Seminar presentation of HVAC cooling load calculation. Change rate = air changes per hour 60 = conversion from hours to minutes 3,600 = conversion from hours to seconds The crack method is a little more complex and is based upon the average quantity of air known to enter through cracks around windows and doors when the wind velocity is.
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No notes for slide. The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience. The Trane Air Conditioning Clinic series is one means of knowledge sharing. It is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning.
We have taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text. This particular clinic introduces the reader to cooling and heating load estimation. It is intended to introduce the concepts of estimating building cooling and heating loads and is limited to introducing the components that make up the load on a building, the variables that affect each of these components, and simple methods used to estimate these load components. It is not intended to teach all the details or latest computerized techniques of how to calculate these loads. If you are interested in learning more about the specific techniques used for cooling and heating load estimating, this booklet includes several references in the back. The three basic principles of heat transfer discussed in this clinic are: 1) Heat energy cannot be destroyed; it can only be transferred to another substance.
To produce cooling, heat must be removed from a substance by transferring the heat to another substance. This is commonly referred to as the principle of 'conservation of energy.' Ice cubes are typically placed in a beverage to cool it before being served. As heat is transferred from the beverage to the ice, the temperature of the beverage is lowered.
The heat removed from the beverage is not destroyed, but instead is absorbed by the ice, melting the ice from a solid to a liquid. 2) Heat energy naturally flows from a higher-temperature substance to a lower-temperature substance, in other words, from hot to cold. Heat cannot naturally flow from a cold substance to a hot substance. Consider the example of the beverage and the ice cubes.
Because the temperature of the beverage is higher than the temperature of the ice cubes, heat will always flow from the beverage to the ice cubes. 3) Heat energy is transferred from one substance to another by one of three basic processes: conduction, convection, or radiation. The device shown is a baseboard convector that is commonly used for heating a space. It can be used to demonstrate all three processes of transferring heat. Hot water flows through a tube inside the convector, warming the inside surface of the tube. Heat is transferred, by conduction, through the tube wall to the slightly cooler fins that are attached to the outside surface of the tube. Conduction is the process of transferring heat through a solid.
The heat is then transferred to the cool air that comes into contact with the fins. As the air is warmed and becomes less dense, it rises, carrying the heat away from the fins and out of the convector. This air movement is known as a convection current. Convection is the process of transferring heat as the result of the movement of a fluid. Convection often occurs as the result of the natural movement of air caused by temperature (density) differences.
Additionally, heat is radiated from the warm cabinet of the convector and warms cooler objects within the space. Radiation is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects. An interesting thing about radiated heat is that it does not heat the air between the source and the object it contacts; it only heats the object itself. In the I–P system of units, the unit for measuring the quantity of heat is the British Thermal Unit (Btu). The Btu is defined as the quantity of heat energy required to raise the temperature of 1 lb of water 1°F.
Similarly, in the Systeme International (SI) system, heat quantity can be expressed using the unit kiloJoule (kJ). A kcal is defined as the amount of heat energy required to raise the temperature of 1 kg of water 1°C. One kcal is equal to 4.19 kJ.
In heating and cooling applications, however, emphasis is placed on the rate of heat transfer, that is, the quantity of heat that flows from one substance to another within a given period of time. This rate of heat flow is commonly expressed in terms of Btu/hr—the quantity of heat, in Btu, that flows from one substance to another during a period of 1 hour. Similarly, in the SI metric system of units, the rate of heat flow is expressed in terms of kilowatts (kW).
One kW is equivalent to 1 kJ/sec. One kilowatts describes the quantity of heat, in kJ, that flows from one substance to another during a period of 1 second. Finally, the rate of heat flow may often be expressed in terms of watts (W).
One kW is equivalent to 1000 W. The process of comfort heating and air conditioning is simply a transfer of energy from one substance to another. This energy can be classified as either sensible or latent heat energy. Sensible heat is heat energy that, when added to or removed from a substance, results in a measurable change in dry-bulb temperature.
Changes in the latent heat content of a substance are associated with the addition or removal of moisture. Latent heat can also be defined as the “hidden” heat energy that is absorbed or released when the phase of a substance is changed. For example, when water is converted to steam, or when steam is converted to water. Heating and air conditioning systems use the principles of heat transfer to maintain comfortable indoor conditions for people.
Human or Thermal comfort refers to the range of temperature, humidity and air movement conditions that most people feel comfortable most of the time. According to ASHRAE, the comfort temperature lies between 78F ( maximum in summer to 68F (minimum in winter).
The relative humidity lies between 30%-60%. The term “comfort” is often used to define a broader set of conditions than just temperature and humidity. Air movement, adequate fresh air, cleanliness of the air, noise levels in the space, adequate lighting, and proper furniture and work surfaces, are just a few of the other variables that contribute to making a space comfortable for its occupants. This clinic, however, will focus only on the aspects of thermal comfort. Thermal comfort depends on creating an environment of dry-bulb temperature, humidity, and air motion, that is appropriate for the activity level of the people in the space. This environment allows the body’s rate of heat generation to balance with the body’s rate of heat loss.
