Project Undertaken for Automobile Engineering (MEE 428) 2012-2013 Analysis of a Vapor Absorption Machine to Air-Condition the Cabin of a Car, using the exhaust gas heat from the Diesel Engine. By: Amna Nashit (10BEM0064) Shadab Khan (10BEM0103) Kanav (07BEM) Project guide: Prof. Ramesh Kumar C
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7/28/2019 Analysis of a Vapor Absorption Machine to Air
The materials that meet the refrigerant absorbent pair must meet the following criteria [ASHRAE, 1993]:
1. Absence of solid phase: The refrigerant-absorbent pair must not form a solid phase over the range of
composition and temperature to which it might be subjected.
2. Volatility ratio. The refrigerant must be much more volatile than the absorbent so the two can beeasily separated.
3. Affinity. The absorbent should have strong affinity for refrigerant under working conditions. This
affinity (1) causes a negative deviation from Rault’s Law and results in an activity coefficient of less
than unity for the refrigerant; (2) reduces the amount of absorbent to be circulated and consequentlythe waste of thermal energy from sensible heat; (3) reduces the size of liquid heat exchanger that
transfers heat from absorbent to pressurized refrigerant-absorbent solution in practical cycle.
4. Pressure. Operating pressures, largely established by the physical properties of the refrigerant,should be moderate.
5. Stability. Almost absolute chemical stability is required, because fluids are subjected to severe
conditions over many years of service. Instability can cause undesirable formation of gases, solids, or
corrosive substances. 6. Corrosion. Since the fluids or substances created by instability can corrode materials used in
constructing equipments, corrosion inhibitors should be used.
7. Safety. Fluids must be non-toxic and non-inflammable if they are in an occupied dwelling.
8. Transport properties. Viscosity, surface tension, thermal diffusivity and mass diffusivity are all
important characteristics of the refrigerant-absorbent pair. 9. Latent heat. Refrigerant’s latent heat should be high so that circulation rate of solution can be kept at
minimum.
No known refrigerant-absorbent pair meets all requirements listed. But LiBr-water pair has been found to contain
several advantages over others. It’s high volatility ratio, high affinity, high stability, high safety and high latent heat
make it ideal for our system. Also, less pump work is needed compared to other units due to operation at vacuum
pressures. Evaporator temperatures are limited to above 0 degrees as the refrigerant (water) freezes above that
which is ideal for our cause (air-conditioning).
Although, the tendency of crystallization of LiBr salt at moderate concentrations ( >0.65 kg LiBr/Kg of solution) and
the systems need to be designed in hermetically sealed units since they operate at vacuum pressures do pose a
challenge.
7/28/2019 Analysis of a Vapor Absorption Machine to Air
State 1: High pressure, high temperature pure refrigerant (here water vapor) enters the condenser.
State 2: Water vapor loses heat to the surrounding, in an isobaric process, in the condenser and turns into low
temperature, high pressure liquid.
State 3: After passing through the throttle valve, the fluid undergoes isenthalpic expansion. The final state is low
temperature, low pressure liquid.
State 4: Heat gain in the evaporator is an isobaric process. It results in the change of state of the refrigerant from low
pressure liquid to low pressure gas.
State 5: High concentration LiBr solution (low concentration in water) is sprayed over the incoming water vapor inthe absorber. The resulting solution is a low concentration LiBr solution (high concentration in water). Heat is
produced during this exothermic absorption process which is continuously extracted by cooling water circulation.
State 6: High pressure weak solution (high concentration of water)
State 7: Temperature increases from state 6
State 8: Very high temperature and highly pressurized strong solution of LiBr (weak in water)
State 9: Temperature is lower than that of state 8. Constant pressure heat exchanger.
State10: High temperature, low pressure strong LiBr solution (weak in water).
7/28/2019 Analysis of a Vapor Absorption Machine to Air
Now, in a generator the normal equations that apply to heat exchangers break down as the Reynold’s number and
Nusselt number equations are valid, by definition, only to flows where either sensible heating or latent heating is
involved. As in a generator both sensible and latent heat exchange takes place, it was the author’s opinion that pool
boiling equations would be more suitable for such a scenario. Having said that, the author would also like to point out
that this is an a-typical case of boiling as the concentration of solution in vapor phase and liquid phase are different
and so are their temperatures. Here we deal not only with heat but mass transfer effects. Mass transfer tends to
reduce nucleate boiling heat transfer coefficients and, in some cases, may reduce the value of the heat transfer
coefficient by up to 90%. Detailed reviews of mixture boiling are given by Thome and Shock (1984) and by Collier and
Thome (1994).The mass transfer effect on bubble growth can be explained in simple terms as follows. Since the equilibrium
composition of the more volatile component is larger in the vapor phase than in the liquid phase, the more volatile
component preferentially evaporates at the bubble interface, which in turn reduces its composition there and
induces the formation of a diffusion layer in the liquid surrounding the bubble. The partial depletion of the more
volatile component at the interface increases that of the less volatile component, which increases the bubble point
temperature at the interface. This incremental rise in the local bubble point temperature can be denoted as∆Ө.
Hence, to evaporate at the same rate as in a pure fluid, a larger superheat is required for a mixture.
