International Conference on Mechanical, Industrial and Energy Engineering 2014 26-27December, 2014, Khulna, BANGLADESH

ICMIEE-PI-140254

Design, Construction and Performance Test of Generator and Condenser for a Small Capacity Vapor Absorption Cooling System Faisal Ahmed, DipayanMondal, Mohammad Ariful Islam Department of Mechanical Engineering, Khulna University of Engineering & Technology, Khulna-9203, BANGLADESH

ABSTRACT The most commonly used refrigerant-absorbent pair is water/Li-Br where water acts as the refrigerant and the Li-Br acts as the absorbent. Generator and condenser of a small capacity Li-Br/water vapor absorption system was designed and constructed to compare actual performance with designed conditions, of 2 kW capacity and 50% Li-Br solution at constant pressure 9.66kpa. Properties of water/Li-Br solution are evaluated from standard curve fitting system. Construction material for condenser and generator coil are copper tube and rectangular stainless steel chamber for corrosion resistance and the system is kept under 9.66kpa pressure with absolutely leak proof and well insulated. The system was run for about 100 minutes. The performance is tested for variable mass flow rate and constant mass flow rate of water at the generator and condenser. The obtained minimum temperature is 25oC which avoid crystallization temperature of Li-Br solution. Key Words: Vapor absorption, Generator, Condenser, water/Li-Br refrigerant, intermittent cooling.

1. Introduction An absorption refrigeration system uses heat source to provide the energy needed to drive the cooling system. Absorption refrigeration system is a popular alternative to regular compressor refrigeration system .Generator and condenser are two major part of an absorption system. Generator is used to supply heat to the refrigerant water and the absorber. This generator is varying for the cooling effect of the absorption system. Heat is supplied to the refrigerant water and absorbent lithium bromide solution in the generator from the hot water. The water becomes vaporized and moves to the condenser, where it gets cooled. As water refrigerant moves its pressure is reduced along with the temperature. This water refrigerant then enters the evaporator where it produces the cooling effect [1].A condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically cooling it. The latent heat is given up by the substance, and will transfer to the condenser coolant. The condenser water is used to cool the water refrigerant in the condenser and the water-Li Br solution in the absorber. 2. Working Procedure of a Single Effect Li-Br/Water Cooling System A single-effect, Li-Br /water cycle is illustrated in Fig (1). With reference to the numbering system shown in figure, at point (1) the solution is rich in refrigerant and a pump forces the liquid through a heat exchanger to the generator (3). The temperature of the solution in the heat exchanger is increased. In the generator thermal energy is added and refrigerant boils off the solution. The refrigerant vapor (7) flows to the condenser, where heat is rejected as the refrigerant condenses. The condensed liquid (8) flows through a flow restrictor to the evaporator (9). In the evaporator, the heat from the load evaporates the refrigerant, which flows back to the

absorber (10). A small portion of the refrigerant leaves the evaporator as liquid spillover (11) which is pumped back to the evaporator inlet again. At the generator exit (4), the steam consists of absorbent-refrigerant solution, which is cooled in the heat exchanger. From points (6) to (1), the solution absorbs refrigerant vapor from the evaporator and rejects heat through a heat exchanger. At point (1) the solution is reach in refrigerant and a pump (2) forces the liquid through a heat exchanger to the generator (3). The temperature of the solution in the heat exchanger is increased [4].

Fig.1 P-T diagram of LiBr-water absorption cooling cycle. 3. Design of a single effect Li-Br/Water absorption cycle system To perform designing of equipment size and performance evaluation of a single-effect Li-Br/water absorption cooler basic assumptions are made. The basic assumptions are:  The steady state refrigerant is pure water.

Corresponding author. Tel.: +88-01922988336 E-mail address: [email protected] ICMIEE-PI-140254-1

     

There are no pressure changes except through the flow restrictors and the pump. At points 1, 4, 8 and 11, there is only saturated liquid. At point 10 there is only saturated vapor. The pump is isentropic. There are no jacket heat losses. The capacity of the system is 2kW.

Table 1 Design parameters for the single effect LiBr/water absorption cooler Parameter Symbol Value Capacity Qe 2 kW Generator solution exit T4 80 0C temperature Weak solution mass X1 50% Li-Br fraction Strong solution mass X4 55% Li-Br fraction Solution heat exchanger T3 65 0C exit temperature Generator vapor exit T7 80 0C temperature Table 2 Data for system Point h (kJ/kg) 1 92.4 2 92.4 3 145.4 4 212.2 5 154.3 6 154.3 7 2628 8 185.3 9 185.3 10 2519.2 11 40.35

single effect Li-Br/water cooling m (kg/s) 0.0129 0.0129 0.0129 0.0118 0.0118 0.0118 0.00017 0.00017 0.00017 0.0009 0.0002

P (kPa) 1.227 9.66 9.66 9.66 9.66 1.227 9.66 9.66 1.227 1.227 1.227

T ( 0c) 34.9 34.9 65 90 59.93 44.5 85 44.3 10 10 10

X (% LiBr) 55 55 55 60 60 60 0 0 0 0 0

Table 3 Energy flows in generator and condenser of the system Description Symbol KW Capacity Qe 2 Heat input to the generator Qg 3.45 Condenser heat rejected Qc 2.62 4. System heat exchangers sizing Equations In the heat transfer analysis, it is convenient to establish a mean temperature difference (ΔTm) between the hot and cold fluids such that the total heat transfer rate Q between the fluids can be determined from the following expression:

Where, A (m2) is the total heat transfer area and U (W/m2-°C) is the average overall heat transfer coefficient, based on that area. (

)

⁄

(2)

F= Correction factor. The overall heat transfer -coefficient (U) based on the outside surface of the tube is defined as [6] ⁄

⁄

⁄

⁄

⁄

⁄

(3)

For the design of the heat exchangers, the cooling water inlet and outlet temperatures are assumed. The cooling water inlet temperature depends exclusively on the available source of water, which may be a cooling tower or a well. The Petukhov-Popov equation [5] or turbulent flow inside a smooth tube gives: ( ) Nu =

(4) ( )

Friction Factor, f = [1.82 log (Re) -1.64]-2 Constant k1=1.34; k2= 11.7+ Nusselt’s analysis of heat transfer for condensation on the outside surface of a horizontal tube, gives the average heat transfer coefficient as [6] [

]

(5)

The logarithmic mean temperature difference is [6] (6)

For the average Nusselt number Churchill and Chu proposed [6] a correlation in free convection boiling regime on horizontal tube. The correlation is: (7) here range

[10-4

1012]

Num and RaD are based on pipe diameter. Namely

(1)

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Table 4 Design Parameters for generator & condenser Tube Outer diameter Do= 12.7 mm dimension Inside diameter Di=10.7 mm Chamber Pressure 9.66kpa (Vacuum) Cooling water Inlet=25°C inlet Outlet=28°C temperature Condensed From 80°C to water 44.3°C Condenser temperature Mass flow rate 0.21 kg/s of cooling water(m) Condensed 0.00107 kg/s water mass flow rate Load 2.62kW Entering: 50% Generator LiBr at 65°C solution Leaving: 60% Generator LiBr at 80°C Generator water vapor mass flow 0.16507 kg/s rate (m) Load 3.45 kW 5. Condenser Design The overall heat transfer coefficient is given by Eq. (3) For this equation, the value of the fouling factors (Fi,FO) at the inside and outside surfaces of the tube can be taken as 0.00009m2°C/W [6] and k for copper = 383.2 (W/m-°C). The heat transfer coefficients, hi ,ho, for the inside and outside flow need to be calculated. The Petukhov-Popov equation Eq.4 applies for Reynolds numbers 104
ν = 0.8365×10-6m2/s Pr= 5.85 µ= 0.86×10-3 kg/m.s

Qc equals to 2.62 kW, therefore

kl= 0.613 W/m-°C μl= 801.4×10-6kg/m-s Tv= 45.01°C

= 997kg/m3 3 = v 0.04125 kg/m Tw = 26.5°C Do = 0.0127 m

hm= 9958.03 W/m2-°C By substituting the above values in Eq. (3) a resulting overall heat transfer coefficient of U=1657.932W/m2-°C is determined. Finally, LMTD from Eq. (6) The tube length L is determined by writing an overall energy balance, [6]

Which gives, L=2.14 m 6. Generator Design The generator provides sensible heat and latent heat of vaporization. The heat of vaporization consists of the heat of vaporization of pure water and the latent heat of mixing of the liquid solution [15]. Typically, the heat of mixing is about 11% of the heat of vaporization for water/ lithium bromide. For designing of the generator following designing parameters are used as shown in Table 4.The heat transfer coefficients, hi,hsfor the inside and outside flow need to be calculated. Assume, Hot water inlet T1= 90 Water outlet at T2= 85

