Innovative Energy Engineering



HVAC - Heating Ventilation and Air Conditioning

Basics

Steps to design a HVAC System

Pre-Selection

Deciding on System

For the remaining choices perform an energy simulation, estimate up-front cost, maintenance cost, and life expectancy. Future replacements not only cost money, it also disrupts operations and adds cost if access is not good or if downtime is expensive. Central heating or cooling systems locate maitnainable equipment away from occupants and air and/or water is heated or cooled and moved to the zone in question. The zone device typically is simple and requires little maintenance. Central equipment typically is larger, less expensive and has longer life than smaller units. Sometimes zone units (e.g. water source heatpump) are required. disadvantages of those include noise and maintenance requirement in occupied areas. Replacement also is more difficult and life of smaller units typically is shorter.

The best system won't operate effectively and efficiently if it is not maintained properly. The personnel maintaining the system need to "buy in". Most mechanics like what they already know, but also should be open to newer technologies. In retrofits the equipment they "know" may be 30 years old and current equipment will be "different" and require training. Maintenance should be possible with little impact on occupants and all equipment should be easily accessible. Roof top equipment should be avoided - it never fails when the weather is nice. Design the system as if you would maintain it and responsible for repairs. Typical mistakes include but are not limited to:

Moving Air or Water?

Power required to move a fluid depends on the volumetric flow and the pressure differential. The more volume and more pressure is required, the more energy is used. Water has a higher density and heat capacity than air. In fact, it requires 3,268 times the volumetric airflow to move the same energy as with water at given dT. Even when accounting for about up to 100 times the pressuredrop (compared to air systems) in water systems, moving air still requires 32 times more energy at given dT. This also explains why piping systems typically are significantly smaller than duct systems. In addition, air systems have a certain leakage, requiring even more air to be moved, and the larger surface of ducts allows for more heat losses and gains.

Heating

Heating efficiency increases with low supply temperatures. This applies to combustion, heatpump and solar heating systems alike. Efficient condensing combustion devices take advantage of the latent heat in the exhaust stream (water vapor from combustion). Typically design temperatures for boilers are 60°C (140°F) supply temperature with 20°C (30°F) dT. For part load operation and to increase real-life efficiency available turn-down needs to be high (up to 1:20).

Gas-fired Unit Heaters (UH) should have 2-stages and a stainless-steel heat exchanger. Heating at lower heat lowers DAT, which increases the ability to heat at floor level. Condensing models are also available. The specific throw needs to be evaluated when selecting a model. Many UH are installed too high and the warm air never reaches the ground. hydronic Unit Heaters have more flexibility with DAT because water flow and supply temperature can be decided on.

Acoustics

Noise is unwanted sound and airborne or structure borne. It is responsible for lost productivity, stress and other and health problems. Noise can be reduced by minimizing sound at the source and reducing sound in the path from sender to receiver. In HVAC slower fluid movement, lower pressures and slower fan speeds reduce noise. More efficient fittings blades also reduce noise and increase efficiency. Noise and efficiency are not mutually exclusive.

Unit-mounted silencers are more effective "silencing" and cause less pressure drop and regenerated noise than duct-mounted silencers.

Additional Information

only search this site

Information about: