AIR CONDITIONING-GENERAL INTRODUCTION Scope Of

Service Application Manual SAM Chapter 630-14 Section 8A

AIR CONDITIONING-GENERAL By: John H. Spence

INTRODUCTION Air Conditioning has been defined as the process for treating air, so as to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space.

Even though the foregoing paragraph describes air conditioning in its broadest terms, this section is not intended to cover air conditioning too theoretically or in all details necessary to fulfill all the objectives indicated in the definition. To do so would completely bewilder those who have not had a thorough training in engineering and thermodynamics.

It is intended to present the theory of air conditioning in everyday language and to give such data as required to actually estimate the average commercial type of air conditioning close enough for all practical purposes. scope of air conditioning

In great-grandfather’s day, air conditioning was confined to winter comfort; that is, to heating the home to a comfortable temperature in the winter by means of open fireplaces or stoves. Later, decided improvements were made in the heating equipment by the introduction of steam and hot water heating, but air conditioning was still confined to winter comfort. Less than fifty years ago human beings were content to be comfortable in winter and terribly uncomfortable during the sweltering humid heat of summer.

Until recently, modifying the summer heat as well as warming the house in the winter was an unknown factor in our daily lives.

As air conditioning has developed, the many improvements and advantages, such as removing dust from the air to create a thoroughly sanitary condition, have widened its scope and use, not only for human comfort, but also in many manufacturing processes where the use of air conditioning has become so extensive that it is a “must”.

By cleaning the air, controlling the amount of moisture in the air, and maintaining the air at a proper temperature, great improvement is shown in the quality of the products and savings are made possible by the reduction of material spoilage attributed to dust, bacteria or fluctuating temperatures.

The scope of air conditioning in industry is enormous. It is well worth it to specialize in commercial air conditioning, because industrial plants of all kinds require properly engineered and adjusted air conditioning systems. air conditioning requires proper knowledge

Although practically all manufacturers of air conditioning equipment furnish application engineering charts, data, tables, graphs, or some rule-of-thumb information in order to provide short cuts to salesmen, everyone involved in selling, installing, or servicing air conditioning should acquire a knowledge of the fundamental principles and terms used in air conditioning in order that he may be able to at least talk intelligently and recognize the problem involved.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -1- Those in the past who have attempted to sell or install air conditioning equipment without having proper knowledge have been responsible for poor operating results and unsatisfactory reactions and experiences by the owners of the equipment.

A person should know the terms used by architects and engineers and be able to communicate the general requirements of any proposed HVAC system. Therefore, this article will start with definitions of terms used in air conditioning.

DEFINITION OF TERMS

BRITISH THERMAL UNIT-The quantity of heat required to change the temperature of one pound of water, one degree Fahrenheit. It is also approximately the quantity of heat required to change 55 cu ft of air one degree F, under average conditions.

DAMPERS-Adjustable metal plates installed inside of a duct to restrict, control volume, balance, or by-pass air, as required in an air conditioning system.

DAMPER, LOUVRE-A damper made-up of several vanes operated by gravity or motor control. This type of damper is generally used to control volume of outside air intake or exhaust to the atmosphere.

DAMPER, SPLITTER- A damper used to change air flow from one duct to another.

DEHUMIDIFICATION- The process of reducing the moisture content of air.

DEHUMIDIFIER-A device to remove moisture from the air.

The dry bulb and wet bulb temperatures, or the dry bulb temperatures and relative humidity, specified to be produced inside at the time of occurrence of the design load.

DESIGN CONDITIONS, INSIDE-The dry bulb and wet bulb temperatures, or the dry bulb temperature and relative humidity, specified for design load conditions.

DESIGN LOAD-The capacity required to produce specified inside conditions when specified outside conditions of temperature and humidity prevail and when all sources of heat load taken at the maximum occur simultaneously.

It should be recognized that most installations require careful analysis for load conditions, other than maximum, under which the air conditioning apparatus will be required to perform. DUCTS-Conveyors of air from one location of a system to another.

EVAPORATION-The change of state from a liquid to a vapor. For example when water changes to steam, the liquid water evaporates.

