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.
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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)
ICMIEE-PI-140254-2
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
