Mechanical and Electrical Equipment for Buildings (MEEB): Chapter 12 Active Climate Control (pg.421-477)
Terms in this set (152)
What are active climate control systems?
Thermal comfort and indoor air quality modification systems that:
1. Require the use of purchased energy
2. Involve numerous single- purpose components
3. Are typically only lightly integrated into the overall building fabric
4. Normally designed by a consultant other than the architect
What are active climate control systems opposite of?
A passive systems approach to climate control
What is the use of active systems dictated by?
Realities of climate or building function
What should a designer of buildings be familiar with?
Active systems for:
1. Climate control
3. Vertical circulation
4. Fire protection
What does HVAC stand for?
Heating, Ventilating and air conditioning
What is heating?
A process whereby the temperature of something (such as the air in a building) is increased
What is ventilation?
A process that moves or circulates air
What should the air conditioning system be able to do?
1. Control air temperature (by heating and/ cooling as required)
2. Control the humidity of air (perhaps bi- directionally)
3. Control air distribution (speed and direction)
4. Control indoor air quality
List the key aspects of HVAC system acquisition in the order that would be typical for a new construction project:
1. Establish HVAC- related owner's project requirements
(OPR: design issues/ intents/ criteria: including code/ standard compliance)
2. Establish zoning requirements
3. Make a preliminary system selection based on the OPR and zoning requirements
4. Calculate design heating/ cooling loads
5. Select appropriate source equipment (to meet loads, intent, and context)
6. Select appropriate distribution approach (to meet intents and fit context)
7. Coordinate HVAX components with other building systems
8. Rough- size equipment (fans, pumps, valves, dampers, pipes, ducts, condensers, air- handlers, tanks, etc)
9. Run energy analyses to optimize equipment selections and system assemblies
10. Final size equipment based upon optimization studies
11. Coordinate final individual equipment selections into a cohesive whole
12. Develop appropriate control logic and strategies
13. Develop commissioning test protocols and check lists
14. Witness systems installation and verifications
15. Develop systems manuals for the owner
16. Provide benchmark (new system) performance data for the owner
17. Assist in initial operations to maintain the owner's project requirements
Can HVAC system selection/ design be done by an architect alone?
Yes or with assistance
How can a high performance outcome be achieved with HVAC design?
Through an integrated design process
What is a potential problem faced by one attempting to understand HVAC systems?
What is one of the most basic functions of a building?
To provide shelter
"to interpose itself between people and the natural environment..... to remove the gross environmental load from their shoulders"
How does a building surface influence comfort?
Through its surface temperature
A surface can modify air temperature
What is the determinants of comfort in a cooling situation?
Air motion and relative humidity
What is usually important for building occupants?
How are air and surface temperatures manipulated?
By passive means (a combination of building form, surface material, and informed user response
How are determinants of air motion and relative humidity and the health and comfort considerations of air quality manipulated?
Requires mechanical systems
How will an architect respond if the control of air properties (motion, moisture, pollutant content) becomes critical to a project?
With a sealed building excluding outdoor air except through carefully controlled mechanical equipment
Passive Systems v. Active Systems?
Passive systems can be historic, but have limitations. Active systems are more recent and seem to have few if any limits on capabilities, but do have serious environmental impacts.
What did Roman hypocaust systems exhibit?
Basic characteristics that mirror today's central hot air heating systems
Is it true that US codes rarely mandate a code that requires that an HVAC system be provided for a building?
How does the design of any building system start with?
With a clear understanding of the outcomes required and/ or expected by the client, by users/ occupants, and by society
What will a good OPR address?
1. Thermal comfort requirements for all occupied spaces
2. Indoor air quality requirements for all occupied spaces
3. Minimum acceptable energy performance
4. Environmental preferences for constituents
5. First cost
6. Life cycle costs
9. Building space and volume demands
The laws of thermodynamics are?
1. Zeroth Law
2. First Law
3. Second Law
4. Third Law
What is the Zeroth Law?