EVAPORATIVE COOLING-The process of cooling by means of the evaporation of water into the air.

HEAT-A form of energy. It is due to molecular motion within a substance. Temperature, when measured with a thermometer, reveals the intensity of the energy in degrees.

Heat always flows from a higher temperature body to a lower temperature, until the surrounding temperatures are equalized.

Heat flows by three methods - CONDUCTION, CONVECTION, AND RADIATION.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -2- There are two types of heat in air conditioning and refrigeration. We must recognize and take into consideration both sensible heat and latent heat.

HEAT, LATENT-The heat added to or taken from a substance to change its state, but not its temperature. For example, if 32° ice is melted into water at 32°, it requires 144 Btu per pound of ice melted. If one pound of 32° water is frozen into one pound of 32° ice, 144 Btu of heat must be extracted from the water. Cited in these examples are changes of state without a change of temperature. Different substances have different latent heat characteristics.

HEAT, SENSIBLE-The heat which produces a temperature change, but does not induce a change of state. For example, when water is heated from 60° to 90° Fahrenheit, there is a 30° change in temperature, but the state of the water remains the same, that is, in liquid form.

HEAT, SPECIFIC-Using water as a standard (1 Btu per lb per 1°), the specific heat of a substance is the ratio of the heat required to raise the temperature of a substance 1° to that required to raise the same weight of water 1°.

The specific heat varies slightly, with the temperature, but for practical purposes this need not be taken into consideration. For example, the specific heat of fresh beef is approximately .7 requiring .7 Btu per pound per degree temperature change. Under average conditions air has a specific heat of .241 Btu per pound per degree. It is well to remember this figure, or where to find it, as it will be used many times in air conditioning.

HEAT, TOTAL-As applied to air conditioning, the sum of the sensible heat and the latent heat of a mixture of air and moisture. For example, in cooling air over a coil, the weight of air cooled times the range is the measure of sensible heat, and if moisture collects on the coil, this is the result of actual latent heat extraction of moisture from the air.

Total heat is also known as enthalpy.

HEAT TRANSFER-The movement of heat from a substance of a higher temperature to a substance of lower temperature. In air conditioning, as in refrigeration, heat is transferred by one or more of the following methods:

CONDUCTION-The heat that actually flows through the material by having a temperature difference on the two sides of the material. For example, heat leaking through a wall is conducted heat. A brick wall has far more rapid conduction than a wood or insulated wall. The rate of heat flow through any material is referred to as “Rate of Conduction”. The rate of conduction is usually expressed in Btu per hour, per sq. ft., per inch thickness, per degree temperature difference between the two surfaces. The rate of conduction through a material such as insulation is also referred to as the K factor of a given material.

CONVECTION-The transmission of heat by the circulation of a liquid or a gas. Convection may be natural or forced. For example, gravity type in coils in a walk-in meat cooler is an example of natural convection, while the forced circulation of air from a unit cooler or grille in a duct as used in air conditioning is an example of forced convection.

RADIATION-The transmission of heat through space by wave motion. The rate of radiation is dependent upon the temperature, exposed area, nature of the material, etc. A heating radiator in a home is an example of radiation and, in a lesser degree, the human body, which has a temperature approximately 20° above air conditioning temperature, also gives off heat by radiation.

HUMIDIFICATION-The process of adding moisture to air by the evaporation of water.

HUMIDIFIERA-device to add moisture to air.

HUMIDSTAT-A control device sensitive to relative humidity.

HUMIDITY, ASBOLUTE- The weight of water, usually expressed in grains of moisture, per cubic foot of air. A pound of moisture is 7,000 grains. To realize the load imposed on a compressor for reducing the humidity, it requires approximately the same amount of refrigeration to remove one grain of vapor from one cubic foot of air as to lower the temperature of one cubic foot of air eight degrees.

HUMIDITY, PERCEPT OF- The percentage of moisture contained in one pound of dry air to the amount one pound would hold if completely saturated at the same temperature; sometimes confused with relative humidity. For example, if a given sample of air contains 100 grains at a certain temperature when completely saturated, and a second sample of the same volume has 40 grains at the same temperature, the percent of humidity is 40 divided by 100, or 40%.

