HVAC - Heating Ventilation and Air Conditioning
Basics
- "Heat" itself already is defined as the transfer of energy across the boundary of a system. But the incorrect term "heat transfer" is common. This transfer process can happen via radiation, convection or conduction.
- Energy naturally transfers from the warmer (high potential) to the colder (low potential) medium. Energy is defined by both quantity and quality. According to the First Law of Thermodynamics the amount of energy is conserved when we transform energy. An exception is nuclear energy where mass is converted to energy and vice versa. All energy has different "ability to perform work", which means it could potentially lift a weight. This is defined by "availability" or "exergy" and depends on the absolute process operating temperatures (° Rankine or Kelvin instead of Fahrenheit or Celsius). Ideally all processes converting energy to mechanical energy happen at high temperatures and transfer heat to very low temperatures. Although it is wrong to say "energy is lost", we still use the phrase to declare that energy is "lost to us" and released to the environment.
- Power defines how much energy is converted or transferred over a period of time.
- Sensible heating loads require to raise the temperature of fluids or bodies (without phase change). Examples include heating of a building or of a closed water stream.
- Sensible cooling loads require to lower the temperature (without water change). Examples include cooling of electronics equipment or fan heat added to airstreams.
- Latent heating loads require to evaporate liquid water to vapor (phase change). Examples include use of a humidifier to raise space humidity or drying of a wet floor.
- Latent cooling loads require to condense water vapor to liquid (phase change). An Example is drying of air by cooling.
Steps to design a HVAC System
- Determine (and reduce) Ventilation Load
- Determine (and reduce) Envelope Load including Infiltration Load
- Determine (and reduce) Lighting Load
- Determine (and reduce) Internal Loads
- Determine Comfort Design Requirements using ASHRAE standard 55
- Determine Ventilation Requirements
- Select Cooling Equipment
- Select Heating Equipment
- Select a System
- Design the Air Side System
- Design the Water Side System
- Optimize Controls
- Commission the System
Pre-Selection
- Only systems providing acceptable comfort levels should be considered. An uncomfortable system may be less expensive to install and operate, but discomforting occupants easily can cost more than any potential savings.
- Avoid technologies that are not suitable for the climate, or the application.
- Eliminate options that are not proven or where service and spare parts may be lacking.
- Evaluate space constraints. For example, duct work and air handlers may be too large, thus requiring an alternative system. All system components should be able to be replaced through all the available hallways, doors etc.
- Consider all necessary components and required redundancy. For example, heat pumps often require auxiliary heating systems and ventilation systems.
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:
- equipment above stairs requiring ladders
- equipment with frequent maintenance need (i.e. filters) in occupied spaces or even above furniture
- spare parts not readily available at the location
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
- "Heat Transfer" by Gregory Nellis and Sanford Klein
- "Thermodynamics" by Sanford Klein and Gregory Nellis
- The 4 ASHRAE Handbooks (Fundamentals, HVAC Applications, Refrigeration, HVAC systems and Equipment)
- "Principles of Heating, Ventilation, and Air Conditioning in Buildings" by John W. Mitchell, James E. Braun
- Architectural Acoustics by Marshall Long
- Energy Design Resources
- Taylor Engineering
- Eng-Tips.com
- Trane Education
- Trane Engineer Newsletter
- Trane Air Conditioning Clinic
- EngineeringToolbox.com
- a tutorial to read psychrometric charts
- AHRI (Air Conditioning, Heating and Refrigeration Institute)