Two systems that are in thermal equilibrium with a third system are in thermal equilibrium with each other.
This law defines the notion of temperature
What is First Law?
Heat and work are forms of energy transfer. Energy is always conserved thus all energy associated with a system must be accounted for
What is Second Law?
A system attempts to move toward a state of thermodynamic equilibrium which increases entropy
What is Third Law?
The entropy of a system approaches a constant value as the temperature approaches zero
What is energy?
The ability to do work. In HVAC system design, the amount of energy required to accomplish a defined work task is often of substantial interest to the design team
What is power?
Power is an instantaneous snapshot of work (over a very short time increment)
What is power and energy's relationship?
Energy is the integration of power over time
What is effectiveness?
A qualitative descriptor that describes success in meeting defined objectives. An HVAC system that delivers the Owner's Project Requirements would be properly termed effective
What is efficiency?
It is defined as a ratio that is a quantitative assessment of the ability of a system to provide some effect relative to the resource cost of doing so. An HVAC system that provides a lot of useful heat while requiring just slightly more heat as input would be termed efficient.
What is enthalpy?
It is a measure of the total energy (latent and sensible) content of a sample of air
What is entropy?
A measure of the disorder of a system. Without counterbalancing energy inputs, most building systems lose order over time (thus increasing in entropy). A hot water pipe loses energy when not in use. A layer of paint degrades over time.
What is exergy?
Generally not-well-understood measure of the utility of an energy exchange. In the future we may evaluate the exergy of competing HVAC system solutions to judge their appropriateness. As an example, the exergy involved when using a passive solar thermal system to accomplish the same task
What is conservation?
Implies the use of less of something (energy, water, materials, cash). Conservation is seen as unnecessarily doing with less. Conservation is necessary concept when efficiency makes no sense.
What is psychometrics?
The study and manipulation of properties of moist air
What are the eight distinct psychrometric processes?
1. Sensible heating
2. Heating and humidifying
3. Heating and dehumidification
6. Sensible cooling
7. Cooling and dehumidification
8. Evaporative cooling
What is sensible heating?
Under this process, room air temperature is increased with no change in absolute humidity. Relative humidity will decrease as the air is heated, but there is no change in air moisture content.
What is heating and humidifying?
This process is desired in smaller buildings located in cold climates. There is not single HVAC devices that will produce this effect; sensible heating equipment must be paired with a humidification device
What is heating and dehumidifying?
This process is not commonly desired or encountered process. No single HVAC device will intentionally produce this effect
What is humidification?
This process adds moisture to the air without intentionally changing air temperature. A device called a humidifier can accomplish this effect
What is dehumidification?
This process removes moisture from the air without intentionally changing air temperature. A device called a dehumidifier can accomplish this effect.
What is sensible cooling?
This process reduces air temperature without a change in absolute humidity. Some HVAC systems will produce sensible cooling through a part of their operating range.
What is cooling and dehumidification?
This combined sensible and latent cooling process is commonly desired in a wide range of building situations and is produced by the common vapor compression cooling process. A cooling coil is brought to a temperature below the dew point of the room air such that moisture condenses and leaves the air while the air is reduced in dry- bulb temperature
What is evaporative cooling?
This process involves simultaneous sensible cooling and latent heating (cooling and humidification) is an intriguing HVAC option. The evaporative cooling process occurs along a line of near- constant enthalpy; sensible heat is exchange for latent heat at a little net energy cost.
How is the work to be done by an active air conditioning equipment indicated?
By the total change in enthalpy that must occur as the air is treated by the equipment
What is the cooling process?
The conditioned air must be introduced to the space at a lower DB temperature and lower RH than the desired indoor condition so that the supply air can "soak up" heat and moisture. Outdoor air is cooled without a loss of moisture
What is the heat- recovery process?
Can be incorporated in cooling equipment so that the reheat is provided by heat that was removed and would otherwise be wasted
What kind of metrics will be encountered?
