Published on November 14, 2016
1. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” Luigi Nalini, Speaker firstname.lastname@example.org Energy saving through evapora7ng cooling in comfort and industrial applica7ons
2. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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.
3. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
4. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 0 5 10 15 20 25 30 35 40 45 50 14000 12000 10000 8000 6000 4000 2000 0 temperature -‐ °C Vapor pressure -‐ Pa 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
5. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 pressure of the several gaseous 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
6. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 0 1000 2000 3000 4000 5000 6000 0 5 10 15 20 25 30 35 40 temperature -‐ °C Vapor pressure -‐ Pa 10% 25 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
7. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
8. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
9. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” Isothermal Humidiﬁca1on Adiaba1c humidiﬁca1on Air Hea1ng Air Cooling Temperature Enthalpy Absolute Humidity Rela1ve Humidity 0 5 10 15 20 25 30 0 5 10 15 20 25 30 35 40 30 15 10 5 0 25 20 20 100 70 80 90 60 30 40 50 DRY BULB TEMPERATURE -‐ °C ABSOLUTE HUMIDITY – g/kg PATM = 101.325 Pa 10% Process Trend Psychrometric Chart and Basic Processes ISENTHALPIC LINES
10. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 [%] 100% 60% 40% 80% 20% PVS – PV [Pa] 4 2 6 0 Qsp [J/kg of air] 10 5 15 0 20 saturation Vapor pressure deﬁcit Air rela7ve humidity Cooling eﬀect ON GOING EVAPORATIVE COOLING PROCESS 20,6 25 30 35 40 15 10 5 0 Vapor par7al pressure -‐ kPa absolute humidity – gv/kga 38 1 2 0 0,5 2,5 1,5 water evapora1ve cooler 38°C 20% RH 20,6 °C 100% RH Adiaba7c Cooling 2/2
11. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” However, along 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 Direct Satura7on Eﬀec7veness
12. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
13. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 T2Tamb Direct Evapora7ve Cooling from the space to the space from outdoor to outdoor
14. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 GI CD PC HU CC RC GI-‐GE 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
15. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
16. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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.
17. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
18. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
19. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
20. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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.
21. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” To Conclude, let Us Men7on a Few Case Studies
22. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
23. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” Internal Views
24. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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.
25. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” COMPARATIVA ESTACIONES 4/09/06 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 12.30 13.00 13.30 14.00 14.30 15.00 15.30 16.00 16.30 17.00 17.30 18.00 18.30 19.00 19.30 20.00 20.30 21.00 21.30 22.00 22.30 23.00 23.30 HORA ºCY%HR 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 ecpa 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
26. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
27. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
28. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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
29. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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 reliability demanded.
30. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” 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-Net fan-coil gatew ay Ethernet (cross cable) fan-coil BUS SUPERVISION BUSpLAN 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.
31. EinB2016 – 5th Interna1onal Conference “ENERGY in BUILDINGS 2016” Luigi Nalini, Speaker email@example.com