Mechanical systems (T5) Flashcards
mechanical energy systems design
The principle purpose of a building mechanical system is to maintain thermal comfort and indoor air quality for occupants
A building mechanical system maintains thermal comfort by balancing the heat lost or gained from external weather and internal occupants.
A building mechanical system maintains indoor air quality by bringing in fresh air from the external environment. The assumption is that the outdoor air is more free of pollutants than the indoor environment.
mechanical systems have losses
we must account for heat losses, for example in the pipes carrying heat, and system inefficiencies to determine the overall system size.
how to design a mechanical system
one must (1) estimate the energy demand for each space, (2) select the “emitter” for each space that will meet the demand, (3) determine the routing for system equipment, and (4) select the final heating/cooling generator
Heat exchange occurs from 4 major phenomena
Heat Exchanged from External Surfaces
this includes conduction, convection and radiative heat transfer. For conduction, the heat transfer is a function of the U-value. For convection, this is the heat transferred from the movement of the wind, and radiation is the solar radiation hitting the external surfaces
Heat exchange occurs from 4 major phenomena
Heat Exchanged from Infiltration
No building is 100% sealed. Infiltration describes the heat that is exchanged between the inside air and the air that seeps in through the building’s external walls.
Heat exchange occurs from 4 major phenomena
Heat Exchanged from Internal Heat Gains
People, lights, and equipment (bakery, computers, 3D printer) all give off heat to their environments which will affect the temperature
Heat exchange occurs from 4 major phenomena
Energy Stored in the Building Construction
Sometimes energy does not directly pass through an element. The energy can be stored and released at a later time. Energy is stored in the air within a building and in the external and internal constructions.
Energy Balance Equation relates each heat transfer exchange phenomena
(how to equal energy storage)
energy storage = internal gains + surfaces + infiltration + HVAC
Using energy simulation what can see regarding buildings
- Visualise how changes in various building elements affects the room temperature and resulting energy needed
- Building Construction
- Internal Gains • Infiltration
what are heating systems made up of?
heat generator = distribution = heat emitter
boiler
efficiency = useful energy out / energy in (55-90%)
natural gas, electricity, fuel oil, biomass
typical domestic boiler
Majority of boilers in the UK use natural gas as a fuel
Gas is burned to heat up water in pipes to the desired temperature ~80 C
Water heated is used to provide hot water to radiators and to water taps
Average Efficiency ~ 80%
condensing boiler
• Gain additional energy by preheating water with the flue gases
• Because of this addition, condensing boilers are more efficient than non- condensing
• Efficiencies as high as
90%
Modular commercial boilers
• Boilers are less efficient when not operating at their maximum operating points
• Large commercial buildings can
see a range of demands
throughout the season. High Periods and low Periods
• The solution is to have multiple
boilers that fire up sequentially
to meet the load leading to higher efficiencies
COP
coefficient of performance
- ratio of useful heating/cooling to work required
high COP = low costs
COP = Tsink / Tsink - Tsource
air source heat pump
- Air source heat pumps use energy from the outside air to provide heating
- The efficiency and heat output are affected by the seasonal variation in the outdoor air
ground source heat pump
- Ground source heat pumps use energy from the ground to provide heating
- Ground temperatures vary less than air temperatures leading to higher COP’s throughout the year
- Below 2m the ground temperature is fairly constant and equal to the annual mean temperature
- Can have horizontal or vertical coils. Horizontal requires more land area
- Capital Costs of these systems are larger due to the need to place the coils underground
water source heat pump
- Water source heat pumps use energy from a near by body of water to provide heating
- Water temperatures vary less than air temperatures leading to higher COP’s throughout the year
- Body of Waters: Aquafers, rivers, lakes
- Must make sure not to affect the environmental conditions through the water use
solar thermal heat generators
- Renewable Source of Hot Water
- Two types: Flat panel & Evacuated tube
- Efficiency: Conversion of incident radiation to heat
CHP generators
- Combined Heat and Power: the simultaneous production of electricity and heat
- Typically waste heat is recovered from an electricity generator and used to provide a heating service
- If all of the heat can be used, CHP system can increase overall efficiency by using waste heat that would otherwise not be used
- Used frequently to supply heat to large heat districts e.g. Loughborough Campus
heat emitters
- provides heat in the space directly
factors to consider
• Physical Dimensions: Size
• Architectural considerations: Appearance and positioning
• Occupant thermal Comfort: Convective vs Radiant Heating
• Speed of Response: Requirement for rapid heating
• Exposed Surface Temperatures: Can be limited for Safety
• Noise levels: Some have fans
radiators - heat emitters
• Most frequently used emitter
• Available in a wide variety of shapes, sizes and
output ranges. Can be placed in any room
• Operate with exposed surface temperatures
higher than the room temperature
• Temperatures may be limited for occupant
safety
• Water Supply Temperature: 80 C
• Transfer heat predominately by convection i.e.
transferring heat to the air
natural convectors
heat emitters
Produce virtually 100% convective heating
Use finned elements to transfer heat
Often used where there is insufficient space for mounting radiators
Typically base-board or trench heating
fan convectors
heat emitters
• Produce high heat output from compact units from forced circulation • 100% convective • Comprised of a heating coil, fan, inlet and outlet grilles • Can take air from the room or use outside air • Quick response to change
low temperatures radiant panels
heat emitters
• Comprise a flat metallic surface facing the room
• Heated by a serpentine
water heating pipe connected to a large system
• Heat output is mostly radiant
• Water Supply Temperature: 60 – 80 C
high temperature radiant panels
heat emitters
• Produce 100% radiant heating • Typically gas fired or electrically heated (Stand alone system) • Very high surface temperatures > 100 C • Need to be located a safe distance for occupants • Particularly suited for large halls and manufacturing facilities
underfloor heating
heat emitters
• Uses the floor surface itself as the emitter • Circulation of hot water through pipes underneath the floor • Low surface temperatures (21 – 28 C) • Low temperature good to pair with heat pumps • Because of the low temperatures, response to change is slow • However room temperature is more even throughout the space
what makes up a cooling system
cooling source - distribution - terminal unit
cooling sources
air cooled chiller
- Rejects heat to the atmosphere
- Placed Outside on the roof or near the building
- COP 2.8 – 3.2
cooling sources
water cooled chiller
- Rejects heat to the atmosphere via water
- Placed Outside on the roof or near the building
- COP 3.2 – 3.8
cooling sources
absorption chiller
- Uses heat to provide cooling
- Placed Outside on the roof or near the building
- COP 0.7 – 1.7
- Best with waste heat applications
fan convectors
terminal unit
• Produce high cooling output from compact units from forced circulation • 100% convective • Comprised of a cooling coil, fan, inlet and outlet grilles • Can take air from the room or use outside air • Quick response to change
overall building mechanical design
- determine the heating and cooling loads for each space
- select an emitter/terminal unit for each space
- determine your distribution route and system losses
- determine the size of your heat/cooling generator
