Air Conditioning System Designing Procedure

Air conditioning has become very important nowadays, because global warming and other environmental conditions have made uncomfortable atmosphere around us. So air conditioning system will be the very important part of a building in future.

As a mechanical engineers, we must know the process of designing air conditioning system. That design process has been linked with thermodynamics and also fluid dynamics. So engineers should have sound knowledge around those two field. 

Cooling load estimation is the first step to controlled air-conditioning. Air-conditioning is utilized to supply a controlled atmosphere to public buildings such as offices, halls, homes, and industries for the comfort of human being or for the proper performance of some industrial processes. When considering cooling load, it becomes very important to consider the way that building heat gain occur. It can be clearly illustrated as following.

Figure No 01 : Building Total Heat Gain Factors

Full air-conditioning implies that the purity, movement, temperature and relative humidity of the air be controlled within the limits imposed by the design specification. For any air conditioning system to perform satisfactorily, equipment of the proper capacity must be selected based on the instantaneous peak load requirements. The type of control used is dictated by the conditions to be maintained during peak and partial load. Undersized equipment will not provide the required conditions while a greatly oversized one will lead to operating problems such as “hunting”. Load estimating in air-conditioning system design has been carried out manually in many quarters in developing country .A lot of time and energy are wasted when estimating the cooling loads in complex and intricate buildings of modern time. But here manual calculation process is described

There are two steps to be done to install good air conditioning system, they are

• Estimating cooling load of the building
• Duct designing process

Estimating cooling load of the building

Cooling load estimation is a very important and complex task when it is done manually. There are three methods to estimate cooling load for a particular building, they are 

• Transfer Function Method (TFM) : This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet.
• Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF) : This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data.
• Total Equivalent Temperature Differential/Time-Averaging (TETD/TA) : This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method.

For strictly manual cooling load calculation method, the most practical to use is the CLTD/CLF method as described in the 1997 ASHRAE Fundamentals. This method, although not optimum, will yield the most conservative results based on peak load values to be used in sizing equipment. It should be noted that the results obtained from using the CLTD/CLF method depend largely on the characteristics of the space being considered and how they vary from the model used to generate the CLTD/CLF data shown on the various tables.

Considerations and Assumptions

• Design cooling load takes is done by making lot of assumptions some of them can be mentioned as following.
• Weather conditions are selected from a long-term statistical database. The conditions will not necessary represent any actual year, but are representative of the location of the building. ASHRAE has tabulated such data.
• The solar loads on the building are assumed to be those that would occur on a clear day in the month chosen for the calculations.
• All building equipment and appliances are considered to be operating at a reasonably representative capacity.
• Latent as well as sensible loads are considered.
• The latent heat gain is assumed to become cooling load instantly, whereas the sensible heat gain is partially delayed depending on the characteristics of the conditioned space. According to the ASHRAE regulations, the sensible heat gain from people is assumed 30% convection (instant cooling load) and 70% radiative (delayed portion).
• The ventilation rates are either assumed on air changes or based on maximum occupancy expected.
• Design dry blub of 2.5% in summer and 97.5% in winter for Chittagong city is considered. 2.5% means that temperature exceeds the value (330C) only 2.5% of all the hours in summer months. Windows are externally shaded.

As above mentioned manner Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF) is the most accurate and most widely used method. Following this method it is very easy to find cooling load for any space of a particular building. After estimating cooling load, next step is duct designing.

Air flow rates should be calculated for each and every space of the building. It is done by  using estimated cooling load values. To do that  psychrometric chart can be used.

In here both sensible and latent heat transfer take place. For sensible heat transfer, the driving potential is the temperature differential. For the latent heat transfer the driving potential is partial pressure difference or the corresponding specific humidity difference. By considering energy balance for latent or sensible heat transfer we can find mass flow rate for particular building space (room). By following same method all the required mass flow rates can be calculated. After that air flow rates can be calculated

Duct designing process

Duct designing is the most critical task in air conditioning system designing. Engineers have responsibility to design very effective duct system by considering several factors. The chief requirements of an air conditioning duct system can be mentioned as following manner.

