Air Conditioning System Using Evaporative Cooling

Published Date: 02 Nov 2017

This paper is s study on an innovative approach for efficient air conditioning system using evaporative cooling. The system uses only water and dry air as fluids for air conditioning. The whole idea evolved during the air flushing activity of the pipeline in C2C3 Plant as part of pre-commissioning activity.

Flushing of pipeline was carried out using air with a dew point of -50 degrees C. Means this is a very dry air. It was observed that, when dry air was passed through a pipeline having some residual water left, it caused chilling effect, as a result, the out coming air was found to be very chilled. Also there was some sweating on the pipe line which means the water inside the pipeline is also getting chill.

From this phenomenon an idea was conceived that if a very dry air is passed through water with sufficient contact time, it can be used for air conditioning effect.

The air carrying moisture with it will still be not very humid as initially it is very dry. The beauty of the process lies in the fact that only at the cost of dryness both water as well as air is getting cooled. This can be effectively used in a closed water loop and there would be no use of any refrigerant.

2. Basic principle involved

Bulk air [Temp, Hv (Enthalpy)]

Hi

Water drop

Hi = Enthalpy of water drop

Interface.

Fig1: Micro level heat transfer model of a water droplet

Modeling of process inside chiller is being carried out where heat transfer accompanied by mass transfer takes place.

There are three parallel heat transfer process going on here:

1) Latent heat transfer owing to vaporization of water. This is the energy involved in phase change.

2) Sensible heat transfer owing to temperature difference between air and water. This is the smallest part of heat transfer.

3) Heat transfer accompanied by mass transfer (diffusion) of cold water to air. This becomes the major mode of heat transfer once equilibrium is established.

Referring to the above fig 1,

As vaporization occurs it causes a loss of energy in the water and that is being gained by air, the whole being a closed adiabatic system. This causes water to cool sharply. Now consider a situation where mass transfer is high. This cool liquid vaporizing to air will carry cold energy along with it. This is what we have referred in (3) mode. The mass transfer of cold liquid from water stream to air stream is causing increase in humidity as well as cooling of air.

In this modeling we assume the water droplet to be surrounded by a thin film of air and enthalpy difference (Hi – Hv) between the film and the surrounding provides the driving force for the cooling process. This H being a function of humidity(x) as well as air temperature (T) gives a combined effect of both modes of heat transfer.

3. PROCESS FLOW DIAGRAM

A small process flow diagram of the evaporative cooling is shown below.

Fig2: Block diagram of process

Air in (1)

Water line

Chiller Air

38 deg C

RH=60%

Ambient

Air Out (2)

65 degC, 2.2kg/cm2

RH=60-80%

Chilled (4)

1.8kg/cm2 kg/cm2

RH= 45-60%

T= 17-27 degC

(a)

1.8kg/cm2

Dry Air(3)

T=50-55 degC

Dryer

RH=2-4%

Dp= -7 to +5degC

1.9kg/cm2 22-26 degC

(b)

Make up Water (e)

30degC;

0.48-0.57m3/hr

Heat Exchanger

(f)P=2.0kg/cm2

(5)

1.6 Kg/cm2 (c)

Suction P= 1.8kg/cm2

Water to be recycled

15-25 degC;

RH=45-60%

Air in to chiller

Air out from Chiller

Water circulation rate(m3/hr)

Make up water

Reqd.(m3/hr)

Air flow rate(scfm)

T= 50-55degC

Dew point= -7 to +7degC

RH = 2-4 %

T=17-27degC

RH = 45-60%

1.0

0.48-0.57

20,000

(C2-C3 control room)

Design basis: Equivalent to process Control room HVAC system

Air blower is used which compresses the air to 2.2kg/cm2.This is essentially a high volume low pressure centrifugal type blower. Above we have shown the operation for ambient temp 38degC and absolute humidity 60%.When passed through the dryer (silica gel) the humidity reduces to 2-4% relative humidity and temp reaches approx 50-55degC. Silical gel or any low cost dryer is used because our dryness requirement is less. Even a dry air with dew pt -7 to +5 degC will be good enough for this process. Next this dry air is passed through an adiabatic chiller where water splashes over the dry air for interaction and water particles diffuses into air thereby increasing its humidity. This chiller is a normal spray tower with a high equivalent mass transfer interaction area. The number of equivalent equilibrium stages is 3.The air coming out of chiller is moderately chilled and having relative humidity(RH) of 45-60%(For a normal HVAC system comfortable zone of operation is between 45-60% relative humidity)

Humidity and temperature of the outlet can be controlled by mass flow rate of water. The beauty of the process is that both the water as well as air gets cooled due to loss of enthalpy of vaporization and bulk movement of water droplets from water to air.

At the final stage of the process we have used a heat exchanger so as to utilize the cold energy of chilled water coming out of chiller. This ensures water to run in closed loop for cold energy utilization.

4.Thermodynamic feasibility

Chilling

Pressure

(Kg/cm2)

Dryer inlet

Blower inlet

Temperature (degC)

Fig 3: P-T curve for the whole process

For the above mentioned P-T diagram, thermodynamical study for the entropy change was done for the adiabatic chilling part.

Entropy change for the above mentioned process was derived as follows: ∆S (change in entropy) > 0 and ∆H =0;

As there is a net increase in entropy, this proves that this process is thermodynamically feasible process.

5.Specifications:

Basis: Air flow rate of 20,000 scfm.

Size of conditioning area = Process control room, C2C3 Plant Dahej

Specifications of the major equipements used for the above mentioned process:

1. Centrifugal type air blower:

Volume of air per min at fan outlet conditions = 20,000cfm.

Total pressure = 2.2 kg/cm2.

Velocity pressure =0.1 kg/cm2.

Static pressure= 2.1kg/cm2.

2. Dryer design requirements:

Drying from RH 70% at 65 deg C to RH=2-4%, 55degC

Pressure drop across it = 0.3 kg/cm2.

3. Chillers: Spray type chillers

No. of equivalent equilibrium stage= 3;

Pressure =1.8kg/cm2;

SIMULATION BLOCK(using ASPEN 11.1)

Fig 4: Block arrangement of simulation

As there is no standard air - water contacting chiller shape available in ASPEN,the simulation was carried using a RADFRAC standard vessel with zero recondensor and reboiler duty. By troubleshooting, optimum number of equilibrium stage was found to be 3.

FOR INLET CONDITION-1(Normal operation)

Air in to chiller: Chiller pressure= 1.8 kg/cm2;

Dp = 0 degC;

Temperature = 55degC

Temperature (degC)

Relative humidity(%)

Fig5: Variation of Air-out humidity with Air-out temperature

FOR INLET CONDITION-2

Air in to chiller: Chiller pressure = 1.8 kg/cm2;

Temperature= 55degC;

Humidity (Dp) = +7 degC;(Not so dry air)

Fig6: Variation of Air-out humidity with Air-out temperature

Keeping chiller P=1.8kg/cm2

Fig 7: Variation of Air-out temperature with Chiller pressure

For Air–in [Humidity (Dp = 7 degC); Temp=55degC]

Fig 8: Variation of Relative humidity (%) of Air-out vs Chiller pressure (kg/cm2)

for Air-In [humidity (DP = +7degC); Temp=55degC]

Fig9: Simultaneous dependence of

Chiller Pressure(kg/cm2) – Air out RH(Dp)- Air out Temp(degC)

[For Air inlet to chiller: Dp= +7degC; Temp =55 degC.]

APPENDIX: Snapshots of Simulation inputs and results