A mandated minimum performance threshold that is embedded in a building code
What are some of the commonly encountered performance metrics that will affect HVAC system selection and design for buildings:
2. Annual fuel utilization efficiency (AFUE)
3. Coefficient of performance (COP)
4. Seasonal coefficient of performance (SCOPC)
5. Energy efficiency ratio (EER)
6. Integrated part load value (IPLV)
7. Seasonal energy efficiency ratio (SEER)
8. Heating seasonal performance factor (HSPE)
9. Energy utilization index (EUI)
The ratio of system output to system input when both values are presented in consistent units. It is expressed as a decimal value or percentage
Annual Fuel Utilization Efficiency (AFUE)?
The ratio of annual fuel output energy to annual input energy which includes any off-season pilot input loss
Coefficient of Performance (COP)?
Defined slightly differently, depending upon the task. For cooling, it is the ratio of the rate of heat removal to the rate of energy input in consistent units; for a complete cooling system, as tested under a nationally recognized standard or designated operating conditions. For heating, to is the ratio of the rate of heat delivered to the rate of energy input in consistent units, for a complete heat pump system as tested under designated operating conditions.
Seasonal coefficient of performance for cooling (SCOPC) or heating (SCOPH)?
The formulation of COP that considers the total output of a device during its normal operating season (versus an instantaneous output value of COP)
Energy efficiency ratio (EER)?
The ratio of net equipment cooling capacity in BTU/h to the total rate of electric input in watts under designated operating conditions (When consistent units are used, the ratio is the same as COP)
Integrated part load value (IPLV)?
A single number figure of merit based on part- load EER or COP expressing part- load efficiency for air conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment
Seasonal energy efficiency ratio (SEER)?
The total cooling output of an air conditioner during its normal annual usage period for cooling, in BTU, divided by the total electric energy input during the same period, in watt- hours.
Heating seasonal performance factor (HSPF)?
The total heating output of a heat pump during its normal annual usage period for heating, in BTU, divided by the total electric energy input during the same period, in watt- hours.
Energy utilization index (EUI)?
Energy use intensity is an indicator of total annual building energy usage normalized per unit floor area; BTU/ ft^2 yr; for a typical building HVAC energy is a large part of EUI
What is a thermal zone?
An area of a building that must be provided with separate control if thermal comfort expectations are to be met. Often the key basis for HVAC selection and zoning decisions can play a critical role in occupant comfort responses.
What is often a key basis for HVAC system selections?
Thermal zones. Fewer zones will result in some level of discomfort. More zones than necessary will increase the first cost of the project
What does the concept of thermal zone apply equally well to?
Passive and active climate control systems
What is the intent of zoning?
To set up a control scenario that can respond to changing room loads and maintain thermally desirable conditions
How is thermal control initiated?
By a thermostat that senses room air temperature
How are thermal zoning decisions driven?
By differences in the timing of loads from one room to another
What question will be asked once the basics of system zone determination have been established?
How similar should the interior environments of buildings be?
Where are the advantages of uniformity most evident?
In ease of design and construction that, through, mass production and speed, often brings lower first costs
What are some of the issues to be considered early in the HVAC placement process?
Whether the system will be a local (requiring no central space allocation), central, or district (where source equipment is remote from the building)
What is the advantage of a central mechanical room?
Minimize distribution component runs to fan rooms, minimize electrical distribution runs from the building service, allow for easy connection to cooling towers/ air- cooled condensers, simplify noise control, minimize structural system costs, and (as necessary) simplify provision of fuel storage
Where would a central mechanical room be placed?
A single central mechanical room in a good location might serve an entire building, with area and height sufficient for both heating/ cooling and air- handling equipment
Where should a fan room be located?
A fan room should be logically located relative to the areas being served and have easy access to the exterior for connection to an outdoor air intake. Fan rooms do not require the same ceiling height as a central mechanical room
What are the four main categories of functional elements will be encountered in the typical HVAC system?