HUMIDITY, RELATIVE- The ratio of the vapor pressure of the air to the vapor pressure of air at the same temperature, saturated with moisture.

INFILTRATION-The leakage of air into a building or space.

PSYCHROMETRY-The science and practices associated with atmospheric air mixtures, their control, and the effect on materials and human comfort.

SLING PSYCHROMETER-An instrument equipped with both wet and dry bulb thermometers which, when whirled in the air using a handle attached, indicate simultaneously the wet and dry bulb temperatures.

TEMPERATURE-The measurement of heat intensity as indicated on a thermometer, in degrees, either Fahrenheit or Centigrade.

TEMPERATURE, DEWPOINT-The temperature at which a specified sample of air with no moisture added or removed would be completely saturated. The temperature below which air on being cooled gives up moisture or dew, it is sometimes described as the point at which moisture condenses out of the air onto a substance or material that is cooler than the surrounding atmosphere.

TEMPERATURE, DRY BULB (D.B.) The air temperature as indicated by a standard thermometer. (Fahrenheit scale is used in all commercial applications involving air conditioning.)

TEMPERATURE, WET BULB (W.B.)The temperature of air as measured by a thermometer with the bulb covered with a water-saturated wick. The wet bulb temperature is measured by whirling a wet bulb thermometer at 15 feet or more per second. The rate of evaporation from the cloth determines the wet bulb temperature. The lower the amount of moisture in the air, the more rapid the evaporation, and the more the wet bulb temperature is depressed, resulting in a lower relative humidity.

For example, using average conditions in St. Louis, the wet bulb would go down to 78°, but in Denver, under average conditions, it would go down to around 64°. Average conditions in Denver in summer are 90° D.B. and 64° W.B., or 22% relative humidity. The design condition for St. Louis in summer is 95° D.B. and 78° W.B., which is 46% relative humidity. This means that this 95° air would have 46% as much moisture in it as if it were completely saturated.

TEMPERATURE, EFFECTIVE-A relative value for the degree of comfort felt by the human body, affected by temperature, humidity, and air motion. Many tables have been compiled in the industry by actually having people sit in the space cooled to various temperatures and humidities to see what is a comfortable condition. While the results of this type of a test vary somewhat according to different individuals, it has been found that the “average” person feels about the same degree of comfort at 75° D.B. and 50% R.H. as is experienced at 80° D.B. and 10% R.H. - an effective temperature in either case being 70°. As was explained in the paragraph “Absolute Humidity”, it requires the same refrigeration capacity to remove one grain of moisture as is required to lower one cubic foot of air eight degrees. It is easily seen that the same effective temperature

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -4- may be produced with less cost by using a lower dry bulb temperature and a fairly high humidity, than with a high dry bulb temperature and lower relative humidity.

THERMAL CONDUCTION-The passage of heat from one point to another by transmission of molecular energy through a conductor from one particle to another particle.

VENTILATION-The process of supplying or removing the proper quantity of air from a given space by natural or mechanical means.

MEASURING THE PROPERTIES OF AIR HEAT

Quantity

1. Heat flow is measured in Btu per hour or Btu per sq ft of radiating surface.

2. Cooling is measured in Btu per hour or in tons of refrigeration, based on the heat necessary to melt one ton of ice in one 24 hour period. One ton of refrigeration = 288,000 Btu ÷ 24 = 12,000 Btu per hour.

Intensity

1. Dry bulb temperature, measured in degrees, is an indication of sensible heat.

2. Wet bulb temperature, measured in degrees, is an indication of the total heat.

3. Dew point temperature, measured in degrees, is an indication of latent heat.

MOISTURE

Quantity

1. Absolute humidity is indicated in grains per pound of air (7,000 grains equal 1 pound) or in grains per cu ft of air.

2. Relative humidity is the amount of moisture in the air as compared with the amount the air could hold if fully saturated.

Volume Flow

Cubic feet per minute is the usual measurement.

Velocity

It is measured in feet per minute velocity. Intensity is sometimes measured in the number of air changes per hour (within an air conditioned space).

