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02 August 2016

1. POLIMI laboratory test rig scheme

Indoor I : T=35°C UR=30% Outdoor A : T=30°C UR=40% Outdoor B : T=30°C UR=50% Outdoor C : T=36.8°C UR=27.2%

You can compare the outdoor conditions by : • Dry bulb temperature[°C] • Wet bulb temperature [°C] • Absolute humidity [g/kg]

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ATU

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CA

- °C

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10% RELATIVA UMIDITÀ

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90%

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5

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30 TEMPERATURA BAGNATA DI LAMPADINA - °C

30

0,78

0,80

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0,86 VOLUM

E - CUBIC METER PER KG

DRY AIR

0,88

0,90

0,92

0,94

RAP

POR

TO D

I UM

IDIT

À - U

MID

ITÀ

DI G

RAM

MI P

ER L

' AR

IA A

SCIU

TTA

DI C

HIL

OG

RAM

MO

IA

B

C

R R

NUMERO DELLA TABELLA DI ASHRAE PSYCHROMETRIC1TEMPERATURA NORMALE

PRESSIONE BAROMETRICA: 101,325 kPaCopyright 1992

SOCIETÀ AMERICANA DI REFRIGERAZIONE DEL HEATING E DEGLI ASSISTENTI TECNICI DEL AIR-CONDITIONING, INC

LIVELLO DEL MARE

The water mass sprayed is in the range 30 l/h (0.014 kg/kg) and 62 l/h (0.043 kg/kg). The following tests are carried out using nozzles of 0.8mm of diameter

2. Boundary conditions

3. Humidification efficiency before the heat exchanger

ηℎ𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢 =𝑇𝑢𝑑,𝑢𝑢 − 𝑇𝑢𝑑,𝑢𝑢𝑢𝑇𝑢𝑑,𝑢𝑢 − 𝑇𝑤𝑑

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

20 25 30 35 40 45 50 55 60 65

η_hu

mid

ifica

tion

Water sprayed [l/h]

A @ 1200 A @ 1800

The humidification measurements are made with nozzles 150mm far from the HX entrance and a plenum of 500mm of length. The nozzle are installed in counter flow configuration. Has been noticed that the plenum plays an important role in the humidification process. Bigger is the plenum higher is the water absorbed by the air.

The test of the measurement of the dry bulb of the air flow has been carried out sampling the air in 4 different points with sample tubes just before the entrance of the heat exchanger. The air is mixed in a collector and then measured.

4. Effect on pressure drop

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0 5 10 15 20 25 30 35

Proc

ess

pres

sure

dro

p [P

a]

Water flow rate [l/h]

ΔP as a function of water mass sprayed and in dry condition with vsN equal to 10.9 m/s

It possible to state the pressure drop is not affected by the presence of the thin water layer on the surface in the wet condition: the film on the plate is so thin that do not affect the frontal section of the channels

5. Configuration 1

5. Configuration 1: Effect of water nozzles arrangement

Wet bulb effectivness

Wet bulb effectiveness as a function of water mass sprayed for velocity of the secondary air stream equal to 3.7 m/s (Figure A) and 5.7 m/s (Figure B).

The counter flow arrangement is clearly more effective than the parallel flow one, due to the mixing of water droplets in the air stream, which leads to a uniform distribution of water in the heat exchanger

𝜀𝑤𝑑 =𝑚𝑝̇ 𝑇𝑝,𝑢𝑢 − 𝑇𝑝,𝑢𝑢𝑢

𝑚𝑝,̇ 𝑚𝑠̇ 𝑢𝑢𝑢𝑇𝑝,𝑢𝑢 − 𝑇𝑤𝑑,𝑠,𝑢𝑢

5. Configuration 1: Effect of water nozzles arrangement

Wet bulb effectiveness and fraction of evaporated water as a of water sprayed for different counter flow nozzles arrangement. Condition A and vsN equal to 3.7 m s-1.

Nozzle A: water flow of each nozzle equal to 3.52 l h-1 at 10 bar. Nozzle B: water flow of each nozzle equal to 7.50 l h-1 at 9 bar.

It is possible to state the wet bulb effectiveness and the ratio of evaporated vapor mainly depend on water flow rate, regardless of type and number of nozzles.

5. Configuration 1: Effect of secondary air conditions

Case A T=30°C RH=40% Case B : T=30°C RH=50% Case C : T=36.8°C RH=27.3%

ΔTp as a function of water mass sprayed for different type of nozzles and secondary air condition.

At a given water flow rate, wet bulb effectiveness is strongly influenced by secondary air condition. The dependency is not directly linked with dry or wet bulb temperature but with the combination of them. The model developed based on several tests can give the answer in al wide range of thermo hygrometric conditions typical of the Data Center application.

5. Configuration 1: Effect of secondary air conditions

Wet bulb effectiveness as a function of water mass sprayed for Condition A, vsN equal to 3.7 m/s and 5.7 m/s.

