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All main topics / Mechanical Engineering / General

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Sound Shadows
Sound Shadows are ground level barriers that block sound from ground level occupants however, their shadow means noise is not blocked at upper levels.
Under what climate conditions are the following passive cooling strategies most suitable?: Natural Ventilation; Evaporative Cooling; High Mass Cooling; High Mass Cooling with Night Ventilation; Conventional Air-Conditioning
Natural Ventilation is best with higher relative humidities, but less than 32 degrees C.

Evaporative Cooling is good with low relative humidities, such as less than 60% and less than 40 degrees C.

High Mass Cooling is good with high dry bulb temperatures, and less than 35 degrees C

High Mass Cooling is good with high dry bulb temperatures, and 42 degrees C

Conventional Cooling is suitable for less than 48 degrees C.
High Mass Cooling
High Mass Cooling is good for warm, dry summers. The mass needs a heat sink to offload the heat. Common strategies include roof ponds on 1-2 storey buildings or earth heat sinks, such as earth covered walls and roofs.

Water is sprayed on the roof at night for cooling.

This method is also good for winter heating at lower latitudes.  Insulation panels are used to control heat storage and offload.
High Mass Cooling with Night Ventilation
High Mass Cooling with Night Ventilation is good for hot, dry summers. During the day the building is thermally closed to prevent any heat gain. In the evening, the building is flushed with with cold, exterior air. The less cold air available, the more mass is needed. For example, cities with both warm days and evenings will require more mass.

Quick ventilation is also needed to reduce the number of building flushing hours.

This strategy is good when coupled with passive solar gain strategies (direct gain), thermally massive buildings and large and high buildings.
Evaporative Cooling
Evaporative Cooling uses fan-driven air and water to cool the building. It resembles many HVAC systems.

A concern is the fan noise, higher relative humidity, and smell of wet material.
Direct Gain Heating Strategy
Direct Gain Heating Strategy is the most common passive solar heating system. Sun is admitted which heats the thermal mass surfaces.  It is a simple strategy which offers large amounts of daylighting. It is a good strategy when coupled with night ventilation.

The thermal mass surface is typically 3x the southern glass area (avoids sunny overheating). The mass is minimum 4" thick.

Problems include glare, sunny-day overheating, large night heat-losses and faded furnishings.
Indirect Solar Gain Strategy
Indirect Solar Gain Strategy is the rarest of the passive solar strategies. The sun strikes the thermal mass first and heat is passed to the space behind (no real window).  This system is also known as a Trombe Wall (TW)

The mass is a dense, highly conductive material - such as 8-12" thick concrete, dense brick or water. Ventilation can be introduced between the glass and mass to flushout some heat build-up.

Problems include space allocation and layout:
-allow for an accessible space between the glazing and wall for maintenance and cleaning
- mass wall cannot have any blockings or other material which distrupts heat transfer into the secondary space
- less daylight and southern view since the thermal mass must be placed there, this makes it a less popular option

Benefits include less glare and less overheating on sunny days. It also offers large radiant heat gains even into night.
Greenhouse Effect
Greenhouse Effect
Gases that block heat (long-wave radiation) from escaping the earth collect in the atmosphere, therefore contain this heat, causing global warming.

These gases are primarily CO2, methane, and nitrous oxide, chlorofluorocarbons (CFCs). A majority of these gases are from burning fossil fuels. Other gases which are a byproduct of combustion are toxic.
Single Duct, Variable Air Volume
Single Duct, Variable Air Volume has a central boiler and chiller plant. Air is drawn to a fan room, conditioned and distrubted. Air-volume is controlled by a thermostat (with VAV terminal) at each outlet. Air is exhausted and reused.

Conditioning depends on the volume of air entering the space. It is the most versatile and common system as it is a relatively cheap method for good temperature control and is economical to operate. It virtually self-balances, but has a limited range of heating and cooling. It also cannot simultaneously heat and cool two areas.

A large amount of space is needed for ductwork. They are typically used a building perimeters with large temperature difference.
Single Duct, VAV Reheat
Single Duct, VAV Reheat has a reheat coil at the diffuser which boosts the final temperature. It offers a greater heating and cooling variation with closer temperature control. The thermostat controls the reheat coil temperature. The reheat coil uses electricity or hot water. It is more efficient than a CAV since it reheats only when needed.
VAV Induction
VAV Induction uses a smaller air volume and circulates it at a higher velocity. At the outlet it inducts room air and mixes it with ducted air. It uses less duct space, however potentially more fan energy to run the air at a higher velocity. It encourages air movement to spaces with low demand for heating and cooling.
++Dual Duct VAV+
Dual Duct VAV uses a pair of ducts to feed heating and cooling to zones. Outlets mix the heating and cooling feeds to the required temperature, the mix is controlled by the thermostat. It offers very good local control but is also very expensive and space consuming (two ducts). It is not an efficient system.
Single Duct, Constant Air Volume (CAV)
Single Duct, CAV has a central fan room which conditions the air and distributes it. A constant volume of conditioned air is fed to the space. A Master thermostat controls the temperature of the air generated in the fan room.

This system suits large, open spaces with uniform loads and few windows. For example, lobbies, theatres, department stores and exhibition halls.

It is easy to maintain and offers good IAQ control, but offers no individual control (one zone only). It can be a packaged system.
Single Duct, CAV Reheat
Single Duct, CAV Reheat has reheat coils at the outlet which run on steam, hot water or electricity. Since it cools air and then reheats it is not an energy efficient system and therefore rarely specified for new buildings.

It is used where constant and precise temperature is needed, such as laboratories, hospitals and operating rooms.
Single Duct, CAV Multi-Zone
Single Duct, CAV Multi-Zone has a central zone which feeds several ducts per zone. The zone's thermostat mixes the air temperature at the central zone. It may include reheat coils in the fan room. Large duct spaces near the central area are needed. It only feeds a small number of zones with short ducts. Packaged units are available.
Flashcard set info:
Author: wyu
Main topic: Mechanical Engineering
Topic: General
Published: 25.04.2010
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