Research studies have been conducted to show that, with a specific amount of air movement, thermal comfort can be produced with certain combinations of dry-bulb temperature and relative humidity. When plotted on a psychrometric chart, these combinations form a range of conditions for delivering acceptable thermal comfort to 80% of the people in a space.
This “comfort zone” and the associated assumptions are defined by ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy. Determining the desired condition of the space is the first step in estimating the cooling and heating loads for the space. In this clinic, we will choose 78ºF 25.6ºC dry-bulb temperature and 50% relative humidity (A) as the desired indoor condition during the cooling season. The selection of heating, ventilating, and air conditioning (HVAC) system components and equipment should always be based on an accurate determination of the building heating and cooling loads. During this period we will estimate the cooling loads for a single space in a single-story office building. In Period Four we will estimate the heating loads for this same space.
As stated in the preface, this clinic is intended to introduce the concepts of estimating building cooling and heating loads and is not intended to cover all of the details. The Cooling Load Temperature Difference/Solar Cooling Load/Cooling Load Factor (CLTD/SCL/CLF) load estimation method., used throughout Period Two, is a simplified hand calculation procedure developed long ago by ASHRAE. Because of its simplicity, it is the most common method used for basic instruction on estimating cooling loads. Reference: 1997 ASHRAE Handbook–Fundamentals, Chapter 28, Table 29. The space cooling load is the rate at which heat must be removed from a space in order to maintain the desired conditions in the space, generally a dry-bulb temperature and relative humidity.
The cooling load for a space can be made up of many components, including: Conduction heat gain from outdoors through the roof, exterior walls, skylights, and windows. (This includes the effects of the sun shining on these exterior surfaces.) Solar radiation heat gain through skylights and windows.
Conduction heat gain from adjoining spaces through the ceiling, interior partition walls, and floor. Internal heat gains due to people, lights, appliances, and equipment in the space. Heat gain due to hot, humid air infiltrating into the space from outdoors through doors, windows, and small cracks in the building envelope. In addition, the cooling coil in the building HVAC system has to handle other components of the total building cooling load, including: Heat gain due to outdoor air deliberately brought into the building for ventilation purposes. Heat generated by the fans in the system and possibly other heat gains in the system.
Throughout this period, we will assume that the space has no plenum (the space between the ceiling and roof). Therefore, all of the heat gain due to the roof and lighting affects the space directly. These load components contribute sensible and/or latent heat to the space. Conduction through the roof, exterior walls, windows, skylights, ceiling, interior walls, and floor, as well as the solar radiation through the windows and skylights, all contribute only sensible heat to the space. The people inside the space contribute both sensible and latent heat. Lighting contributes only sensible heat to the space, while equipment in the space may contribute only sensible heat (as is the case for a computer) or both sensible and latent heat (as is the case for a coffee maker). Infiltration generally contributes both sensible and latent heat to the space.
The cooling coil has to handle the additional components of ventilation and system heat gains. Ventilation contributes both sensible and latent heat to the coil load.
Other heat gains that occur in the HVAC system (from the fan, for example) generally contribute only sensible heat. One of the more difficult aspects of estimating the maximum cooling load for a space is determining the time at which this maximum load will occur. This is because the individual components that make up the space cooling load often peak at different times of the day, or even different months of the year.
For example, the heat gain through the roof will be highest in the late afternoon, when it is warm outside and the sun has been shining on it all day. Conversely, the heat gain due to the sun shining through an east-facing window will be highest in the early morning when the sun is rising in the east and shining directly into the window.
Determining the time that the maximum total space cooling load occurs will be discussed later in this clinic. Room 101 is the space that we will use as an example throughout this clinic. The windows face west and the solar heat gain through these windows will peak in the late afternoon when the sun is setting and shining directly into the windows.
Because of this, we will assume that the maximum cooling load for our example space occurs at 4 p.m. For this example, the following criteria will be used as a basis for estimating the space cooling and heating loads. Open-plan office space located in a single-story office building in St. Louis, Missouri.
Floor area = 45 ft x 60 ft 13.7 m x 18.3 m. Floor-to-ceiling height = 12 ft 3.7 m (no plenum between the space and roof). Desired indoor conditions = 78ºF 25.6ºC dry-bulb temperature, 50% relative humidity during cooling season; 72ºF 22.2ºC dry-bulb temperature during heating season.
West-facing wall, 12 ft high x 45 ft long 3.7 m x 13.7 m, constructed of 8 in. 203.2 mm lightweight concrete block with aluminum siding on the outside, 3.5 in. 88.9 mm of insulation, and ½ in.
12.7 mm gypsum board on the inside. Eight clear, double-pane (¼ in. 6.4 mm) windows mounted in aluminum frames. Each window is 4 ft wide x 5ft high 1.2 m x 1.5 m. Flat, 45 ft x 60 ft 13.7 m x 18.3 m roof constructed of 4 in. 100 mm concrete with 3.5 in. 90 mm insulation and steel decking.
