This paper describes the occasion attainable inside the topic of auto aircon based totally on vapour absorption cooling. The cooling impression is produced by waste heat energy recovered from engine exhaust. The advantages of such a system are drastic low cost of gasoline over consumption and emissions associated to automobile air conditioner utilization. The current aircon system utilized in automobile relies on vapour compression cycle which primarily consists of a compressor pushed by engine output and thus will improve gasoline consumption cost and air air pollution proportion. The introduction of auto aircon using vapour absorption cycle eliminates the need for compressor; proper right here compressor is modified by generator and absorber unit. Engine waste heat from exhaust gases is used as heat provide for generator of vapour absorption system. This paper describes the occasion attainable inside the topic of auto aircon based totally on vapour absorption cooling. Some limitations are outlined and options for future enchancment are recognized.
Key phrases – Vapour Absorption Refrigeration Cycle, Vapour Compression Refrigeration Cycle.
Introduction Motivating elements for the designing this method is regular optimization of the effectivity of inside combustion engines and the rising utilization of aircon in vehicles, as a result of it reaches the standing of vital need for up to date life. Interior combustion engines are potential energy sources for absorption refrigeration strategies, as about one third of the ability availability inside the combustion processes wasted by the use of the exhaust gasoline. Thus, use of the exhaust gasoline in an absorption refrigeration system can enhance the final system effectivity.
An automobile engine makes use of solely about 35% of accessible energy and rests are misplaced to cooling and exhaust system. If one is together with typical aircon system to automobile, it further makes use of about 5% of the general energy. Subsequently automobile turns into costlier, uneconomical and fewer surroundings pleasant. It moreover decreases the lifetime of engine and can improve the gasoline consumption. For very small vehicles compressor desires three to 4 bhp, a significant ratio of the ability output. Retaining these points in ideas, a car aircon system is proposed from restoration of engine waste heat using engine exhaust as provide of generator for VARS.
Introduction to VARS: –
Decide 1 Schematics of Ammonia Water Absorption Refrigeration System
Fig. 1 reveals a schematic of the important aqua-ammonia refrigeration cycle. Extreme pressure ammonia vapor enters the condenser, the place it transfers heat to the neighborhood. Liquid ammonia leaves the condenser and passes by the use of an development valve, reaching the evaporator pressure. The refrigerant then enters the evaporator, the place it receives heat from the chilly provide, turning into low pressure vapor. Throughout the sequence, ammonia vapor enters the absorber, the place a weak reply of water and low focus ammonia absorbs the refrigerant and, on the same time, transfers heat to the neighborhood. The reply has now a extreme ammonia focus, and is pumped to the vapor generator, the place it receives heat from an exterior provide. The ammonia inside the reply then evaporates, separating from water and flowing to the condenser to start a model new cycle. A weak water-ammonia reply leaves the vapor generator and enters the absorber to absorb ammonia vapor from the evaporator. A heat exchanger between the absorber and the vapor generator transfers heat from the weak reply leaving the vapor generator to the extreme ammonia focus reply going into the vapor generator.
The coefficient of effectivity (COP) of the absorption system is generally lots lower in magnitude then the compression system. Nevertheless this low price of the earlier should not be of lots significance as a result of it makes use of the waste heat corresponding to engine exhaust heat. Essential issue about VARS is even when the evaporator temperature falls, the equivalent COP is perhaps maintained by elevating the generator temperature .Subsequently the potential of the system stays nearly the equivalent.
Design course of of heat extraction system: –
Since VARS is heat operated cycle we would like heat extraction system to extract heat from extreme temperature provide and to ship this heat to the generator of system. As a means to enhance the effectivity of the refrigeration cycle we now have to optimize the design of the Heat Extraction system. Resulting from its simplicity in operation, a lot much less arrange along with repairs value, we select Heat Exchanger as heat extraction system.
Figure2- Schematic of Heat Exchanger
As a approach to uncover the size of the Heat Exchanger now we now have to think about certain cooling functionality of the cooling system. Let’s assume it as 2.5 kW that is Qref = 2.5 kW. Calculations for Heat Extraction Machine that is in our case a Heat Exchanger are as follows
Calculation for Ammonia site: –
Calculation of mass circulation cost
Qref = á¹*cp*(Tg – Te ) —– (1) The place , cp – explicit heat functionality
Tg – Temperature of the generator
Te – Temperature of evaporator
á¹ – Mass circulation cost
From this equation we’re capable of resolve mass circulation cost of refrigerant.
Calculation of velocity
á¹=A*v*Ï —– (2)
The place, Ï = density of the refrigerant (taken from design information e-book)
A – Transfer House for Refrigerant Ammonia
As a approach to uncover A now we now have to take diameter of the tube in step with availability on the market. “v” is the speed of the refrigerant .
Calculation of Reynolds amount
Re = (Ï*v*D)/µ —– (three)
The place D = diameter of the refrigerant tube
µ = dynamic viscosity of refrigerant at indicate temperature (taken from design information e-book)
Calculation of Prandtl amount
Pr = (µ*Cp)/ okay —– (4)
The place, Cp = explicit heat of the refrigerant
okay = thermal conductivity of the refrigerant
Calculation of Nusselt amount
Calculation of the Nusselt amount relies on the co-relations and the variety of the co-relation relies on the magnitude of the Re ,Pr , and the character of the heat change flooring. In our particular scenario we select Gnielinski co-relation, as a result of it consists of a lot much less uncertainty (6%) so mathematical finish end result is perhaps additional appropriate.
Nu = ((Æ’/2)* (Re – 1000)*Pr)/ (1+12.7* (Æ’/2)half of*((Pr)2/three – 1)) —– (5)
The place, Æ’ = friction problem its price relies upon the Re
Æ’= Zero.079(Re)-Zero.25 4*103< Re<105 —– (6)
Calculation of convective heat change co-efficient
Nu = (hr*D)/okay —– (eight)
The place, hr = Convective heat change co-efficient
okay = Thermal conductivity of the ammonia
Associated methodology we’re capable of uncover out these parameters for exhaust gases by following the equivalent course of.
Calculation of Log Indicate Temperature Distinction (LMTD)
âˆ†Tm = (âˆ†T1-âˆ†T2) / ln(âˆ†T1/âˆ†T2) —– (9)
The place, âˆ†T1 = Temperature distinction between the exhaust inlet temperature and refrigerant exit temperature
âˆ†T2 = Temperature distinction between exhaust outlet temperature and refrigerant inlet temperature
Calculation of Full Thermal Resistance
Qref =U*A *âˆ†Tm —– (10)
Since âˆ†Tm / Qref is the general thermal resistance we’re going to get the price of 1/ U*A
Calculation of the dimensions of heat exchanger
Rtotal = Rconv.+Rcond.+Rconv. —– (11)
1/(U*A)= 1/(he*A) + (ln( ro/ri)) /(2Ï€*L*okay) + 1/(href*A) —– (12)
The place, he = Convective heat change coefficient of the exhaust gasses
href = Convective heat change coefficient of the refrigerant
A = Heat Change House
From equation (12) we’re capable of merely calculate optimum price of the dimensions of heat exchanger.
Now Effectiveness of Heat Exchanger changes as Inlet Temperature Distinction between scorching exhaust gases and funky refrigerant varies. Effectiveness of Heat exchanger is perhaps calculated by following course of.
Calculation of the effectiveness of the heat exchanger
In case of the counter circulation the effectiveness É› is given by
É› = (1-EXP((-1+C)*NTU) /(1- C* EXP( (-1-C)*NTU)) —– (13)
The place, NTU = Number of Change Unit
NTU = (U*A)/(á¹ * CP)min. —– (14)
C = Functionality Ratio
C = (á¹ * CP)min / (á¹ * CP)max —– (15)
Calculation of amount of heat change to the generator
É› = (exact heat change / most heat change) —– (16)
Most heat change Qmax = á¹*CP*âˆ†Tmax —– (17)
The place, âˆ†Tmax = Most temperature distinction between scorching exhaust gases and chilly refrigerant
Using Equations (14) & (15) we’re capable of calculate exact heat transferred to the generator of the VARS.
