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Chapter 1: Problem 122

What is stratification? Is it likely to occur at places with low or high ceilings? How does it cause thermal discomfort for a room's occupants? How can stratification be prevented?

Short Answer

Expert verified
Answer: Stratification is the separation of air layers with different temperatures in a room, causing temperature gradients due to differences in air density. It can be prevented by using ceiling fans, proper air distribution through air vents and diffusers, utilizing radiant heaters, insulating the building envelope, and designing buildings with lower ceilings and open spaces to minimize the separation of warm and cold air layers.

Step by step solution

01

Definition of Stratification

Stratification is the phenomenon in which layers of air with different temperatures separate, causing temperature gradients (varying temperatures) in a room. This occurs mainly due to the differences in air density, with warmer air being less dense and rising to the top, while colder air is denser and settles at the bottom.
02

Stratification in Low or High Ceiling Places

Stratification is more likely to occur in places with high ceilings as there is more space for the warm air to rise and the cold air to settle. In addition, high ceilings often have more insulation, trapping the heat at the top of the room, which exacerbates the stratification.
03

Thermal Discomfort Due to Stratification

Thermal discomfort occurs when there is a significant temperature difference between various heights in a room. People frequently experience discomfort when their upper body is exposed to the warmer air near the ceiling, while their lower body is in contact with colder air near the floor. This creates an uneven thermal environment, which can result in discomfort and dissatisfaction with the indoor climate.
04

Preventing Stratification

Several measures can be taken to prevent or reduce stratification inside rooms. These include: 1. Ceiling fans: Installing ceiling fans can help circulate the air, mixing the warm and cool air layers to even out the temperature distribution. 2. Proper air distribution: Installing air vents and diffusers at various heights can help distribute warm and cool air evenly, reducing temperature gradients. 3. Radiant heaters: Utilizing radiant heaters instead of convection heaters can minimize stratification by directly heating objects and people, rather than only warming the air. 4. Insulating the building envelope: Ensuring proper insulation of walls, ceilings, and floors can prevent heat loss to the exterior and reduce stratification. 5. Proper building design: Designing buildings with lower ceilings and open spaces can minimize stratification by limiting the space for warm air to rise and separate from cooler air. This can be achieved by reducing the height of individual rooms or using vaulted/tray ceilings that reduce the volume of air.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Temperature Gradients
When we talk about temperature gradients in the context of a room or building, we're referring to the way temperature changes from one area to another – typically from the floor to the ceiling. This occurs due to the natural tendency of warm air to rise and cool air to settle due to differences in density. This can create layers, or 'strata', of air with varying temperatures, known as stratification. For occupants, this means the air temperature could be considerably warmer near the ceiling compared to the floor.

Understanding temperature gradients is crucial in addressing thermal comfort issues within a space. The gradients can affect how efficiently heating or cooling systems work and determine the types of heating solutions that should be implemented to achieve a uniform indoor temperature.
Thermal Discomfort
Thermal discomfort arises when occupants of a room experience uneven temperatures, often as a result of thermal stratification. In a stratified room, a person's head could be in a warmer layer of air, while their feet are in a cooler layer. This inconsistency can lead to discomfort and dissatisfaction, as the body struggles to maintain a balance. It's not just a matter of preference; thermal discomfort can affect productivity and health.

Several factors contribute to thermal discomfort including air temperature, humidity, air movement, and radiant temperatures. Effective thermal management not only aims to maintain a comfortable temperature range but also seeks to minimize temperature differentials within a space to avoid these comfort issues.
Air Circulation Methods
Proper air circulation is key to combating temperature gradients and ensuring a comfortable indoor environment. There are several methods to promote effective air circulation:
  • Ceiling fans: An ideal way to mix air layers and promote a uniform temperature. They force warm air down during the winter and can also create a cooling effect in the summer.
  • Air Distribution Systems: Correctly placed vents and diffusers can evenly distribute air across different vertical levels of a room.
  • HVAC Systems: Modern systems can be designed with variable air volume controls that adjust the flow based on the temperature in different zones.

