Outdoor design conditions for summer and winter
Outdoor design conditions are determined from published data for the specific location, based on weather bureau or airport records. For outdoor design condition dry-bulb, wet-bulb temperature, relative humidity and daily range refer to Ashrae climate data, where most countries outdoor weather design conditions are mentioned.
Normal design conditions for summer
Normal design conditions are recommended for use with comfort and industrial cooling applications where it is occasionally permissible to exceed the design room conditions. These outdoor design conditions are the simultaneously occurring dry-bulb & wet-bulb temperatures and moisture content, which can be expected to be exceeded a few times a year for short periods. The dry-bulb is exceeded more frequently than the wet-bulb temperature. And usually when the wet-bulb is lower than design. When cooling and de-humidification are performed separately with these types of applications, use the normal design dry-bulb temperature for selecting the sensible cooling apparatus and use a moisture content corresponding to the normal design wet-bulb temperature and 80 % relative humidity for selecting the dehumidifier. Daily range is the average difference between the high and low dry-bulb temperatures for a 24-hr period on a design day. This range varies with local climate conditions.
Normal design conditions for winter
Normal winter design conditions are recommended for use with all comfort and industrial heating applications. The outdoor dry-bulb temperature can be expected to go below the listed temperatures a few times a year, normally during the early morning hours. The annual degree days listed are the sum of all the days in the year on which the daily mean temperature falls below 65 F db, times the number of degrees between 65 F db and daily mean temperature.
Corrections to outdoor design conditions for time of the day and time of the year
Correction to outdoor design conditions for time of the day and time of the year must be made during design phase. Table 2 lists the approximate corrections on the drybulb and wet-bulb temperatures from 8 a.m. to 12 p.m. based on the average daily range. Dry-bulb corrections are based on analysis of weather data, and the wet-bulb corrections assume a relatively constant dew-point throughout the 24-hr period.
Table 3 lists approximate corrections of the dry-bulb and wet-bulb temperatures from March to November, based on the yearly range in dry-bulb temperature (summer normal design dry-bulb minus winter normal design dry-bulb temperature). These corrections are based on analysis of weather data and are applicable only to the cooling load estimate.
Example for Corrections to Design Conditions Given:
Let's say we have comfort application in Texas (United States). Let's find the approximate dry-bulb and wet-bulb temperatures at 12:00 noon in October.
Solution:
Normal outdoor design conditions for Texas in July at 3:00 p.m. are 99 F db, 71 F wb (From Ashrae outdoor design weather conditions)
Daily range in Dubai is 20 F db.
Yearly range in Dubai = 99-0 = 99 F db.
Correction for time of day (12:00 noon) from Table 2:
Dry-bulb = -5 F
Wet-bulb = -1 F
Correction for time of year (October) from Table 3:
Dry-bulb = -16 F
Wet-bulb = -8 F
Design conditions at 12:00 noon in October approximately:
Dry-bulb = 99 - 5 - 16 = 78 F
Wet-bulb = 75 -1 - 8 = 66 F
Inside comfort design conditions for summer
The inside design conditions listed in Table 4 are recommended for types of applications listed. These conditions are based on experience gathered from many applications, substantiated by ASHRAE tests. Optimum or deluxe conditions are chosen where costs are not of prime importance and for comfort applications in localities having summer outdoor design dry-bulb temperatures of 90 F or less. Since all of the loads (sun, lights, people, outdoor air, etc.) do not peak simultaneously for any prolonged periods, it may be uneconomical to design for the optimum conditions.
Inside comfort design conditions for winter
For winter season operation, the inside design conditions listed in Table 4 are recommended for general heating applications. With heating, the temperature swing (variation) is below the comfort condition at the time of peak heating load (no people, lights, or solar gain, and with the minimum outdoor temperature). Heat stored in the building structure during partial load (day) operation reduces the required equipment capacity for peak load operation in the same manner as it does with cooling.
