- Give an overview of the historical development of the heating and ventilation system and introduction of the air-conditioning (A/C) system.
- Provide the reader with a case study on the design and optimisation of an air-conditioning (A/C) system.
- Enable the reader to understand the fundamental principles and operation of the heating, cooling, ventilation and air-conditioning system.
- Introduce the possible replacement refrigerant/system to R134a
The early history of transportation systems starts mainly with the horse drawn carriage.This was eventually surpassed by the invention of the automobile. Early automobiles had cabin spaces that were open to the outside environment.This means that the occupants had to adjust there clothing to allow for different weather conditions. Closed cabin spaces were eventually introduced which required heating, cooling and ventilating to meet customer expectations. Early heating systems included heating clay bricks and placing them inside the vehicle or using simple fuel burners to add heat to the vehicle’s interior.Ventilation inside the vehicle was achieved through opening or tilting windows or the windscreen; vents were added to doors and bulkhead to improve air circulation and louvred panels were the equivalent to our modern air ducts. Air flow was difficult to control because it was dependent upon the vehicle speed and sometimes would allow dirty, humid air which contained fumes to enter the interior from the engine compartment. Cooling could be as simple as having a block of ice inside the vehicle and allowing it to melt! Eventually a number of design problems were overcome, these included air vents at the base of the windscreen for natural flow ventilation and electric motors to increase the flow at low speeds. Eventually heat exchangers were introduced which used either the heat from the exhaust system or water from the cooling system as a source, to heat the inside of the vehicle cabin. Early cabin cooling systems were aftermarket sourced and worked on evaporative cooling.They consisted of a box or cylinder fitted to the window of the vehicle. The intake of the unit would allow air to enter from outside and travel through a water soaked wire mesh grille and excelsior cone inside the unit. The water would evaporate due to absorbing the heat in the air and travel through the outlet of the unit which acted as a feed to the inside of the vehicle.The water was held in a reservoir inside the unit and had to be topped up to keep the cone wet otherwise the unit would not operate.The air entering the vehicle would be cool if the relative humidity of the air entering the unit was low. If the relative humidity of the air was high then the water could not evaporate.When the unit was working effectively it would deliver cool saturated water vapour to the inside of the vehicle which raised the humidity levels.These units were only really effective in countries with very low humidity.
In 1939 Packard marketed the first mechanical automotive A/C system which worked on a closed cycle. The system used a compressor, condenser, receiver drier and evaporator (fitted inside the boot/trunk) to operate the system. The only system control was a blower switch. Packard marketing campaign included:‘Forget the heat this summer in the only air-conditioned car in the world.’ The major problem with the system was that the compressor operated continuously (had no clutch) and had to have the belt removed to disengage the system which was generally during the winter months.Over the period 1940–41 a number of manufacturers made vehicles with A/C systems but these were in small volume and not designed for the masses. It wasn’t until after World War II that Cadillac advertised a new feature for the A/C system that located the A/C controls on the rear parcel shelf, which meant that the driver had to climb into the back seat to switch the system off. This was still better than reaching under the bonnet/hood to remove the drive belt. In 1954–55 Nash-Kelvinator introduced air-conditioning for the mass market. It was an A/C unit that was compact and affordable with controls on the dash and an electric clutch.
Case study – the air handling system
Experimental approach
Experimental approach
In the past, the only way to evaluate a proposed air handling system design was to build a prototype and test it in the laboratory.The air handling components were placed on a test stand, conditioned air was supplied at the inlet and the airflow and temperature distribution at critical locations were measured.This approach takes a considerable amount of time and requires the construction of expensive prototypes. In addition, it provides little or no understanding of why a design performed the way it did. In particular, testing is unable to detect details of recirculating areas, turbulence, temperature stratification and constrictions that adversely impact performance and pressure loss. In addition, the performance of the system usually needs to be evaluated in many different configurations. For example, it sometimes is necessary to evaluate the air handling system in different modes of operation – vent, floor, defrost and mixed – at each of eight different temperature controls.
