1ve cooler designed for air condi1oning purposes reduces the processed air temperature but increases its humidity content; this should be considered in order to keep the hygrothermal room condi1ons within the limits required for each applica1on.

Therefore room air condi1oning by means of an evapora1ve cooler is not viable just recircula7ng internal air because the indoor humidity would soon approach the satura1on condi1on.

Instead, it requires the introduc7on of outside air to which obviously must correspond an equal rate of exhaust air.

Evapora1ve cooling equipment can be direct or indirect.

40% RH 80% RH

100% RH 100% RH

YES

NO

Adiaba7c Cooling Requires Air Changes


GI Total air flow rate Gvent Min fresh air flow EC Evapora1ve cooler CC Cooling coil GO Outside air flow CD Combined dampers PC Pre-‐hea1ng coil RC Re-‐hea1ng coil

The free cooling by Direct Evapora7ve Cooling (DEC) is got cooling (and humidifying) outdoor air and introducing it straight into the space: this is therefore viable whenever the temperature T2 of the outdoor air downstream the adiaba1c cooler is lower than the indoor temperature Tamb. In fact, for the same air flow, the cooling capacity is propor1onal to the air flow rate and to the difference (T2-‐Tamb).

GI

GO

GI

CD PC EC CC RC

AHU UNIT WITH DIRECT ADIABATIC COOLER AND MOTORIZED DAMPERS TO ADJUST THE AIR FLOW RATES

T2 Tamb

Direct Evapora7ve Cooling

from the space

to the space

from outdoor

to outdoor


GI Total air flow rate Gvent Min fresh air flow EC Evapora1ve cooler HU Humidifier GO Outside air flow α Ra1o GO/GE HE Heat exchanger CC Cooling coil GE External air flow CD Combined dampers PC Pre-‐hea1ng coil RC Re-‐hea1ng coil

GO (= r * GE); TO; HO

GE; TE; HE

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CD PC HU CC RC

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HE

GI ≥ GE ≥ Gvent

AC

GO ; TC; HA

GE ; TX; HX

GO ; TA; HA

GI

AHU UNIT WITH INDIRECT ADIABATIC COOLER AND MOTORIZED DAMPERS TO ADJUST THE AIR FLOW RATES

The Indirect Evapora7ve Cooling (IEC) occurs by cooling air in an adiaba1c humidifica1on process, and then in turn using the same air to reduce – via a heat exchanger – the temperature of a second stream of air, whose moisture content remains unchanged.

Indirect Evapora7ve Cooling


Based on a mass transfer process, an adiaba1c cooler should have: •  enough air velocity to create a sufficient turbulence and the removal of

vapor molecules from the water surface; •  enough interface surface between cooled air and evapora1ng water. There are two basic ways to expand the surface: 1)  by using a solid wet media with an extended surface that, if kept wet,

act as a vast water-‐air interface area; 2)  by introducing into the air stream water in the form of minute droplets

using a process known as nebulisa7on, pulverisa7on or atomisa7on.

Features of Most Used Adiaba7c Coolers


In these adiaba1c humidifiers the air is passed through weWed pads, i.e.: honeycomb structures of resin-‐impregnated cellulose or glass fiber offering a wide interface area.

The pads, placed ver1cally, are kept wet by a water flow distributed on their upper edge.

Wet Media Humidifiers 1/2

In ducted HVAC systems wet media humidifiers are generally placed inside of air handling units; the wet pad is made using modules. This makes possible to adapt the front surface and the depth of the wet media according to the available space, the air flow rate, the efficiency, the allowed pressure loss.


Only part of the water drawn from a boWom tank by a recircula1on pump and distributed onto the pads evaporates when the rest is recirculated.

The evapora1on process increases the concentra1on of salts which may build up on the surface, forcing to clean or replace the pads when clogged. Furthermore they should be periodically controlled because the presence of a warm water recircula7on poten7ally promotes a risky bacterial growth.

Last but not least, the air side pressure drop of the pads requires an addi7onal energy consump7on even when no humidifica7on is needed.