1. Source components
2. Distribution components
3. Delivery components
4. Control components
These produce heating effect and/ or cooling
These circulate the heating/ cooling effects from the source(s) to the various conditioned zones (in a local system this component category is minor or nonexistent)
These introduce the heating/ cooling effect into the various spaces being conditioned
What must an HVAC system that will offset building heat losses have?
These provide for beneficial operation of a system, such as on- off functionality temperature control, energy efficiency, freeze protection, fire response, etc.
What are the four main categories of functional elements that will be encountered in the typical HVAC system?
1. Source components: produce heating effect and/or cooling effect
2 Distribution effect: circulate the heating/ cooling effects
3. Delivery components: introduce heating/ cooling effect into various spaces
4. Control components: provide for beneficial operation of a system
What are the four conceptual and physical means of introducing heat into a building?
1. On site combustion
2. Electric Resistance
3. Heat transfer
4. Energy capture
On site combustion?
A fuel of some type is burned on site, usually within the building: heat is produced as an outcome of the combustion process. The fuel may be delivered to the building site upon demand or to be delivered in bulk for on- site- storage.
Electricity is passed through an electrical resistance element to produce heat; the electricity my come from an off site or on site source. Electricity is delivered to a building on demand.
This approach is similar to heat transfer, but an energy form other than heat is tapped into and converted to heat for use in a building. Solar energy is by far the most common such energy resource
Finding some heat already on site and moving its heat to a place where it is more useful is the essence of heat transfer. An air- to- air heat exchanger (whereby heat in exhaust air is transferred to incoming fresh air) is an example of this approach; a heat pump
What is on- site combustion?
A fuel of some type that is burned on site
What is electric resistance?
Electricity is passed through an electrical resistance element to produce heat
What is heat transfer?
Finding some heat already on site and moving this heat to a place where it is more useful
What is a boiler?
Produce hot water or steam. Boilers provide the heating effect required to elevate water temperature to a point where it can be used for building heating (or to evaporate water to steam)
How is a boiler chosen?
Depends upon the size of the building heating load, the heating fuels available (and desired), the desired efficiency of operation, and whether single or modular boilers are the most reasonable.
What are options of boilers?
1. Fire tube boilers
2. Water tube boilers
3. Cast- iron boilers
4. Steam boilers
5. Modular boilers
Fire tube boilers?
The hot gases of the fire are taken through tubes that are surrounded by the water to be heated
Water tube boilers?
The water to be heated is taken through tubes that are surrounded by the boiler's fire. They hold less water than the fire tube models and so respond faster and can generate steam
Cast iron boilers?
Often used in residential and commercial applications, these boilers operate at lower pressure and lower efficiency
What is a flue?
What is a vertical flue section?
What are the advantage of electric boilers?
The elimination of a need for combustion air, a flue, and air pollution mitigation at the building site
What are the disadvantages of electric boilers?
The use of a high grade energy source for a relatively low- grade task and the pollution impact at the electric generating plant.
The primary advantage of modular boilers is efficiency. Boilers achieve maximum efficiency when they are operated continuously at their full rated fuel input. Eliminate the initial cost of oversizing heating equipment. In cold climates, conventional boiler systems often use two or three large boilers to ensure that heat is available if one boiler fails
Operate with lower water temperatures so that water temperatures so that water vapor in the flu gas can condense and drain
Oil- fired steel boiler?
A refractory chamber receives the hot flame of the oil fire. Combustion continues within the chamber and the fire tubes.
How are hot water boilers rated?
According to heating capacity using several metrics
What is heating capacity?
The rate of useful heat output when the boiler is operating under steady- state conditions
What does the term "useful heat" refer to?
Assumes that the boiler is within the heated envelope of the building thus any heat that excepts through the boiler walls is available to help heat the building
What should the storage space to be provided for fuel oil or propane depend on?
The proximity of the supplier and the space available at the building site
Gas fired cast- iron hot water boiler?
Cast- iron sections contain water that is heated by hot gases rising through these sections.
High- output, package type steel boiler?
One or several such boilers may be installed in a large building that uses steam as a primary heating medium
When is efficiency highest?
When boiler equipment is operating near its capacity
What is a furnace?