The design process of modern vehicle systems improved with the introduction of Computer Aided Design (CAD), Computer Aided Engineering (CAE) and Computer Aided Manufacturing (CAM). CAD allows designs to be generated and visually appreciated on a computer. Standard components can be shared among manufacturers and suppliers to ensure that components assemble correctly. Designs can be sent to clients for verification and feedback. Designs can be modified and rechecked within short periods of time in a number of different formats, e.g. an STL file (stereolithography). Complex parts and assemblies can often be manufactured very quickly using rapid prototyping facilities (CAM). CAD also includes the facility to provide virtual testing.This is generally provided using additional modules or add-ins converting CAD to CAE.The software is even now used among a number of secondary schools in the UK who have the use of Solidworks as a CAD package for their technology departments which include add-in modules like Cosmos Works for Finite Element Analysis and Computational Fluid Dynamics. Finite Element Analysis (FEA) is basically mechanical stress analysis and Computational Fluid Dynamics (CFD) analyses the flow of a fluid like air through or over complex geometry. These additional features are all computer-based and use mathematical equations built into the software to predict variables like the stress distribution of a component or assembly (FEA) or the flow of air through an air vent (CFD). All these tests would have originally been carried out manually with continual adjustments being made to a model to optimise it.
In 1939 Packard marketed the first mechanical automotive A/C system which worked on a closed cycle. The system used a compressor, condenser, receiver drier and evaporator (fitted inside the boot/trunk) to operate the system. The only system control was a blower switch. Packard marketing campaign included:‘Forget the heat this summer in the only air-conditioned car in the world.’ The major problem with the system was that the compressor operated continuously (had no clutch) and had to have the belt removed to disengage the system which was generally during the winter months.Over the period 1940–41 a number of manufacturers made vehicles with A/C systems but these were in small volume and not designed for the masses. It wasn’t until after World War II that Cadillac advertised a new feature for the A/C system that located the A/C controls on the rear parcel shelf, which meant that the driver had to climb into the back seat to switch the system off. This was still better than reaching under the bonnet/hood to remove the drive belt. In 1954–55 Nash-Kelvinator introduced air-conditioning for the mass market. It was an A/C unit that was compact and affordable with controls on the dash and an electric clutch.
The A/C system begins life as an idea driven by consumer needs and government legislation. This leads to a specification.The specification will include minimum performance requirements, temperatures, control zones, flow rates etc. This will lead to a number of concept designs. The designs will have a number of computer generated models which will be presented as possible solutions to the original specification. These need to be tested for their performance.
As part of the analysis, a designer may change the geometry of the system or the boundary conditions such as the inlet velocity, flow rate etc. and view the effect on fluid flow patterns.
CFD is an efficient tool for generating parametric studies with the potential of significantly reducing the amount of physical experimentation required to optimise the performance of a
design.
A fan performance curve can be inputted into a model. Without this feature, the user has to guess the flow within the fan enclosure, calculate the pressure using CFD and see if it matches
the fan’s characteristics. If the pressure doesn’t match, then another guess has to be made. Normally, at least three iterations (test runs) are required to make a match. The software has the facility to enter a fan performance curve directly into the model. Each analysis run then interacts with the fan curve to determine the precise operating conditions of the fan as part of the regular analysis. Using this technique, engineers can easily determine what
As a typical example of improvements consider the typical design specification of the HVAC system with respect to the temperature dial on the instrument panel. In other words, moving the dial from position one to position two should have the same impact on temperature as moving from position two to position three. In the past, the linearity of the temperature dial could not be estimated until full vehicle prototypes were constructed.At that point, changes were costly and the testing data provided little or no input on what type of changes were required.
Now, engineers can determine the linearity of a proposed design as soon as the solid model in CAD has been created in a matter of days. They typically set up a series of analysis runs that evaluate eight different temperature settings at each of the three HVAC system modes. In less than a week, they can determine outlet air temperature at each setting. Once all CFD modelling is complete the prototypes are made to ensure the physical models operate as predicted by the computer models.The accuracy of simulated and actual system performance can vary up to 10–15%. Generally, lead times are reduced and designs can be evaluated much quicker allowing more time to optimise their working performance.
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