Physics - Heat 2
Explain the environmental design strategies for building heat loss control during the cool period of the year (when Tout < Theat-balance)?
1. Compact building form and spatial organization (increasing the volume-to-surface (V/S) ratio) 2. Use of transitional spaces as "thermal buffers" (e.g., attached greenhouses) 3. Thermal insulation (increasing thermal resistance of the building fabric) and careful detailing of building elements (to avoid thermal bridging effects) 4. Airtight construction (i.e., lowering the building fabric's infiltration rate)
List FOUR sources of heat loss in the thermal equilibrium of buildings.
1. Heat loss through wall, roof and windows (by conduction, convection and radiation through the building enclosure (via opaque and transparent building elements) 2. Air infiltration and ventilation 3. Heat loss through ground slab 4. Indoor water evaporation As an extra, you want to add the FIFTH one which is heat loss due the cooling system (mechanical cooling systems). Without such systems it would not be possible to maintain thermal equilibrium at certain (hotter) times of the year.
List FOUR sources of heat gain in the thermal equilibrium of buildings.
1. Solar gain through the walls and roof (opaque building elements) 2. Solar gain through the windows (transparent building elements) 3. Heat gain from the residents (occupants) ( latent + sensible gains) 4. Heat gain from lighting and other equipment (appliances) As an extra, you want to add the FIFTH one which is heat gain from the heating system (mechanical heating systems). Without such systems it would not be possible to maintain thermal equilibrium at certain (colder) times of the year.
List the factors that affect the amount of heat exchange during convective heat transfer.
1. Temperature difference (Delta-T) 2. Heat flow direction (from top to bottom, from bottom to top, or horizontal direction) 3. Air speed (m/s) 4. Surface conditions (shape, roughness) 5. Flow condition in the boundary layer (laminar vs. turbulent)
Define "Black Body" and describe its characteristics.
A Black Body is an idealized physical body that absorbs and emits all incident radiant energy. In other words, a blackbody is conceptual opaque object emitting thermal radiation perfectly (all incoming radiation is emitted back). A Black Body is the best possible emitter of thermal radiation. Its emissivity is 1.0 (which is the maximum possible level for emissivity, all other objects have emissivity levels that are always smaller than that of blackbody emittance which is 1.0).
What is a low-E coating of window? What is its function?
A low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. Low-emissivity (low-E) coatings control heat transfer through windows with insulated glazing. Low-E coatings reflects long-wave infrared energy (or heat) so when the interior heat energy tries to escape to the colder outside during the winter, the low-e coating reflects the heat back to the inside thus reducing the radiant heat loss through the glazing surface.
What effect does thermal mass have on the relationship between peak heat gains and peak cooling loads due to thermal mass? Why should we consider this difference when we calculate cooling load?
Because of the effect of thermal mass, there is usually a delay and a time lag between peak heat gains and peak cooling loads. These must be considered when calculating cooling load, because the load required for the space can be lower than the instantaneous heat gain being generated, and the size of HVAC system may be significantly affected.
What types of information do we need to calculate building heating- cooling loads?
Calculating building heating-cooling loads requires detailed building design information and weather information, which can be classified into six categories: 1) Building Characteristics: Building materials, component size, external surface colors, and shape are usually determined from building plans and specifications. 2) Building Configuration: Determine building location, orientation, and external shading from building plans and specifications. Shading from adjacent buildings can be determined from a site plan. 3) Building Areas: Use consistent methods for calculation of building areas. For fenestration, the definition of a component's area must be consistent with associated ratings. 4) Outdoor Design Conditions: Obtain appropriate weather data, and select outdoor design conditions. Dry-bulb temperature, wet-bulb temperatures and prevailing wind velocity are usually necessary. 5) Indoor Design Conditions: Select indoor dry-bulb temperature, indoor relative humidity, and ventilation rate. Include permissible variations and control limits. 6) Internal Heat Gains and Operating Schedules: Obtain planned density and a proposed schedule of lighting, occupancy, internal equipment, appliances, and processes that contribute to the internal thermal load.
Briefly describe the three basic mechanisms of heat transfer.
Conduction: The transfer of energy between objects that are in physical contact. Convection: The transfer of energy between an object and its environment, due to fluid motion. Radiation: The transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation.
Define convection and provide two examples of this phenomenon
Definition: The transfer of heat through a fluid (liquid or gas) caused by molecular motion. Examples: a. Warm air rises to displace cooler air (over a heated road surface) b. Mixing hot and cold water in a water heater
Define Emissivity and its unit of measurement.
Emissivity is the ratio of the spectral radiant power from an object to that from a blackbody at the same temperature and the wavelength. Emissivity can also be defined as an ability of a material to emit (give off) thermal radiation to the surroundings. It is a function of wavelength. Unit: Emissivity is a ratio with no units (it is a ration of actual objects emissivity to that of blackbody emissivity, which is always 1.0).
What is the one major difference between radiation and the other two heat transfer mechanisms?