Quality and Quantity

1. When measuring dust quantity it is measured by the number of particles in one cubic foot of air.

2. Bacteria count in air is measured by the colonies of bacteria in a cubic foot.

Intensity

1. When measuring smoke intensity it is measured by the average size of particles in terms of a micron (1,000 of a millimeter).

2. There is no theoretical measurement of the intensity of odors; they can be detected by a person’s sense of smell only.

DETERMINING AIR CONDITIONING LOAD

When determining air conditioning loads, the heat gained from six major sources must be included. They are as follows:

1. HEAT GAINED THROUGH WALLS, FLOOR AND CEILING.

The heat gained due to transmission is the amount of heat which flows into the conditioned space from surrounding spaces. Normally, the air conditioned space is lower in temperature than the space surrounding it, hence, heat flows into the conditioned area through walls, partitions, floor and ceiling. To maintain the lower temperature inside the conditioned area, the air conditioning equipment must have a sufficient capacity to absorb all the heat from this source.

There is an additional heat flow from sun exposed walls or roof due to the heat given off by the sun’s rays (solar radiation).

2. HEAT GAINED THROUGH GLASS

As in heat gained through walls, floors and ceilings, heat gained through glass includes heat flow due to difference in temperature and the additional heat given up by the sun’s rays. The amount of heat flow due to temperature difference varies with the type of glass used. The amount of heat gained due to the sun varies with the type of protection used, such as awnings, Venetian blinds, shades, etc.

3. HEAT GAINED DUE TO OCCUPANTS

Normally the human body dissipates heat at the rate of approximately 330 Btu per hour while at rest. This amount may vary, depending upon the type of activity. It is customary to use 400 Btu per hour per person for the occupant load on the usual air conditioning installation. For unusual activity, such as dancing, the amount of heat dissipated may be approximately double that for normal activity.

4. HEAT GAINED DUE TO LIGHTS

The electrical energy used in lighting is dissipated in the form of heat. This heat must be considered in obtaining the total Btu load, at 3.412 Btu per hour per watt consumed.

Heat gained from appliances, shower baths, and baking is often encountered in homes. Heat gained from grilles, toasters, coffee urns, and other heat generating appliances in restaurants, as well as hair dryers and permanent wave machines in beauty parlors, are important sources of heat gain. The heat from each appliance must be included in obtaining the total Btu load, at the rate of 3.412 Btu per hour per watt.

For estimating approximate heat gain from electric motors in conditioned spaces, use the factor 3.41 Btu per watt. This is accurate enough for practical use.

6. HEAT GAINED DUE TO VENTILATION

It is necessary to introduce some outside air to the conditioned space in air conditioning installations in order to eliminate objectionable odors. This outside air (called ventilation) must be cooled from outside to room conditions, and the Btu load included in the total load. The Btu load for ventilation is a very important factor in air conditioning.

Where it is not practical to introduce outside air for ventilation purposes, it is necessary to add heat gained due to natural infiltration of air.

Natural ventilation is one of the great variables in air conditioning heat gain loads. These variables include such factors as types of building construction (brick, concrete block, stone, or wood frame), whether or not wood frame buildings have an outside vapor seal, weather stripping around windows and doors, and the size of family. A home occupied by a man and his wife would not require the opening and closing of doors as often as one where there are several small children.

Wind velocities must be considered, as wind creates temperature differences due to increased outside pressure against structures, although the structure may be well built, properly sealed, and well insulated.

COMFORT AND EFFECTIVE TEMPERATURE

There appears to be no rigid rule as to the best atmospheric conditions for comfort for all people. Under the same conditions of temperature and humidity, a healthy young man may be slightly warm, while an elderly person may feel too cool. An individual who steps into a conditioned store from the blazing heat of the street experiences a welcome sense of relief, while the active clerk who has been in the store for several hours may find the conditions a trifle too warm for perfect comfort. Waitresses in a restaurant may feel somewhat warm, while the patrons seated at the tables find the atmosphere comfortable, or perhaps slightly cool.

The comfort of an individual is affected by many variables, such as health, age, sex, activity, clothing, food and degree of acclimation, all of which play their part in determining the elusive “best comfort conditions” for any particular person. Hard and fast rules that will apply to all conditions and to all people cannot be given; the best that can be done is to approximate those conditions under which a majority of the occupants of a room will feel comfortable.