Each of these methods has its place and effectiveness depending on the building's requirements and the specific thermal challenges presented by the space.
Building Insulation
Building insulation is a critical component in managing temperature gradients and ensuring energy efficiency. The role of insulation is to reduce heat transfer between the building interior and the exterior environment. By doing so, it helps maintain consistent indoor temperatures, lowers energy consumption and costs, and reduces the prevalence of stratification.

Key insulation areas include walls, ceilings, and floors. Insulation materials and their proper installation play a pivotal role in the overall thermal performance of a building. When insulation is correctly applied, it traps warm air during cold seasons and blocks heat from entering during warmer periods, contributing to an even temperature distribution throughout the space.
Building Design
The design of a building can significantly influence the amount of thermal stratification that occurs within its spaces. For example, high ceilings may lead to increased temperature gradients, while lower ceilings can help reduce stratification. Architectural features such as vaulted or tray ceilings also play a role by altering the volume of air within a room.

Design elements such as window placement, orientation, materials used in construction, and overall space layout impact how heat is distributed and retained. Integrating passive solar design principles can further enhance comfort and efficiency. As such, architects and engineers must carefully consider how the design of a space will affect air flow, light entry, and temperature control to create environments that remain consistently comfortable for its occupants.

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Most popular questions from this chapter

The inner and outer surfaces of a \(25-\mathrm{cm}\)-thick wall in summer are at \(27^{\circ} \mathrm{C}\) and \(44^{\circ} \mathrm{C}\), respectively. The outer surface of the wall exchanges heat by radiation with surrounding surfaces at \(40^{\circ} \mathrm{C}\), and convection with ambient air also at \(40^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of \(8 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Solar radiation is incident on the surface at a rate of \(150 \mathrm{~W} / \mathrm{m}^{2}\). If both the emissivity and the solar absorptivity of the outer surface are \(0.8\), determine the effective thermal conductivity of the wall.

An ice skating rink is located in a building where the air is at \(T_{\text {air }}=20^{\circ} \mathrm{C}\) and the walls are at \(T_{w}=25^{\circ} \mathrm{C}\). The convection heat transfer coefficient between the ice and the surrounding air is \(h=10 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). The emissivity of ice is \(\varepsilon=0.95\). The latent heat of fusion of ice is \(h_{i f}=333.7 \mathrm{~kJ} / \mathrm{kg}\) and its density is \(920 \mathrm{~kg} / \mathrm{m}^{3}\). (a) Calculate the refrigeration load of the system necessary to maintain the ice at \(T_{s}=0^{\circ} \mathrm{C}\) for an ice rink of \(12 \mathrm{~m}\) by \(40 \mathrm{~m}\). (b) How long would it take to melt \(\delta=3 \mathrm{~mm}\) of ice from the surface of the rink if no cooling is supplied and the surface is considered insulated on the back side?

How does forced convection differ from natural convection?

Solar radiation is incident on a \(5 \mathrm{~m}^{2}\) solar absorber plate surface at a rate of \(800 \mathrm{~W} / \mathrm{m}^{2}\). Ninety-three percent of the solar radiation is absorbed by the absorber plate, while the remaining 7 percent is reflected away. The solar absorber plate has a surface temperature of \(40^{\circ} \mathrm{C}\) with an emissivity of \(0.9\) that experiences radiation exchange with the surrounding temperature of \(-5^{\circ} \mathrm{C}\). In addition, convective heat transfer occurs between the absorber plate surface and the ambient air of \(20^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of \(7 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Determine the efficiency of the solar absorber, which is defined as the ratio of the usable heat collected by the absorber to the incident solar radiation on the absorber.

An electric heater with the total surface area of \(0.25 \mathrm{~m}^{2}\) and emissivity \(0.75\) is in a room where the air has a temperature of \(20^{\circ} \mathrm{C}\) and the walls are at \(10^{\circ} \mathrm{C}\). When the heater consumes \(500 \mathrm{~W}\) of electric power, its surface has a steady temperature of \(120^{\circ} \mathrm{C}\). Determine the temperature of the heater surface when it consumes \(700 \mathrm{~W}\). Solve the problem (a) assuming negligible radiation and (b) taking radiation into consideration. Based on your results, comment on the assumption made in part ( \(a\) ).
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