For conduction, heat transfer process occurs mainly in solid materials, and for convection the major medium of heat transfer is fluids (liquids and gases in molecular motion). However, there is no medium necessary for radiation transfer (think about how we are getting the heating effect of sun (solar heating) on earth, there is no medium in space (vacuum) but radiation takes places even in space).
Calculate the radiation heat transfer per unit area of two "perfectly" black panels with temperatures at 150°C and 25°C respectively. Stefan-Boltzmann Constant,s =5.67 ́10-8W/m2K4)
Given data: T1 = 150°C = 423oK T2 = 25°C = 298oK Assume: ε = 1.00 (blackbody emittance due to the term "perfectly" in the question) s =5.67 ́10-8W/m2K4 Q=es(T4 -T4)=5.669 ́10-8(4234 -2984)=1.37kW/m2
Calculate the heat transfer between a stream of air at 10°C flowing over an internal wall surface of a building of area (A) 100m2 with a surface temperature maintained at 25°C. The convective heat transfer coefficient (hc) is known to be 30 W/m2K in this specific instance. What factors may influence this convective heat transfer coefficient?
Given data: h = 30 W/m2K Tw = 25°C T∞ = 10°C A = 100 m2 Q = h A (Tw - T∞) = 30 ́100 ́(25-10) = 45kW Factors affecting the convective heat transfer coefficient (hc) could be the air speed, air velocity and pressure (i.e., fluid properties) and the building surface geometry.
What is Building HVAC Energy Consumption?
HVAC Energy Consumption is the amount of energy consumed by building HVAC system to maintain the desired indoor air environment (Unit: Wh or kWh). HVAC Energy Consumption depends on both the building properties and the HVAC system properties and efficiencies. If a building installs different HVAC system, the HVAC Energy Consumption can be quite different.
What is the general nature of transport phenomenon of heat transfer, mass transfer, and momentum transfer? Which law of thermodynamics do these transfer mechanisms obey?
Heat Transfer: High temperature > low temperature Mass Transfer: High concentration > low concentration Momentum Transfer: High velocity> low velocity The transfer mechanisms obey the 2nd law of thermodynamics (stating the irreversibility)
Name three different periods in a year that classify the building conditioning states.
Heat deficit period (Heating season) Free running period (Swing season) Heat excess period (Cooling season)
What is Heat Gain, what is the unit of measurement for heat gain?
Heat gain is the rate at which heat enters a space, or heat generated within a space during a time interval. Unit is Watt.
Why are Heating and Cooling Load calculations so important in HVAC system design?
Heating and Cooling Load calculations affect the size of piping, ductwork, diffusers, air handlers, boilers, chillers, coils, compressors, fans, and every other component of the systems that condition indoor environments. Cooling and heating load calculations can significantly affect the first cost of building construction, the comfort and productivity of building occupants, and operating energy consumption and utility costs.
Explain the difference between natural convection and artificial convection and provide examples for these heat transfer phenomena.
If "fluid flow" is generated by density-pressure differences caused solely by temperature variation, the heat transfer is called natural convection. Example: natural air flow in atmosphere (wind), exchange of air masses over a heated motorway (where cool and high density air sinks and warm and low density air rises). The fluid is forced to flow over the surface by external "energy" source such as fans, stirrers, and pumps and blowers, creating an artificially induced (forced or assisted) convection current. Example: air conditioners (A/C), fans, pumps and blowers.
Why does conduction occur more readily in solid and liquid than that in gases?
In gases, conduction can only be caused by the elastic collision of molecules. Due to the large distance between atoms in a gas, fewer collisions between atoms occur resulting in less conduction.
What does thermal equilibrium in buildings mean?
In order to maintain desired indoor temperature (imposed by thermostat set-points to maintain human thermal comfort) "Heat Gain" should be equal to "Heat Loss" (Heat gain + Heat Loss = 0, thermal equilibrium).
Define indoor ventilation in buildings and why is it important?
Indoor ventilation refers to the exchange of air between and outside and inside of a building - Exhaust pollutants, moisture, and odors from indoor to the outdoor - Bring in outdoor "fresh" air to reduce CO2 concentration
In an experiment to measure the thermal conductivity of a steel panel with an area 1.5 m2 and thickness 0.03m, one of its surface is maintained at 80°C and the other at 40°C and a heat source at the rate of 140 W is supplied across the two surfaces. Determine the thermal conductivity (k) of the panel.
Q=140W L=0.03m A=1.5m2 Steady-State Heat transfer rate is given by T1 =80°C T2 =40°C Q=-kAdT =-kAT -T dx L Þ 140 = -k 1.5(40 - 80) 0.03 21 Hence k (thermal conductivity) = 0.07 W/m K Pay attention that T2 is smaller than T1. The flow of heat is from T2 to T1, this is why we have the -kA(T2-T1)/L equation. "k" is always positive (cannot be a negative value).