Their use, widespread for the limited price, should be carefully evaluated looking also at the opera1ng costs, some1mes surprisingly high.

Wet Media Humidifiers 2/2


These devices are equipped with a volumetric pump which pressurizes the water to values between 70 and 100 bar and delivers it to small nozzles that produce a fine mist (droplets of 10-‐15 micron) easily absorbed by air stream because the

surface offered by 1 liter of water atomized at 15 μm is as high as 400 m2.

PUMPING STATION ATOMIZING NOZZLE

High Pressure Atomising Systems 1/2

The distribu1on piping network that supports and supplies the nozzles is posi1oned in an air duct or placed directly into the environment to humidify.

NOZZLE RACK SUSPENDED TYPE NOZZLE RACK IN AHU SECTION


High pressure atomising systems may reach an excellent level of accuracy (± 2%) of the humidity in the controlled space and very high capaci1es with a negligible electric consump1on absorbed by the pump (<4 W per liter of sprayed water).

Under the hygienic aspect they are not cri1cal because do not promote bacterial growth; infact: §  in the case of direct atomiza1on into the environment, the sprayed

water is fully absorbed by the air; §  in ducted systems the frac1on not evaporated -‐ usually very small -‐ is

drained without recircula1on.

The use of demineralised or sweetened water is recommended to prevent clogging of the nozzles.

High pressure atomising systems are available for capacity up to many thousands of kg/h.

High Pressure Atomising Systems 2/2


Ultrasonic Humidifiers

STAND ALONE UNIT

SMALL SIZE UNIT

DUCTED TYPE UNIT

Ultrasonic humidifiers provide an extra fine atomiza1on of water (≈ 3 μm) by means of the high-‐frequency vibra1on (close to 1,7 Mhz) of a piezoelectric element (or more than one, in parallel); the absorp1on of vapor is immediate due to the wide interface surface (2000 m2 offered by 1 liter of water atomized at 3 μm).

Due to size and cost they are convenient for small and medium installa1ons (0,5 to 15 kg/h).

The use of demineralised water is highly recommended.

Best ultrasonic humidifiers reach excep1onal levels of precision (± 1%) in the en1re range of their rated capacity and, thanks to the high efficiency of absorp1on, they are suitable for the distribu1on of the produced mist directly into the room as well as in ducted systems.


To Conclude, let Us Men7on a Few Case Studies


Evapora7ve Cooling: Datacenter Applica7on

The need: humidity control and evapora1ve cooling

A company has a big data center in Middlesbrough (Newcastle-‐ UK). It has more than 180 global data centers and IT service companies.

Data hall

Atomizing nozzles

Hot exhaust air

to data centre

Coniugated

dampers


Internal Views


Evapora7ve Cooling: Air Cooled Heat Exchangers

In aircooled heat exchangers (i.e.: condensers, radiators, etc.) the intake air is adiaba1cally cooled to improve the performance in hoWest periods. Water may be sprayed in excess in order to wet the finned coil so promo1ng a further evapora1on during air hea1ng along the exchanger.


COMPARATIVA ESTACIONES 4/09/06

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Tª EXT. PEÑAG. Tª EXT. ILUST. Tª AMB. PEÑAG. Tª AMB. ILUST. Tª IMP. PEÑAG. Tª IMP. ILUST.HR EXT. PEÑAG. HR EXT. ILUST. HR AMB. PEÑAG. HR. AMB. ILUST. HR IMP. PEÑAG. HR IMP. ILUST.

El e cp a S.L.Instalaciones y Control

The aim of this solu1on is to provide more comfort, cooling the environment using water as a “source of power”, because it’s considerably more economic than tradi1onal cooling systems (direct expansion) as it consumes less power.

Evapora7ve Cooling: Subway Applica7on

Number of pla�orms: 2 Q = 90,000 m3/h for each pla�orm (ven1la1on) Outdoor air = 100% Discharging air condi1ons: 27-‐28°C/70-‐80% r.H.

Result: In the period 15/july/2006 to 15 sept/2006, the temperature in this sta1on was 3.4°C colder than in other comparable sta1ons.