Calculation of Coefficient of Effectivity (COP) of VARS
COP= (Cooling Impression Produced / Heat Vitality Enter to Generator) —– (18)
Since COP is the function of temperature we’re capable of calculate the COP by way of the usage of following relation moreover,
COP= (Te*(Tg-Ta))/ (Tg*(Ta-Te)) —– (19) The place Te = Temperature of the evaporator
Tg = Temperature of the generator
Ta = Temperature of absorber
Outcomes and Dialogue: –
For optimization of design of heat extraction system, we now have to resolve and restore some parameters. Assume desired heat change to be three kW. Moreover we now have to find out the actual temperature or temperature differ of VARS generator with the intention to have optimum COP.
Graph 1: Generator Temperature Vs COP of VARS
From graph 1, it is clear that VARS system might have most COP inside the generator temperature differ of 118°c (391 Okay) to 127°c (400 Okay). Now we’re capable of restore the refrigerant outlet temperature. Furthermore we won’t in the reduction of the exhaust gasoline temperature beneath certain stage. Sudden drop in exhaust temperature will set off the exhaust gasoline to decelerate. The drop in exhaust temperature is perhaps accommodated by reducing the exhaust pipe diameter.
After fixing the generator temperature i.e. , refrigerant outlet temperature, exhaust gasoline outlet temperature and refrigerant inlet temperature, the one parameter remaining is exhaust gasoline inlet temperature. Exhaust temperature varies with load conditions (no load to full load conditions) and driving conditions (idling to vitality mode). This ends in to change in LMTD, ensuing from which the final heat transferred to refrigerant changes.
On account of change in exhaust gasoline inlet temperature the effectiveness of heat exchanger changes. As exhaust temperature will improve environment friendly heat change house required decreases along with effectiveness of heat exchanger reduces. Optimum heat change house and effectiveness of heat exchanger is represented by graph 2.
Graph 2: Optimum Heat Change House, Effectiveness Vs Inlet Temperature Distinction
Effectiveness is the function of temperature distinction between scorching exhaust gases and chilly refrigerant at inlet. As this temperature distinction will improve, effectiveness of heat exchanger decreases. Effectiveness of heat exchanger should not be of prime concern. We’re capable of protect the effectiveness to certain stage by varied refrigerant inlet temperature come what might, as an example, electrical heating. It ought to protect the temperature distinction between two fluids at inlet. Small amount of energy is perhaps utilized to lift the refrigerant temperature. Prime concern of the study is to accumulate desired cooling impression through the use of exhaust waste heat. So effectiveness of heat exchanger is perhaps compromised to certain stage.
Nearly, COP of system is perhaps lots lower as compared with mathematically obtained values nonetheless ample to supply desired cooling impression successfully. Graph 1 represents the theoretical values of COP obtained by equation (19). We have used these values to search out out optimum generator temperature. After fixing the parameters of heat extraction system (Heat Exchanger), the wise values of COP are obtained by way of the usage of equation (18). Graph three represents the wise values of COP. It is clear from graph three that as generator temperature rises from 118°c to 127°c, COP values drops to 73% and cooling impression obtained at some extent is 2.eight kW and effectiveness of heat exchanger is about 50%.
Graph three: Generator Temperature Vs COP
From the above outcomes we’re capable of say that it is attainable to extract waste heat of the engine exhaust using heat exchanger. As a means to enhance the effectivity of VARS now we now have to perform heat exchanger on the optimum scenario mentions in outcomes. Some precaution now we now have to take care corresponding to on the preliminary phases of engine operation effectivity of VARS is low, due to this fact to get the equivalent cooling impression now we now have heat ammonia generator using heating coil.