Provides the heating effect required to elevate air temperature to a point where it can be used for building heating. Produces hot air that can be distributed to spaces via a central ductwork system; local furnace configurations without ductwork are also available. The heat source for a furnace is most commonly on- site combustion or electric resistance
What can heat transfer do?
Heat transfer can also be used as the heat source for a furnace via a coil connected to a heat pump system.
What are examples of heating equipment?
How is a boiler selected for a project?
Depends on the size of building heating load, heating fuels available, the desired efficiency of operation, and whether single or modular boilers are the most reasonable
What is breeching?
The horizontal enclosure or flue
What is the stack?
The vertical flue section
What is the advantage of an electric boiler?
Elimination of a need for combustion air, a flue, and air pollution mitigation at the building site
What is the disadvantage of an electric boiler?
The use of a high grade energy source for a relatively low grade task and the pollution impact at the electric generating plant
What boiler is preferred for energy conservation?
What is the primary advantage of modular boilers?
What do furnaces provide?
Provide the heating effect required to elevate air temperature to a point where it can be used for building heating
Describe the evolution of wood burning appliances.
Campfire to fire place to wood stove.
Fuel was captured for use rather than wasted to the outdoors
What are wood stoves used as?
Frequently used as the sole active heat source for an entire building, such as a residence or a small commercial building that is passively solar heated
What overcomes many of the disadvantages of a metal wood stove?
Masonry heaters because their footprint is rather small compared to their height.
What are infrared heaters?
They are fired with either natural gas or propane
What are the three ways of introducing coolth into a building?
1. Vapor Compression Refrigeration
2. Absorption refrigeration
3. Evaporative cooling
What is vapor compression refrigeration?
The system acts as a thermal bridge between one water system and another water system. The vapor compression cycle is a mechanical- electrical circuit in which a refrigerant is circulated under temperature conditions that allow it to pick up heat from within a building and dump heat to the outside environment (under weather conditions that won't permit passive heat flow from in-to-out). Vapor compression is by far the most commonly used and encountered means of producing a cooling effect for an HVAC system. All vapor compression systems require an externally placed heat rejection unit (a condenser). The capacity of vapor compression equipment covers a wide range' the specifics of equipment types is also diverse. The vapor compression cycle can be used directly to cool air or to produce chilled water
What is absorption refrigeration?
The absorption refrigeration cycle is diagrammatically similar to vapor compression cycle- but employs a chemical refrigerant flow process driven by hear. AR equipment is less efficient than comparable vapor compression, but may be driven by waste heat, solar hot water, or natural gas. Absorption equipment can produce chilled water in moderately large capacity applications.
What does a heat pump do?
Uses the refrigeration cycle to both heat and cool
What is the primary attractions of a heat pump?
That in heating mode it can deliver more energy than it consumes
What types of heat pumps are more dependably and efficiently?
The pumps that transfer heat from water sources or from the ground
What is evaporative cooling?
This approach to cooling involves a basic psychometric process through passive means. This approach to coolth production can be very energy efficient. Evaporative cooling is dependent upon climate than that of either vapor compression or absorption refrigeration
What is a question that designers ask?
How to get heat to flow from a cooler indoor environment to a hotter outdoor environment- which is the situation faced during the overheated portions of the year.
Vapor compression Cycle?
This system acts as a thermal bridge between one water system (chilled water) and another water system (condensed water). Heat is removed from a building via the chilled water and rejected from the building via the condenser water- thus cooling the building. The cycle operates by a cyclical liquefaction and evaporation of a refrigerant, during which processes the refrigerant releases and absorbs heat
What is the heart of refrigeration?
How do you liquefy a gaseous refrigerant?
First compressed to a high pressure vapor, then by means of a heat sink, latent heat is extracted from the refrigerant, which condenses it to a liquid. The resulting high pressure liquid has a potential for heat absorption only after it is passed through an expansion valve where it is depressurized and allowed to expand back to its gaseous form. During this change of state, the refrigerant must take on latent heat by reducing its own temperature and subsequently drawing heat out of the building
What are refrigerants?