Peñagrande subway sta1on, Madrid


Need: efficiency improvement of a new gas turbine for produc1on of electricity

Petrochemical complex With 23 plants, they operate 400,000 barrels per day of crude oil, produce 18.4 million tons per annum (mpta) of petroleum-‐based products and 2.4 mpta of ethylene and propylene-‐based deriva1ves

Evapora7ve Cooling: Industrial Applica7on

Technical note: Cooling the combus1on air ingested by the turbine – even by a few degrees – can increase power output substan1ally. This because cooled air is denser and therefore gives the turbine a higher mass-‐flow rate and pressure ra1o, resul1ng in increased turbine output and efficiency – as much as 1 % per degree Celsius.

Varia1on of the performance of a gas turbine vs. air intake temperature


The solu7on Project condi7ons: Airflow : 80.000 m3/h From 43°C and 20% R.H. Desired 25 °C with max 85% R.H. Total Rack Capacity : 690 l/h

Turbine Evapora7ve Cooling Diagram


Humidifica7on in a Music Hall in Athens The needs 1)  Control humidity level during all seasons, ie.. instruments made of wood are the most affected and come into contact with non-‐wood pieces, string instruments (guitars, violins, etc.).

2)  Changes in humidity cause the detune problems to singers, during a performance. Room environment must be at show1me condi1ons before musicians being warming up.

The solu7ons Music hall: 4 – adiaba1c mul1zone Master sta1on 10 – adiaba1c mul1zone Slave sta1on 14 – distribu1or rack 14 – drop separators Library: 2 – adiaba1c mul1zone Master sta1on 6 – adiaba1c mul1zone Slave sta1on 8 – distribu1or rack 8 – drop separators

Greek Na1onal Opera, Athens


CAR FACTORY Humidifica1on in most automo1ve paint booths has tradi1onally been accomplished by water spray coils or wet-‐media located in the air houses serving the paint booths.

The needs Desired stable paint booth condi1ons at 65 to 75°F & 65 to 75%rH. The pain1ng booths are supplied with permanently condi1oned air by a ducted system.

Humidifica7on in Water-‐Borne Pain7ng Booths

The results: the system has operated with a precision previously unknown in this industry, achieving set point in 10 minutes from cold startup. From the actual performance graph, from a cold start, the system comes into specifica1on within 10 minutes and then maintains ±1°F and ±2%rH. The old, simple cardboard pads will no longer provide the precision and reliabil ity demanded.


Humidifica7on in a Museum in Venice The need Temperature and Humidity control inside the various rooms with 1ght Temperature and Humidity set point range (24°C Temperature – 50% rH with ± 5% tolerance).

The installed system It was chosen a Direct Expansion Units (Mul1func1on air/water cooled units for climate control + Fan Coils) with its regula1on system for temperature control, temperature and Humidity values recording and remote management via Internet access. Due to historical architecture of the building, it was not allowed the installa1on of water piping for hydronic systems.

Technical Solu7on: Air/Water Units – Fan Coils – Ultrasonic Humidifiers

AG150A

fan-coil

Bus

M-N

et fan-coil

gateway

Ethernet (cross cable)

fan-coil

BU

S

SU

PE

RV

ISIO

N

BU

S pLA

N

BUS GATEWAY

BUS HUMIDIFIERS

RS485 bus GATEWAY RS485 bus pLAN RS485 bus HUMIDIFIERS RS485 bus SUPERVISION

Results •  Reduced energy requirements: 60W per litre of spray per hour, corresponding to about 7% of the energy consump1on of a tradi1onal humidifier.

•  Use of demineralized water eliminates the problem of bacteria improving the air quality.

•  The adiaba7c humidifica7on process decreases the temperature of the air in summer7me, thus reducing the ac7vity of the compressors and saving energy.

•  Extremely fine droplet spray: the water is finely sprayed into extremely small droplets (few microns) easily and quickly absorbed by the air.


Luigi Nalini, Speaker [email protected]

Energy saving through evaporating cooling in comfort and industrial applications

Engineering

Transcript of Energy saving through evaporating cooling in comfort and industrial applications