Effective temperature is not an actual temperature in that it cannot be measured by a thermometer. It is an experimentally determined index of the various combinations of dry and wet bulb temperatures under which most people will feel equally cool or warm. It is not generally an index of comfort.

General design for room conditions for average peak load in comfort air conditioning in the summertime would call for equipment with ample capacity to arrive at 78°F D.B., 65°F W.B., with a relative humidity of 50%, that will result in an effective temperature of 72°F. Many practical applications can be made with equipment that will result in 80°F D.B., 67°F W.B., that result in 51% relative humidity and an effective temperature of approximately 74°F. Where occupancy in an air conditioned space is limited to 20 to 50 minutes, a system can be designed for 82°F D.B., 68°F W.B., with a 49% relative humidity and approximately an effective temperature of 75°F.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -7- Bear in mind the foregoing information on summer air conditioning design conditions is but a brief general average specification; however, these conditions are suitable for most ordinary comfort air conditioning installations.

Many times design specifications must meet requirements of codes and ordinances; therefore, the novice should lean heavily on engineering manuals, experienced engineers, and manufacturer’s recommendations, with their years of accumulative experience.

If air conditioning requires fresh air to be brought into a room, it must be done systematically to counteract or dilute the effects of respiration, moisture, odors, and carbon dioxide. In some areas filters should possibly be employed, especially in manufacturing centers, to remove as much of the dust, fly ash, soot, smoke, pollen, etc. as possible from the fresh or outside air. When involved in problems of air purification, it is suggested that guidance be sought from manufacturer’s of air filters, or reputable engineers qualified in air purification.

QUANTITY OF FRESH AIR REQUIRED

In some states laws have been enacted requiring 30 cubic feet of fresh air per minute, per person, but more recently, with adequate filters available, this amount is usually reduced to the following:

10 cfm per person seated and not smoking.

15 cfm per person seated and light smoking.

20 cfm per person seated or slightly active and heavy smoking.

20 to 30 cfm per person dancing, smoking heavily, or where odors are a problem.

In office buildings where the sensible heat load is high, and few people are concentrated, enough fresh air should be brought into the cooled space to have a pressure inside so that cooled and used air will go out the cracks around doors, etc., rather than have warm, dust-laden, outside air leak in.

However, specifications and results obtained regarding ventilation and infiltration of air into an air conditioned space depends on many factors, such as the type of building construction, the type and design of the building, the tightness of windows and doors, and the location of the air conditioned space within a given structure.

As a rule-of-thumb, 100% of the cubicle contents of the air in a room should be changed per hour. As an example, assume a room to be 40 × 10 × 10, having ten people at rest and no smoking. Using the minimum recommended amount of fresh air per minute of 10 cfm times ten people, times 60 minutes, the result is 6,000 cubic feet per hour, whereas, to provide sufficient outside air to prevent infiltration (one air change per hour), it would require only 4,000 cubic feet per hour. This is found by taking the cubic content of the room, 40 × 10 × 10. However, in any case, the greatest figure should be used.

This air pressure method, ventilation by infiltration, may not be the proper approach to changing all the air in an air conditioned space each hour, if the building is tightly constructed. This method can only be used in buildings where investigation reveals that a sufficient number of doors and windows are in the room, and these openings are not too tightly constructed.

Accurate and satisfactory air conditioning, involving field installed systems, requires adequate air distribution and circulation. This can only be accomplished by proper sized ducts, as well as inlet openings for conditioned air, and correct outlet openings for returning used air to the apparatus to be conditioned again.

The average well designed air conditioning installation will use about 25% to 33-1/3% of fresh air of the total amount circulated. For example, a 10-ton air conditioning installation will require about 4,000 cfm total air circulated, of which at least 1,000 cfm would be fresh air and 3,000 cfm recirculated. Keep in mind this is a rule-of-thumb that works very well

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -8- in estimating most installations, but where requirements are more exacting consult more complete detailed engineering manuals, or enlist the aid of a reputable manufacturer or an experienced competent engineer.