A refrigerant should have desirable temperature- pressure relationships such that it can operate at a condensing temperature above the outdoor air temperature when used in an air- cooled configuration, and at an evaporator temperature below room air temperature when in DX configuration.
What happened in the 1990s?
The refrigerants most commonly used during the 1990s were a serious threat to the atmosphere.
Why do refrigerants pose an environmental threat?
Because they can escape from equipment during operation or repair as gases that mix into the atmosphere. The two key threats are stratospheric ozone depletion and climate change.
What is absorption refrigeration?
This process uses water as the primary refrigerant and lithium bromide as the absorber. Heat drives this chemical process. Absorption refrigeration is less efficient than the vapor compression refrigeration cycle and requires about twice the heat rejection capacity. The input heat required to undertake salt solution regeneration may be provided by solar energy or by relatively high temperature waste heat from an industrial- type process or a fuel cell. Because high- quality energy is replaced by lower quality heat, the absorption cycle can enjoy an exergy advantage over the vapor compression cycle.
What are the variations on the absorption cycle?
1. When combustion fuel such as natural gas is used to provide heat for the generator, the process is termed direct fired.
2. When a non- combustion heat source is used, the process is termed indirect fired.
3. The single- effect uses one heat exchanger between the strong and weak salt solutions
4. A double- effect absorption cycle adds a second generator and condenser that operate at a higher temperature. Doubles the COP of the single effect cycle
5. A triple- effect cycle provides a 50% COP improvement over the double effect cycle.
What is a air- cooled condensers?
Small capacity refrigeration systems often employ an air- cooled condenser by which heat removed from a building is dumped to the outdoor environment. These types of condensers pass refrigerant gas through a coil over which outdoor air flows. The temperature of the refrigerant is higher than that of the outdoor air so that heat flows from the refrigerant to change state or condense while lowering its heat content.
What is the most common coil type?
A fin and tube arrangement
What are water- cooled condensers?
Chillers often employ a water cooled condenser connected to a cooling tower to reject the heat that is removed from the chilled water system.
Explain cooling towers?
Cooling towers are designed to maximize the surface area contact between outdoor air and the condenser water in order to maximize evaporation. In crossflow towers, fans move air horizontally through water droplets and wet layers of fill (or packing); in counterflow towers fans move the air up as the water moves down. The water vapor that discharges from a cooling tower should be kept from the vicinity of fresh air intakes, and away from neighboring buildings or parked cars, where feasible.
Where is the best location for a cooling tower?
The roof is thus a favorite location for cooling towers where wind can disperse the water vapor, noise, and odor are remote from street, and aesthetics are less of a concern
Explain heat pumps?
A heat pump uses the refrigeration cycle to both heat and cool. A split- system air to air heat pump moves heat via refrigerant loop between the outdoor unit and the indoor unit.
What is the attraction of a heat pump?
The heating mode that can deliver more energy than it consumes (as electricity). Although electricity is required to run the cycle, the heat pump draws "free" heat from a source such as outdoor air. Because performance for a given piece of equipment changes with varying outdoor conditions and with indoor loads, a SEER rating metric has been established.
What are ground- source heat pumps?
These are found in several configurations. They often provide domestic hot water in addition to heating and cooling. An environmentally safe heat exchange fluid is circulated through a loop installed underground, taking heat from the soil in the winter and discharging heat in the summer.
What are water source heat pumps?
Interior closed water loop that connects all the heat pumps serving all thermal zones. This system facilitates heating in one zone while another zone is being cooled, because simultaneous heat "deposits" and heat "withdrawals" actually help the system to function most efficiently
What are water loop heat pumps?
This is a special case application of water source heat pumps. Interconnected heat pumps draw heat from a common water circulation loop or discharge heat to the loop.
What are evaporative coolers?
Can be achieved through passive means, via a hybrid system or by means of active system. Active evaporative cooling employs equipment called an evaporative cooler.
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