Here, Solution TempT3=Tsol= 65 Refrigerant TempT4=80

The Eqn.(4) for turbulent flow inside a smooth tube Reynolds numbers 104
Mass flow rate (m) = Reynolds Number, Re =

= 0.16507 Kg/sec = 62160.15

So,f= 0.01992;k1= 1.0677;k2= 13.151; Nu= 207.84hence, hi=13105.28 w/m2 Substituting the above values into Eq. 4 and replacing NuD= hiDi/ K, gives hi = 11162.53 W/m2°C. The physical properties in Eq. (5) should be evaluated at the mean wall surface and vapor saturation temperature. The average temperature of the condensate film is (45.01+26.5)/2=35.75°C and its physical properties are,

Water-LiBr Solution Properties: Percentage of water-LiBr solution X = 50% Pressure P=9.66kpa andSolution Temp T3= 65

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Density: Here X0 = X/100 = 0.5 Tsol = 65 ( ) = 1145.36 + 470.84 Xo + 1374.79 Xo2 – (0.33 + 0.57 Xo) (273 +T)=1515.165 Kg/m3 Absolute Viscosity: Range 45 %
so, Specific Heat :

= Thermal Conductivity: Solution Temperature Tsol=(65+273) K =338K For T ;

so,

Table 5 Obtained heat exchanger size of generator & condenser Parameter Generator Condenser Tube length 5.77 m 2.14 m Tube material

Copper

Copper

7. Construction of the Generator and Condenser Heat exchangers are constructed according to the design. A stainless steel sheet made structure is providing enclose for condenser and generator assembly. Copper tubes are used for construct the generator and condenser coil. 7.1 Generator To reduce the complexity of the system a vacuum box (made of galvanized iron plates) is used as an alternative of generator which has capacity of storing Li-Br/Water solution under desired vacuum pressure and temperature. Li-Br/Water solution should be feed manually into this to run the whole system. The generator coil is made of copper tube of 5.77 m. Length and width of the generator coil are30.48 10.16 cm. Tube spacing is 2.54 cm. Tube spacing are obtained by using several number of copper U bend. Copper tube and U bend are connected by gas welding.

=

For the average Nusselt number Churchill and Chu proposed [6] a correlation in free convection boiling regime on horizontal tube. So for Eq. (7) Prandlt no =7.009 Kinematic Viscosity Mean wall surface Temperature = 65 and Film Temp

Hence Rayleigh number Average Nusselt number So, heat transfer co-effiient on out side of tube and overall heat transfer coefficient of is determined.

Fig.2 Photographic view of the Generator Coil

7.2 Condenser: For the purpose of running a vapor absorption system a condenser is a must. Condenser condenses vapor obtained from. Condenser is made of copper tube of 2.14 m length. Tube spacing is 2.54 cm. Tube spacing are obtained by using several number of copper U bend. Copper tube and U bend are connected by gas welding.

LMTD is The tube length L is determined by writing an overall energy balance, [6]

Which gives, L=5.77 m

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Rate of Condensation (gm/sec)

0.1 0.08 0.06 0.04 0.02 0 0

0.05

0.1

0.15

0.2

Mass Flow rate of Hot water (Kg/sec)

Fig.5 Variation of rate of condensation with hot water mass flow rate

7.3 Chamber construction The chamber is made of stainless steel sheet of 2mm thickness. At first the rectangular structure of the box is made of 0.5 inch stainless steel square bar. Than the sheet are welded around the box. Water should be fed manually along the tube to run the whole system. A thermocouple arrangement and vacuum pressure gauge is installed for taking any adjustments and measurements. Fig shows various component of the system.

The vaporized water from the Li-Br solution has much higher temperature than the condenser fluid. The inlet temperature of the condenser is at the atmospheric temperature. The vaporized water condenses in the condenser by rejecting heat. The heat is rejected to the condenser fluid. The increased flow in the condenser increases the amount of condensing rate. As a result the amount of condensate increases. Rate of Condensation (gm/sec)

Fig.3 Photographic view of the condenser coil

0.1 0.08 0.06 0.04 0.02 0 0

0.05

0.1

0.15

0.2

Mass Flow Rate of Cold water (kg/sec)

Fig.6 Variation of rate of condensation with cold water mass flow rate

Fig.4 Various Component and water circulation system assembled inside the Chamber.