CONTROL OF HUMIDITY

Since humidity is such an important factor in summer comfort air conditioning, control of humidity in the atmosphere is accomplished by dehumidification or by humidification, depending on the amount of moisture in the air.

The means of reducing the humidity (dehumidification) are simple. To remove moisture from the air, it is only required to pass the air over a coil, the temperature of which is sufficiently below the dew point to precipitate the proper amount of moisture from the air. Since the dew point of air with a 80°F dry bulb and 50% relative humidity (67°F wet bulb) is 60°F, and the air temperature leaving the coil is usually around 50°F to 65°F, it is easily seen that a 40°F to 45°F coil will be somewhere near the correct temperature to do the dehumidification. The coil temperature, of course, depends upon the amount and condition of the air passing over the coil and the first design conditions to be met. It is sometimes necessary to reheat the air off the cooling coil to bring it within comfort conditions. This can be accomplished with heating coils or by mixing with air bypassed around the coil.

Air humidification is effected by the vaporization of water, and always requires heat from some source.

Due to the technical involvement’s regarding humidification in air conditioning, this chapter will be concluded with the simple statement that humidification requires the addition of moisture vapor to air.

Furthermore, humidification is used more often in connection with heating than cooling of air, although occasionally humidification is used in summer air conditioning in some areas.

AIR DISTRIBUTION

Air which has been treated in an air conditioner will have had its dry bulb temperature lowered considerably below the room temperature for cooling work, or raised well above the room temperature for heating.

The velocity of this conditioned air, as it enters the room from grilles or nozzles, will be greater than the inner motion required for comfort conditions in the room. If the conditioned air at relatively high velocity has a temperature much different from that in the room, and is allowed to strike occupants, we have undesirable drafts. Drafts of warm air during the heating period are not nearly as objectionable as those formed by cold air during the cooling period. Consequently, it is especially necessary to take all precautions for avoiding drafts in cooling systems.

When a person is at rest (without forced air circulation) there is a form of warm, moist air completely surrounding the entire body. To dissipate the heat of the body in a warm room, there must be air circulation. We know from experience in refrigeration that the greater the circulation, the more rapid the heat transfer, and the more rapid will be the evaporation of the moisture given off by the body. This is borne out by the fact that a piece of meat in a storage cooler will dry out if the air movement is high and the temperature is comparatively low. So, in air conditioning, we would wish to have rapid circulation to accomplish the objective. Unfortunately, instead of extremely high velocity, the velocity must be kept within the limitations of the requirement, with the cooled air being introduced at or near the ceiling, or directed so that it first travels upward toward the ceiling, involving field installed apparatus.

In large rooms, where the cooled air comes through openings in the ceiling, the openings are usually fitted with reflectors which cause the air to first travel sideways toward the walls before dropping down toward the lower levels. If such guides are not provided, the cooled air will fall into a solid, undisturbed column toward the floor and will result in very bad drafts at various points. In other rooms, it is generally possible to introduce the air at grilles and nozzles located high up on the walls so that the stream travels horizontally across the upper part of the room before mixing with the remainder of the air.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -9- In still other systems of distribution, the incoming air passes through an opening shaped like a flat cone near the center of the ceiling and may be deflected toward the walls. Returned air is taken out of the rooms through a central opening of the inlet cone, thus providing quiet, uniform air motion throughout the entire space.

Proper air distribution is probably the toughest problem that arises in designing air conditioning systems. Air should never be discharged so that it strikes occupants of a room at the back of their necks. Wherever possible, have the air discharged toward the faces of the larger percentage of the people. Sometimes this is impossible, and if so, outlets of the square or circular type may be used at low velocities, so that the air thoroughly mixes with the temperate air in the room before actually striking the occupants of the room.

PERFORMANCE GUARANTEED

Occasionally a purchaser of air conditioning will say “I will pay for the job when it is satisfactory”. The buyer may have the best intentions in the world and be 100% honest, yet this type of guarantee is practically impossible.

Ten people may be seated at a table and three may be too cool, three too warm, and four feel satisfied. or they may all be satisfied except two elderly people, who may “feel a draft”. For that reason the following performance standard may be written up so that both buyer and seller may be protected and a real performance accomplished and still not leave acceptance to guesswork or individual peculiarities.