Wet bulb effectiveness as a function of water mass sprayed for condition C, vsN equal to 3.7 m/s and 5.7 m/s

An increase in secondary air flow rate leads to an increase in wet bulb effectiveness if its inlet temperature is lower than the one of the primary air stream. In case of low inlet relative humidity and high inlet temperature of the secondary air stream, the water flow rate should be increased in order to have satisfactory wet bulb effectiveness (εwb> 0.7)

6. Configuration 2

Configuration 1

The configuration with the spray from the top is more effective than the

configuration with the nozzles perpendicular to the gravity. In particular in the

second configuration, the gravity modifies the way of the spray hindering the

wetting of the surface.

At this moment we are trying to separate the phenomenon between the flow

and the gravity vectors. In this way, once got the parametric equation, it will

be possible to predict the performance of bigger plates and different

configurations.

7. Configuration 3

The gravity opposite to the air flow direction should help the maintenance of the water on the HX plates. It has seen from the outlet of the secondary air there are not droplets carried out from the HX. In this application the height of the plate is relevant on the performance, the HX works as a drop separator. In case of big plate (heat exchanger formed by 4 equals modules) the system should be switched into two subsystems to increase performance

8. Configuration 4

In this configuration the heat exchanger has been installed horizontally, with the fins that are parallel to the ground. The performance has been measured with a slightly angle first to the outside and then to inside. NO RELEVANT DIFFERENCE HAS BEEN DETECTED.

Comparing these results and the model developed it is possible to state that the whole surface is wet more than 50%. This is a condition where the outlet temperature of the primary air is very close to the asymptotic condition.

• It is important to mount the unit with a little angle (just few degrees) to help the condensate drain

• The water can stagnate inside the turbulent geometry of the fins.

• The coated surface is antibacterial

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0 0.005 0.01 0.015 0.02 0.025

ΔT_P

rimar

Y[°C

]

Water / Air [kg/kg]

TO OUT A @ 1200 TO IN A @ 1200

9. Configuration 1, 2, 3 and 4, a comparison • A (30°C 40%) @1200m3/h

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0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350 0.0400 0.0450 0.0500

ΔT_p

rimar

io [°

C]

portata di ACQUA/portata di ARIA [kg/kg]

SIDE_1200 Orizzontale_1200 TOP_1200 BOTTOM_1200

9. Configuration 1, 2, 3 and 4, a comparison • A (30°C 40%) @ 1800m3/h

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0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350

ΔT_p

rimar

io [°

C]



9. Configuration 1, 2, 3 and 4, a comparison • C(36°C 27%) @ 1200m3/h

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0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350 0.0400 0.0450 0.0500

ΔT_p

rimar

io [°

C]



9. Configuration 1, 2, 3 and 4, a comparison • C (36°C 27%) @ 1800m3/h

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0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350

ΔT_p

rimar

io [°

C]



10. Others arrangements considerations

This arrangement doesn’t work well because the force of gravity is unable to combat the buoyancy force of the drop inside the air flow, the droplets are carried away by the air.. Tests have been performed with nozzle of 0.8mm of diameter maximum and low pressure adiabatic system.

The distance of the nozzle is empirically under investigation but we can make some considerations. • The distance of the nozzles influences less the performance when the water spray is counter flow. • Higher is the temperature of the secondary air flow better is to cool down the air before the

entrance in the HX • With lower temperature of the secondary air flow than the primary air, as there is not he risk to

heat the primary air, the nozzle can be installed near the frontal area of the HX to reduce the water consumption maintaining a good performance

11. Wet (configuration 1) and dry surface: a comparison

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ΔTp

[°C]

Saturation Efficiency adiabatic system [%]

Wet - Ts30°C, URs40% Dry - Ts30°C, URs40%

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ΔTp

[°C]



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ΔTp

[°C]



The comparison is made with balanced air flow, v=3.7m/s and primary inlet condition equal to 35°C The wet surface increases the performance of the system. With higher temperature and lower relative humidity of the secondary air the difference between the two system decreases but the advantage of the wet surface still remains high. With the wet surface, in these conditions, the approach temperature is always lower than 1.5°C compared to the dry surface (90% of efficiency of the adiabatic system) In the second case (36°C, 27%) the amount of water evaporated in dry condition is 10l/h with 90% of adiabatic efficiency while the consumption of water of the wet system is 30l/h where the evaporated ratio is 40% (13l/h)

Different types of water can be used

The bacteria growth is blocked by the presence of an antibacterial element in the protection film. Never had problem on this topic. The type of water does not affect the performance.

Water Cost Maintenance Heat exchanger

Mains Cheapest It is better to use bigger nozzle and to check and clean very often the rack

Aluminum plate could be used. Side plate with epoxy protection

Softened Average price.

With softened water some dust can settle on the plate. Maintenance should be made often

Protection on the aluminum plate it is needed as epoxy protection on the side plate

Deionized Higher price. Could be 5 times higher than the mains water.