The effect of mass transfer on nucleate pool boiling heat transfer can therefore be explained by introducing the
parameter ∆Ө, which represents the increase in the bubble point temperature at the surface due to preferential
evaporation of the more volatile component. At a given heat flux, the boiling superheat of the mixture is ∆TI+∆Ө while that for an ideal fluid with the same physical properties as the mixture is ∆TI. Thus, the ratio of the mixtureboiling heat transfer coefficient αnb to that of the ideal heat transfer coefficient αnb,I at the same heat flux is [9]:
The value of ∆TI is the wall superheat that corresponds to αnb,I, which is determined for instance using the Cooper
correlation with the molecular weight and critical pressure of the mixture. Hence, as the value of ∆Ө increases, the
ratio αnb/αnb,Idecreases, which means that a larger wall superheat is required in a mixture to transfer the same heat
7/28/2019 Analysis of a Vapor Absorption Machine to Air
flux. As exploited in an early mixture boiling prediction by Thome (1983), the maximum value of ∆Ө is the boiling
range of the mixture ∆Ө bp, which is equal to the difference between the dew point and the bubble point
temperatures at the composition of the liquid.
Starting from a mass transfer balance around an evaporating bubble and simplifying with an approximate slope of the
bubble point curve, the following expression was obtained to predict heat transfer in the boiling of mixtures
where βmL is the mass transfer coefficient in the liquid (set to a fixed value of 0.0003 m/s). The value of αnb,I is
determined with one of the pure fluid correlation suggested by Cooper (1984):
Overall heat transfer coefficient U can be given by
And then the overall heat transferred in the generator is given by
where ∆Tlm is the logarithmic mean temperature in 0C and A in the inner surface area of the pipe in m2.
Result and conclusion:
The following plots were obtained and conclusions drawn.It can be clearly concluded that the COP of the absorption machine, and hence the efficiency of the air-conditioning
system attached to the vehicle, greatly improves as the vehicle gains speed. This is owing to the fact that the
temperature of exhaust gases increase with the increase in speed. Also, the mass flow rate decreases as the speed
increases as can be seen from plot 2. This increases the net time of heat exchange and hence increases the COP. But
this relationship is not linear as can be concluded from the graph and verified analytically from the stated equations.
It is also seen that a well designed heat exchanger can provide enough heat to the LiBr solution to keep the system
running even in no load conditions, i.e., stalling. It can be thus concluded that a vapor absorption machine is an
exploitable mode of running air conditioning in the vehicle and further work must be undertaken to design and
model such a system.
7/28/2019 Analysis of a Vapor Absorption Machine to Air
for k=1:length(M) t1 = M(1,i); %Exhaust gas entry temperature
t2 = M(2,i); %Exhaust gas exit temperature t3 = M(3,i); %Temperature of LiBr weak solution entering the generator t4 = M(4,i); %Temperature of LiBr strong solution exiting the generator Mex = M(5,i); %Mass flow rate of exhaust gas m3 = M(6,i); %mass flow rate of weak solution into the generator m4 = M(7,i); %mass flow rate of Strong solution from the generator m5 = M(8,i); %mass flow rate of refrigerant from the generator Di = M(9,i); %inner diameter of pipe Do = M(10,i); %outer diameter of pipe L = M(11,i); %Length of pipe
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%Refrigerant temperature A = 2.00755; 0.16976; -3.133362e-3; 1.97668e-5; B = 124.937; -7.71649; 0.152286; -7.95090e-4;
for i=1:3 s1 = B(i)*(X^i); s2 = A(i)*(X^i);
end t5 = (T-s1)/s2;
%Pressure C = 7.05; D = -1596.49; E = -104095.5; P = exp( C + (D / (t5+273)) + (E / ((t5+273)^2)));
%Enthalpy F = -2024.33; 163.309; -4.88161; 6.302948e-2; -2.913705e-4;
G = 18.2829; -1.1691757; 3.248041e-2;-4.034184e-4; 1.8520569e-6; H = -3.7008214e-2; 2.8877666e-3; -8.1313015e-5; 9.9116628e-7; -4.4441207e-9; %Enthalpy of weak solution coming into the generator h3=0; for i=1:5
h3= h3 + ( F(i)* (Xw^i) + (G(i) * (Xw^i)) * t3 + (H(i) * (Xw^i)) * (t3^2)); end %Enthalpy of strong solution leaving the generator h4=0; for i=1:5
Prlb = (Cp*MUlb)/Klb; %Prandtl number q = (m5*h5) + (m4*h4) - (m3*h3); %Heat absorbed by the solution
%heat transfer coefficient for LiBr solution side of the heat exchanger %using pool boiling equations Ho1 = 55 * (Prlb^0.12) * ((-0.4343 * log(Prlb))^0.55) * (M^0.5) * (q^0.67);
%accounting for mass transfer due to azeotropic solution beta = 0.0003; %mass transfer coefficient Ho = Ho1 * ((1 + ((ho1 / q) * (Tdp-T) * ( 1 - exp ( -q / (rho*h4*beta)))))^-1);
%Over-all heat transfer coefficient U = ( Hi^-1 + Rfi + (Di/(2*Ksteel))*log(Do/Di) + (Di/Do)*Rfo + (Di/Do)*(Ho^-1))^-1;
%Average heat transfer in a generator A = pi*Di*L; %Inner surface area for heat exchange Tlm = ((t1-t3)-(t2-t4))/log((t1-t3)/(t2-t4)); %logarithmic mean temperature
difference Q(i) = U*A*Tlm;
end
plot (Mex,Q); %Since COP depends directly on Q if capacity is kept constant
Plots:
1. Power input vs Exhaust gas temperature
2. Input power vs COP
3. Exhaust gas temperature vs COP
7/28/2019 Analysis of a Vapor Absorption Machine to Air