AIR TEMPERATURE AND HUMIDITY

The temperature of the conditioned space shall be maintained and not exceed 15°F below the outdoor temperature.

The relative humidity shall not be less than 40% or more than 60% for any sustained period during the operation of the cooling equipment.

AIR QUALITY

The air in such occupied space shall at all times be free from toxic, unhealthy, or disagreeable gases, and shall be relatively free from dust. Filters of the proper size will be used when dust removal is required. Filters shall be sized so as not to restrict proper air velocity.

AIR MOTION

The air in the conditioned space shall at all times be in constant motion, sufficient to maintain a reasonable uniformity of temperature and humidity, but not such as to cause exceptional drafts. A velocity of approximately 35 to 50 feet per minute measured with a velometer in the conditioned area shall be considered good design conditions (except in food stores equipped with open self-service refrigerated merchandising fixtures, in which case air velocities from air conditioning systems must not be high enough to disturb the well of cold air in the open self-service refrigerated cases. Air movement in vicinity of self-service cases must be under 10 feet per minute).

AIR DISTRIBUTION

The standard allowance for concentration of carbon dioxide of one part in 12,000 at three feet above the floor level shall be an indication of air distribution in case any question is raised.

The quantity of fresh air shall not be less than 10 cubic feet of outside air per minute, per person, or one complete fresh air change per hour in the total conditioned space, whichever is the greater.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -10- The foregoing performance standards cover the average design conditions for proper cooling and should be acceptable to the purchaser. If any variations from the above conditions are desired, care must be taken to select the equipment so that it will perform satisfactorily.

SUMMARY

This article has been a brief layman’s treatise on what is to be accomplished in a well designed air conditioning system. The foregoing information has been stripped of as much technical data as possible. While all the information is not 100% accurate to the last detail, it is accurate enough to be practical and useful in specifying equipment components.

Table 51T12A lists but a relatively few of the construction materials used in buildings. The “k” factors shown are approximate, but are suitable for use for all practical purposes. The “k” factor must not be confused with the “C” factor and the “U” factor. The “k” factor is called the conductivity of a material and applies to 1-inch thicknesses of materials. The “C” factor is called conductance and is the heat transfer for the total thickness of a material. The “U” factor is called the overall heat transfer coefficient. It applies to the heat transfer of one sq. ft. of surface.

Table 51T12A

k = Btu/hr/sq.ft/°F Difference/inch Thickness Material k Density Ibs./cu.ft. Asbestos (board) .135 3.65 Asphalt 4.2 132.3 Brick Wall, old 2.05 50.0 Cement Plaster 5.0 116.0 Concrete Wall 8.0 139.7 Corkboard .35 15.6 Fibre Board .30 13.2 Glass 5.0 161.7 Glass Wool .29 4.0 Hollow Tile 3.1 119.9 Linoleum 1.21 73.8 Masonite .33 15.0 Rockwool .29 6.0 Waterproof Paper .50 — Wood, Hard 1.0 45.0 Wood, Soft .80 32.0 When figuring the heat load for a particular application, the values in Table 51T12B may be used. These are average figures and may be relied upon, if the actual number of persons occupying the space is not known.

Table 51T12B AVERAGE SQ. FT. FLOOR SPACE - PER PERSON Cafeteria 10 to 16 sq. ft. Public Dining Room 16 to 20 sq. ft. Private Dining Room 20 to 30 sq. ft. Barber Shops 20 to 35 sq. ft. Beauty Parlors 15 to 20 sq. ft. Small Stores 20 to 35 sq. ft. General Desk Space (Large Offices) 60 to 80 sq. ft. Broker’s Office 30 to 35 sq. ft. Bank Work Room 80 to 90 sq. ft. Private Offices (Minimum of 3 persons) 40 to 60 sq. ft. Heating.

The design inside dry bulb temperature should be not lower than 70°F.

Cooling.

The design inside temperature and humidity for calculation of the cooling load should be not higher than 80°F dry bulb temperature and 50% relative humidity.

