Design Checkpoints of a HVAC System

Every aspect in a HVAC system design like chiller selection, cooling coil selection, control valves , air distribution systems , pressurization systems and even acoustics has to be carefully considered. Below mentioned are few design checkpoints of a HVAC system. Many of them may look simple but they should be respected 

• The characteristics of a control valve in AHU will determine the relationship between the valve spindle lift and percentage flow. Do not assume this will be linear. Also, do not assume the percentage flow and the percentage heating or cooling output will have a linear relationship
• Even chilled water systems need to allow for expansion, as the water in the system may be starting at an ambient temperature of around 20°C or higher if kept idle for long periods in summer times.
• Restrictions in available space may be an obvious factor to consider when sizing a air duct but it is worthwhile remembering to check this, for example, ceiling voids of fixed depth. Also excessively sized risers to take ductwork use potentially lettable floor space.
• Check that both flow velocities and pressure drops in air duct are within acceptable limits at each section of the duct, whether during initial sizing or final sizing.
• Leakage from a duct system is undesirable. If a system leaks too much air the running costs of the system will increase and the system may not provide the required volume and quality of air.
• In case of circular type ducting, the duct sizing chart is logarithmic and inaccurate readings on smaller values can have a considerable impact on pressure drop; for example at duct sizes of 100-150 mm, small changes can increase the pressure drop by 20-50%.
• Using non-standard duct sizes can increase costs. However, there may be situations where using non-standard duct sizes is necessary due to restraints on the allocated space for the ductwork such as ceiling beams and riser shafts.
• Important point to note while designing duct sizes is when drawing lines on charts in order to determine other values the accuracy of the lines drawn and values, read off will depend on pencil thickness and the user’s accuracy.
• Another important design checkpoint in a HVAC system is that, when selecting the fan from fan curve charts, it is important to consider the efficiency of the fan. The fan efficiency curve is often on the same chart or sheet. If the fan selected satisfies the duty and pressure requirements but the efficiency is very low, a better choice would be to select a different fan with better efficiency but still meeting the requirements for the system.
• Check that the specific fan power does not exceed that stipulated in Section 10.3 of the Non-Domestic Heating, Cooling and Ventilation Compliance Guide.
• Make sure that the diffuser grille meets all the criteria required, including throw, velocity, noise level and pressure drop.
• When considering a grille or diffuser type for sizing, consider the air pattern required. Fixed blade devices will not always be suitable for use in areas with low ceiling heights.
• Correction of air density is important. When comparing density at sea level (1.225 kg/m3 at 1013.25 m bar, 15.15°C) and density at 2,000 m above sea level (1.0066 kg/m3 at 795 m bar, 2.2°C), the density has decreased by nearly 18%. When compared to air density at 2,500 m (0.9570 kg/m3 at 746.9 m bar, -1.1°C) the decrease in air density is nearly 22%.
• Where make-up air is required to replace air being extracted, check that the path for the make-up air is achievable. The use of an air flow diagram is a simple way to plot air paths and ensure that there is an airflow balance throughout the building that satisfies the design.
• Make sure that the required pressure in any particular area can be maintained as the operating conditions of other plant vary. A good example is where the air conditioning system is used to over-pressurise an office. If a variable air volume system is used, the return or extract air volume should be matched to the supply so that as the supply volume changes with internal conditions, the extract/return varies accordingly, thus maintaining the same pressure differential in the space.
• The sound level in the room will be determined by not only the source but also the room acoustics. For this reason some rooms will require noise correction factors.
• Equipment operating at lower than specified duty does not necessarily make less noise. This may cause particular problems with plant operating at part load during the night and at weekends