The following is the heat balance equation of a typical building thermal system. Define each variable in the equation. 𝑸𝒊 + 𝑸𝒔 ± 𝑸𝒄 ± 𝑸𝒗 ± 𝑸𝒎 ± 𝑸𝒆 = 𝟎
Qi: Internal heat gain, including gains from electric lights, people, power equipment and appliances, Qs: Solar heat gain through fenestration areas (transparent building surfaces), Qc: Conduction heat gain, through the enclosing elements, caused by a temperature difference between outside and inside (Delta-T), Qv: Ventilation heat gain, due to natural or mechanical ventilation and infiltration, Qm: Heating or cooling provided by the mechanical conditioning systems (HVAC Systems), Qe: Latent heat gain from moisture transfer through permeable building materials, through people's skins and breathing and moisture intake through infiltration and ventilation.
Describe U-value and its unit of measurement. What is the relationship between U-value and R-value?
The U-value which is the overall coefficient of thermal transmission is a measure of the overall ability of a series of conductive and convective barriers to transfer heat from one surface to the other (i.e., across the entire thickness). U-value is not for individual material layers but it applies to entire construction assemblies (i.e., a series of construction materials). Unit of U-value is W/m2K. The lower the U-factor is, the better the construction assembly insulates. R-value is the reciprocal of U-value (1/U-value) which is commonly used to evaluate the effectiveness of insulation and "R" represents "resistance to heat flow". The greater the R-value is, the better the building material's insulation effectiveness (m2·K/W)
What is the "basic rule of thermal insulation", explain?
The basic rule of insulation is that any surface in a building that is directly exposed to the outside climate or an unheated/unconditioned adjacent space (roof, exterior walls, ceiling, floor) is a candidate for THERMAL INSULATION.
Define Convective Heat Transfer Coefficient and its unit of measurement.
The convection heat transfer coefficient (hc) is not a property of material, but a complicated function of the many parameters that influence convection such as fluid velocity, fluid properties, and surface geometry. hcis often determined by experiment rather than theory. (W/m2 oC or W/m2 K)
Describe Newton's Law of Cooling and provide the mathematical equation that represents the law.
The heat flux due to convection is proportional to the temperature difference. Consider a surface at temperature Ts in contact with a fluid at T∞ q = hc · As · (Ts - T∞) where hc= heat transfer coefficient (W/m2 oC) q = heat flow rate (W) As = surface area (m2) Ts = surface temperature (oC) T∞ = temperature of the fluid at a distance far enough from the surface not to be affected by the surface temperature
Describe Fourier's Law and provide the mathematical equation that represents the law.
The time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat is flowing. If temperature is steady, one-dimensional (temperature is a function of "x" - the thickness only), and uniform over the surface, integrating the equation over area A yields: q = - kA dT/dx Where; q = heat transfer rate (W) T = temperature (oC) A = surface area normal to the heat flow (m2) x = material thickness (m)
Describe the Stefan-Boltzmann's Law.
The total energy radiated per unit surface area of a blackbody across all wavelengths per unit time is directly and steeply proportional to the 4th power of the blackbody's thermodynamic temperature (that is T4).
Describe two modes of conduction based on changes of heat flux with time.
The two modes of heat conduction are "Steady-State Conduction" and "Transient Conduction". In steady-state situations, the temperature difference(s) driving the conduction are constant and the spatial distribution of temperatures in the conducting object does not change any further. There is a temperature difference between different faces of a conducting object but this difference is constant (in time). Therefore, at any given time the amount of heat entering any region of an object is equal to the amount of heat coming out (i.e., Qin = Qout). In non-steady-state situations (transient conduction), in which a new temperature change at a boundary or a new source of heat has been suddenly introduced, a system will change in time to reach a new equilibrium (after certain amount of time).
Define Thermal Conductivity and its unit of measurement.
Thermal conductivity, k (or λ), is the property of a material describing its ability to conduct heat. Higher "k" means higher thermal conductivity and vice versa. Unit: W/(m·K)
Which period of the year (cool, warm, swing season or the entire year) would you recommend the use of ventilation and daylighting as effective means of environmental design strategies?
Ventilation & daylighting could be effectively recommended for the entire year (cool + warm + swing seasons) for all climates and most of the building spaces to achieve acceptable levels of evenly distributed illumination levels and to achieve higher indoor air quality (low levels of CO2 and pollutants and odors) within buildings.
The heating radiators in a building may use 80°C hot water in order to deliver 22°C air temperature in spaces. How much energy (quantity with units of measurement) is required to heat the water from 4°C to 80°C for a 2000L boiler? (The density of water is 1000 kg/m3 and the heat capacity of water is 𝟒. 𝟐 ∗ 𝟏𝟎𝟑 𝐉/(𝐊𝐠 °𝐂) )
𝑉𝑜𝑙𝑢𝑚𝑒 = 1 𝑙𝑖𝑡𝑟𝑒 = 0.001𝑚= 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 1000𝑘𝑔/𝑚= 𝑀𝑎𝑠𝑠 = 1000×0.001×2000 = 2000𝑘𝑔 𝑄=𝑐𝑚∆𝑡=4.2×10=×2000× 80−4 =638×10P𝐽