• It should convey specified rates of air flow to prescribed locations
• It should be economical in combined initial cost, fan operating cost and cost of building space
• It should not transmit or generate objectionable noise

Although engineers have achieved major requirements of duct designing, those things will not be enough. Because to install very effective duct system, design engineers must consider following general rules for duct design

• Air should be conveyed as directly as possible to save space, power and material
• Sudden changes in directions should be avoided. When not possible to avoid sudden changes, turning vanes should be used to reduce pressure loss
• Diverging sections should be gradual. Angle of divergence ≤ 20o
• Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4
• Air velocities should be within permissible limits to reduce noise and vibration
• Duct material should be as smooth as possible to reduce frictional losses

Duct systems can be classified as following manner. According to that classification, we have to select suitable air flowing velocity for the system.

• Low pressure systems : Velocity ≤ 10 m/s, static pressure ≤ 5 cm H2O (g)
• Medium pressure systems: Velocity ≤ 10 m/s, static pressure ≤ 15 cmH2 O (g)
• High pressure systems: Velocity > 10 m/s, static pressure 15<p ≤ 25 cm H2O (g)

When considering air velocity, engineers have to consider lot of factors. Because they have to select optimum velocity for the system otherwise lot of problems can occur. There are air velocity standard table for that, so referring those tables air velocity can be calculated. Recommended air velocities depend mainly on the application and the noise criteria. Typical recommended velocities are

• Residences: 3 m/s to 5 m/s
• Theatres: 4 to 6.5 m/s
• Restaurants: 7.5 m/s to 10 m/s

When we have high velocities in the ducts results in

• Smaller ducts and hence, lower initial cost and lower space requirement
• Higher pressure drop and hence larger fan power consumption
• Increased noise and hence a need for noise attenuation

Duct system design become very difficult when we have huge buildings. So engineers should identify the lot of parameters for that. Generally at the time of designing an air conditioning duct system, the required airflow rates are known from load calculations. The location of fans and air outlets are fixed initially. The duct layout is then made taking into account the space available and ease of construction. In principle, required amount of air can be conveyed through the air conditioning ducts by a number of combinations. However, for a given system, only one set results in the optimum design. Hence, it is essential to identify the relevant design parameters and then optimize the design.

The run with the highest pressure drop is called as the index run. From load and psychrometric calculations the required supply airflow rates to each conditioned space are known. From the building layout and the location of the supply fan, the length of each duct run is known. The purpose of the duct design is to select suitable

dimensions of duct for each run and then to select a fan, which can provide the required supply airflow rate to each conditioned zone.

Due to the several issues involved, the design of an air conditioning duct system in large buildings could be a sophisticated operation requiring the use of Computer Aided Design (CAD) software. However, the following methods are most commonly used to design duct systems

• Velocity method
• Equal Friction Method
• Static Regain method

Very effective and easy method to design duct systems is equal friction method, because other to methods can be a result to have large pressure drops. So brief introduction about equal friction method as follows.

In this method the frictional pressure drop per unit length in the main and branch ducts (Δpf/L) are kept same. After that we can follow the procedure, because we have already calculated air flow rates and air velocity for the application.

• Select a suitable frictional pressure drop per unit length (Δpf/L) so that the combined initial and running costs are minimized.
• Then the equivalent diameter of the main duct is obtained from the selected value of (Δpf/L) and the airflow rate airflow rate in the main duct.
• Since the frictional pressure drop per unit length is same for all the duct runs, the equivalent diameters of the other duct runs, can be found by considering frictional pressure drop equation.
• If the ducts are rectangular, then the two sides of the rectangular duct of each run are obtained from the equivalent diameter of that run and by fixing aspect ratio as explained earlier. Thus the dimensions of the all the duct runs can be obtained. The velocity of air through each duct is obtained from the volumetric flow rate and the cross-sectional area.
• Next from the dimensions of the ducts in each run, the total frictional pressure drop of that run is obtained by multiplying the frictional pressure drop per unit length and the length.
• Dynamic pressure losses in each duct run are obtained based on the type of bends or fittings used in that run.
• The total pressure drop in each duct run is obtained by summing up the frictional and dynamic losses of that run.
• The fan is selected to suit the index run with the highest pressure loss. Dampers are installed in all the duct runs to balance the total pressure loss.

After following above mentioned steps, duct system design process will be finished. Then engineers should implement installations process according to the design data.

Figure No 02 : Air Conditioning Systems Layout and Types

References

1. ASHRAE, (1997). ASHRAE Fundamentals Handbook, American Society of Heating, Air-Conditioning and Refrigeration Publishing Service.
2. ASHRAE, (1989). ASHRAE Fundamentals Handbook, American Society of Heating, Air-Conditioning and Refrigeration Publishing Service.
3. C.P Arora, Refrigeration and Air Conditioning, Second Edition.