Space is occupied from 8:00 a.m. Until 5:00 p.m. By 18 people doing moderately active work. Fluorescent lighting in space = 2 W/ft2 21.5 W/m2. Computers and office equipment in space = 0.5 W/ft2 5.4 W/m2, plus one coffee maker. In order to simplify this example, we will assume that, with the exception of the west-facing exterior wall, room 101 is surrounded by spaces that are air conditioned to the same temperature as this space.
Outdoor Design Conditions In Period One, we discussed the indoor conditions required for thermal comfort. The next step toward estimating the cooling load of a space is to determine the highest, frequently-occurring outdoor air temperature. In the summer, for example, when the temperature outside is high, heat transfers from outdoors to indoors, thus contributing to the heat gain of the space. Obviously, HVAC systems would be greatly oversized if cooling load calculations were based on the most extreme outdoor temperature ever recorded for the location. Instead, outdoor design temperatures are based on their frequency of occurrence. Design outdoor conditions for many locations can be found in the ASHRAE Handbook—Fundamentals.
Figure 16, outdoor design conditions for St. Louis, Missouri, includes three columns of dry-bulb temperatures and corresponding wet-bulb temperatures. The first column heading, 0.4%, means that the dry-bulb temperature in St. Louis exceeds 95ºF 35ºC for only 0.4% of all of the hours in an average year (or 35 hours). Also, 76ºF 25ºC is the wet-bulb temperature that occurs most frequently when the dry-bulb temperature is 95ºF 35ºC. The second column heading, 1%, means that the temperature exceeds 93ºF 34ºC for only 1% of all of the hours in an average year (or 87.6 hours).
When the dry-bulb temperature is 93ºF 34ºC, the wet-bulb temperature that occurs most frequently is 75ºF 24ºC. For our example, we will use the more severe 95ºF 35ºC dry bulb and 76ºF 25ºC wet bulb for the outdoor design conditions.
The tables. published by ASHRAE include more weather data that can be useful for sizing certain HVAC system components, but that discussion is outside the scope of this clinic. Reference: 1997 ASHRAE Handbook–Fundamentals, Chapter 26. Conduction through Surfaces Conduction is the process of transferring heat through a solid, such as a wall, roof, floor, ceiling, window, or skylight. Heat naturally flows by conduction from a higher temperature to a lower temperature.
Generally, when estimating the maximum cooling load for a space, the temperature of the air outdoors is higher than the temperature of the air indoors. We will focus on the most common conduction heat gains to a space: through the roof, external walls, and windows. Although often not applicable, a simplifying assumption when estimating the conduction heat gain through an exterior surface is to assume that the surface is completely shaded at all times. With this assumption, the amount of heat transferred through the surface is a direct result of the temperature difference between the space and outdoors. This assumption, however, does not include the additional heat transfer that occurs because of the sun shining on the surface. This will be discussed next. The amount of heat transferred through a shaded exterior surface depends on the area of the surface, the overall heat transfer coefficient of the surface, and the dry-bulb temperature difference from one side of the surface to the other.
The equation used to predict the heat gain by conduction is: Q = U x A x T where, Q = heat gain by conduction, Btu/hr W U = overall heat-transfer coefficient of the surface, Btu/hr.ft2.F W/m2.K A = area of the surface, ft2 m2 T = dry-bulb temperature difference across the surface, ºF C In the case of a shaded exterior surface, this temperature difference is the design outdoor dry-bulb temperature (To) minus the desired indoor dry-bulb temperature (Ti). The overall heat transfer coefficient is also called the U-factor. The U-factor describes the rate at which heat will be transferred through the structure. Walls and roofs are typically made up of layers of several materials.
The U-factor for a specific wall or roof is calculated by summing the thermal resistances (R-values) of each of these layers and then taking the inverse. The ASHRAE Handbook—Fundamentals tabulates.
the thermal resistance of many common materials used in constructing walls, roofs, ceilings, and floors. The wall in our example space is comprised of: aluminum siding (R = 0.61 ft2.hr.ºF/Btu 0.11 m2.ºK/W) 8 in. 200 mm lightweight concrete block (R = 2.0 0.35) 3.5 in. 90 mm of fiberglass insulation (R = 13.0 2.29) ½ in. 12.7 mm gypsum board (R = 0.45 0.08) Additionally, there is a film of air on the outside surface of the wall (R = 0.25 0.044, assuming air moving at 7.5 mph 12 km/hr during the summer) and another film of air on the inside surface of the wall (R = 0.68 0.12, assuming still air).
Reference: 1997 ASHRAE Handbook–Fundamentals, Chapter 24, Table 4. The U-factor of this wall is calculated by summing the thermal resistances of each of these layers and then taking the inverse. The U-factor of the roof in our example is calculated in a similar manner. If the west-facing wall of our example space was completely shaded at all times, the conduction heat gain due to the wall would be 388 Btu/hr 133 W.