Lowest maintenance Deionized water is acid. Aluminum protection it is needed to avoid the fretting.

12. Water considerations

13. Plate material considerations The test has been carried out with mains water with an airbrush Pictures are made after 15s of spraying

Epoxy protection (hydrophobic) Hydrophilic and adsorption coating

The hydrophilic material has the capacity to keep the film on the plate and, with the air flow, to create an even thin water film (test is expected to be carried out at the end of February).

The sample (100x100mm) has been weighted at time zero, immersed in water for 1minute, left to dry for 1 minute and reweighted.

Material Coating adsorption gh2O/mq F18 2000 N (kg h2o) Hydrophobic 4.4 3.3

Hydrophilic and adsoprtion - BBlue 17.3 12.9

The Hydrophilic coating has been tested through EIS to evaluate its resistance in an high corrosive environment. The results show that the coating has got an higher capacity to resist to corrosion than the standard epoxy protection.

13.Plate material considerations Electrochemical impedance spectroscopy (eis) of the hydrophilic coating

14. Mathematical model developed: description

�̇�𝐸𝐸, ℎ𝐸𝐸, 𝑥𝐸𝐸 �̇�𝑤, ℎ𝑤

�̇�𝐸𝐸, ℎ𝐸𝐸, 𝑥𝐸𝐸 �̇�𝑤, ℎ𝑤

�̇�𝑃𝑃𝑃, ℎ𝑃𝑃𝑃


(𝑖 − 1)

(𝑗 − 1)

(𝑖)

(𝑗)

EXAUST AIR (Wet channel)

PRODUCT AIR (Dry channel)

𝑇𝑃𝑃𝑃

𝑇𝑀𝑀𝑀,𝑃𝑃𝑃

𝑇𝑀𝑀𝑀,𝐸𝐸

𝑇𝑤

𝑇𝐸𝐸

𝑑𝑑𝑗

𝑑𝑑𝑢

𝐴

�̇�𝐸𝐸, ℎ𝐸𝐸, 𝑥𝐸𝐸

�̇�𝑤, ℎ𝑤


Hypothesis 1. Steady state, no internal generation 2. Channel height << channel length 3. Fully developed flow 4. Uniform passage geometry throughout the heat exchanger 5. 𝑇𝑤=𝑇(𝑀𝐴𝑇,𝐸𝑋)

�̇�𝑠 ∆𝑥𝑠λ + 𝑐𝑐 𝑇𝑠,𝑢𝑢 − 𝑇𝑠,𝑢𝑢𝑢 =�̇�𝑝𝑐𝑐 𝑇𝑝,𝑢𝑢𝑢 − 𝑇𝑝,𝑢𝑢

Energy balance

The model has been calibrated and tested on 112 different tests

14. Mathematical model developed: water distribution

Percentage of wetted surface on the plate with Vsn equal to 5.7m//s and 50.1l/h with configuration 1

A parametric equation for the wetting of the plate has been found. The surface is never completely wetted but the percentage decreases with the air velocity and increases with the amount of the water sprayed. To increase the water layer on the heat exchanger new configuration and materials have to be investigated.

Velocity Temperature Absolut Umidity Water sprayed

Primary 3.7 m/s 35 °C 10.5 g/kg

Secondary 5.7 m/s 36.7 °C 11.1 g/kg 55.1 l/h

Tp,out 25.9 °C

𝜀_𝑤𝑏 71.8 %

ΔTp 9.1 °C

Ta 3.5 °C

Output

Input

Parity plot of the model

Temperature profile of the primary air, secondary air and wall on plate heigth and length

14. Mathematical model developed: temperature profile

http://authors.elsevier.com/a/1SsUT1M7zGoqpu

http://authors.elsevier.com/a/1SsUT1M7zGoqpu

To sum up • The counter flow arrangement is more effective than the parallel flow • The effectiveness mainly depends on the water flow rate rather than the nozzles

number and size • The secondary air conditions affect the primary supply temperature but the

dependency is not directly linked to the T and UR. • Attention must be paid when Ts>Tp • There is no relevant increase of the pressure drops when the surface is wet • The wet plate can reach lower approach temperature than the dry plate • The horizontal configuration enhances the performance of the system. The

difference is relevant with higher temperature of the secondary air; • The side configuration is always lower than the humidification from the top; • Due to the turbulence before the heat exchanger, the humidification efficiency

can reach really high values (60% .. 80%). • The hydrophilic coating has got a capacity to resist to corrosive at least equal to

the epoxy protection

Next steps • Dynamic tests about the hydrophilic adsorption material • Testing the influence of the fin spacing on the performance • Release in the Rex software the new DLL on the indirect evaporative

cooling process

Thanks for the attention!

presentazione standard di powerpoint - recuperator · 82 84 r 88 90 92 94 a i rapporto di umiditÀ...

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