Commercial inside design conditions
Commercial inside design conditions are recommended for general comfort air conditioning applications. Since a majority of people are comfortable at 75 F or 76 F db and around 45% to 50% relative humidity, the thermostat is set to these temperatures, and these conditions are maintained under partial loads. As the peak loading occurs (outdoor peak dry-bulb and wet-bulb temperatures, 100% sun, all people and lights, etc.), the temperature in the space rises to the design point, usually 78 F db. If the temperature in the conditioned space is forced to rise, heat will be stored in the building mass. With summer cooling, the temperature swing used in the calculation of storage is the difference between the design temperature and the normal thermostat setting. The range of summer inside design conditions is provided to allow for the most economical selection of equipment. Applications of inherently high sensible heat factor (relatively small latent load) usually result in the most economical equipment selection if the higher dry-bulb temperatures and lower relative humidities are used. Applications with low sensible heat factors (high latent load) usually result in more economical equipment selection if the lower dry-bulb temperatures and higher relative humidities are used.
Industrial applications inside design conditions
Table 5 lists typical temperatures and relative humidities used in preparing, processing, and manufacturing various products, and for storing both raw and finished goods. These conditions are only typical of what has been used, and my vary with applications. They may also vary as changes occur in processes, products, and knowledge of the effect of temperature and humidity. In all cases, the temperature and humidity conditions and the permissible limits of variations on these conditions should be established by common agreement with the customer. Some of the conditions listed have no effect on the product or process other than to increase the efficiency of the employee by maintaining comfort conditions. This normally improves workmanship and uniformity, thus reducing rejects and production cost. In some cases, it may be advisable to compromise between the required conditions and comfort conditions to maintain high quality commensurate with low production cost. Generally, specific inside design conditions are required in industrial applications for one or more of the following reasons:
1. Constant temperature level is required for close tolerance measuring, gaging, machining, or grinding operations, to prevent expansion and contraction of the machine parts, machined products and measuring devices. Normally, a constant temperature is more important than the temperature level. A constant relative humidity is secondary in nature but should not go over 45% to minimize formation of heavier surface moisture film. Non-hygroscopic materials such as metals, glass, plastics, etc., have a property of capturing water molecules within the microscopic surface crevices, forming an invisible, non-continuous surface film. The density of this film increases when relative humidity increases. Hence, this film must, in many instances, be held below a critical point at which metals may etch, or the electric resistance of insulating materials is significantly decreased.
2. Where highly polished surfaces are manufactured or stored, a constant relative humidity and temperature is maintained, to minimize increase is maintained, to minimize increase in surface moisture film. The temperature and humidity should be at, or a little below, the comfort conditions to minimize perspiration of the operator. Constant temperature and humidity may also be required in machine rooms to prevent etching or corrosion of the parts of the machines. With applications of this type, if the conditions are not maintained 24 hours a day, the starting of air conditioning after any prolonged shutdown should be done carefully: (a) During the summer, the moisture accumulation in the space should be reduced before the temperature is reduced; (b) During the winter, the moisture should not be introduced before the materials have a chance to warm up if they are cooled during shutdown periods.
3. Control of relative humidity is required to maintain the strength, pliability, and regain of hydroscopic materials, such as textiles and paper. The humidity must also be controlled in some applications to reduce the effect of static electricity. Development of static electric charges is minimized of 55% or higher.
4. The temperature and relative humidity control are required to regulate the rate of chemical or biochemical reactions, such as drying of Varnishes or sugar coatings, preparation of synthetic fibers or chemical compounds, fermentation of yeast, etc. Generally, high temperatures with low humidities increase drying rates; high temperatures increase the rate of chemical reaction, and high temperatures and relative humidities increase such processes as yeast fermentation.
5. Laboratories require precise control of both temperature and relative humidity or either. Both testing and quality control laboratories are frequently designed to maintain the ASTM Standard Conditions of 73.4 F db and 50% relative humidity.
6. With some industrial applications where the load is excessive and the machines or materials do not benefit from controlled conditions, it may be advisable to apply spot cooling for the relief of the workers. Generally, the conditions to be maintained by this means will be above normal comfort.
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