The increase in the mass flow rate increases the amount of heat rejection by generator. As a result more water content from the Li-Br solution gets vaporized. This causes the increase in density of Li-Br solution. Because of being a salt particle, Li-Br present in the solution doesn’t get vaporized. It remains constant while the whole solution is reducing. As a result the weight percentage of the Li-Br present in the solution increases with the increase of mass flow rate inside the generator.

Wt.% of Li-Br Solution

58.00%

8. Result and Discussion Variation of rate of condensation with hot water mass flow rate is shown if Fig. 6. The amount of condensate is dependent on the change of mass flow rate of the generator. As the amount of mass flow rate increases, the amount of condensate also increases. The increase in the mass flow rate in the generator increases the amount of heat rejection. As a result, amount of accumulated condensate increases. The increasing mass flow rate of hot water in generator causes the water present in the Li-Br solution to vaporize in large amount which then condenses in the condenser due to absorption of heat by the condenser.

56.00% 54.00% 52.00% 50.00% 0

0.05

0.1

0.15

0.2

Mass flow rate of Hot water (Kg/sec)

Fig.7 Wt. % of Li-Br Solution against mass flow rate of hot water The amount of heat rejection is dependent on the inlet temperature. The high inlet temperature causes more heat rejection by generator. The more heat rejection causes more water present in the Li-Br solution to ICMIEE-PI-140254-5

Wt.% of Li-Br Solution

vaporize. This increases the density of Li-Br solution. As a result the weight percentage of the Li-Br solution increases with the inlet temperature. 57.00% 56.00% 55.00% 54.00% 53.00% 52.00% 51.00% 50.00% 60

65

70

75

80

Inlet Temp. of hot water (ºC)

Fig.8 Wt. % of Li-Br Solution against Inlet Temperature of Hot water variable

0.1

constant

9. Conclusion A generator and condenser assembly for a small capacity vapor absorption refrigeration system has been designed in this project. Then heat exchangers used for each component are designed. Heat exchanger tube material and dimensions are determined. The unit designed is constructed and each heat exchanger is adjusted to the required output. In this way the designed parameters are ensured. Some performance test is made to evaluate performance of the system. It is so important to maintain each component at required pressure. Experimental result shows that the constructed generator and condenser can able to perform the required purse satisfactorily. 10. Nomenclature cp : specific heat at constant pressure, kJ･kg-1･K-1 Qc : condenser Load KW h : specific enthalpy, kJ･kg-1 p : saturation pressure, kPa : density kg･m-3

0.09

Rate of Condensation (gm/sec)

0.08

11. References

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0

2

4

6

Generator Heat Rejection for variable and constant mass flow rate (KW)

Fig.9 Comparison of the amount of condensate against generator heat rejection between variable and constant condition. From the above graphical representation it can be seen that the amount of accumulation of condensate corresponding to the amount of heat rejection of generator for variable mass flow rate is very much same as for constant mass flow rate. Although it is seen that amount of heat rejection for variable mass flow rate is much higher than the constant mass flow rate. For the same amount of accumulated condensate, variable mass flow rate requires huge amount heat rejection of generator than the constant mass flow rate. The trend line for the variable condition shows the trend line to be less steep. But for constant condition the trend line is much steep. This graphical representation is actually a comparison between variable condition and constant condition.

[1]. Perry, R. H. Perry’s Chemical Engineer’s Handbook, 6th ed.; McGraw-Hill: New York, 1992; Chapter 12. [2]. Marcriss RA, Gutraj JM, Zawacki TS. Absorption fluid data survey: final report on worldwide data, RLN/sub/8447989/3, Inst. Gas Tech., 1988. [3]. Kang YT, Christensen RN. Transient analysis and design model of LiBr-H2O absorber with rotating drums. ASHRAE Trans 1995;101:1163–74. [4]. Priedeman DK, Christensen RN. GAX absorption cycle design process. ASHRAE Trans 1999;105(1):769–79. [5]. ASHRAE, Handbook of Fundamentals, Atlanta, 1997. [6]. Ozisik, M. 1985. Heat Transfer-A Basic Approach. McGraw-Hill Book Company. ISBN 0- 07-066460-9. [7]. Herold, K.E. and Klein, S.A. 1996. Absorption Chillers and Heat Pumps. CRC Press, Inc. USA, ISBN 0-8493-9427-9.

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