For installations which have a load that is predominantly a “people” load, a slightly lower design dry bulb temperature, along with an increase in relative humidity, is permissible to attain a more economical equipment selection. Under these conditions, the design inside dry bulb temperature may decrease to 78°F and the design inside humidity may increase to 55%

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -11- Table 51T12C shows the factors to be used for estimating the sensible heat gain through glass in average rooms. The data in Table 51T12C include haze, set-back and sash area, shade and storage factors, which have been found, by experience, to apply to typical buildings. Also included in Table 51T12C is the transmission gain across the glass at the time of peak solar load.

Factors For Sensible Heat Gain Through Glass* For Average Applications (Btu/Hr/Sq Ft Sash Area) EXPOSURE LOCATION AND OPERATION N NE E SE S SW W NW 30° Latitude 10 Hour Operation… 25 75 90 78 76 88 99 78 24 Hour Operation… 23 56 70 62 60 74 88 70 40° Latitude 10 Hour Operation… 23 67 90 80 77 90 99 75 24 Hour Operation… 22 49 70 63 62 77 88 67 50° Latitude 10 Hour Operation… 23 60 90 82 78 94 98 71 24 Hour Operation… 22 45 70 65 64 81 87 64 * For rooms or zones with more than one exposure, use the North values for all exposures except the one having the largest solar gain (usually greatest glass area). For buildings that are not typical and where extreme accuracy is believed desirable, refer to the current edition of the ASHRAE GUIDE.

The following correction factors may be applied to the values in Table 51T12C in order to obtain the zone load for the air cycle for other than average construction:

10 HOUR OPERATION 24 HOUR OPERATION Very light construction 1.10 1.25 Very heavy construction 0.90 0.90 Apartments & Hotels — 1.25* *This value is used to allow for “pull-down” and individual preference for room conditions on the air side.

Table 51T13 gives factors to be used for estimating the heat gain or heat loss due to transmission through walls, partitions, roofs, ceilings, and floors; and the heat loss due to transmission through windows. For the usual construction, wall transmissions are only a small proportion of the over-all load and, therefore, need not be treated as carefully as glass or roofs exposed to solar radiation. Where extreme accuracy is required, refer to the current issue of the ASHRAE GUIDE

TRANSMISSION GAIN FACTORS* FACTOR ITEM COOLING LOAD HEATING LOAD Windows: Single glass… — 1.13 Double glass… — .50 Walls: Sunlit… 0.30 0.25 Shaded… 0.20 0.25 Partitions… 0.20 0.35 Ceiling under finished room… 0.24 0.24 Floors: Over finished room… 0.24 0.24 Over basement with finished ceiling 0.20 0.20 On ground… 0.00 0.00 No ceiling Ceiling † Under Attic † No Ceiling Ceiling † Under Attic † Roofs: Uninsulated, frame, or heavy masonry… 1.20 1.07 0.93 0.60 0.35 0.30 Uninsulated, light masonry… 1.87 1.60 1.33 0.80 0.40 0.35 Insulated… 0.80 0.67 0.67 0.15 0.13 0.13 * These factors are combination factors. Do not confuse with “U” values (Btu/sq ft/F). †Factors based on flat-roof on flat-roof construction. The design load calculations should be based on the stated average occupancy of the building during the time of maximum design conditions.

Table 51T14 HEAT GAIN FROM OCCUPANTS TOTAL HEAT DEGREE OF TYPICAL ADULT MALE TOTAL HEAT SENSIBLE LATENT ACTIVITY APPLICATION BTU/HR ADJUSTED* BTU/HR HEAT BTU/HR HEAT BTU/HR Seated at rest Theater—Matinee 390 330 180 150 Theater—Evening 390 350 195 155 Seated, very light Offices, Hotels, work Apartments 450 400 195 205 Moderately-active Offices, Hotels, office work Apartments 475 450 200 250 Standing, light work; Dept. store, Retail walking slowly Store, 10¢ Store 550 450 200 250 Walking: seated Drug Store 550 500 200 300 Standing: walking slowly Bank Sedentary work Restaurant † 490 550 220 330 Light bench work Factory 800 750 220 530 Moderate dancing Dance Hall 900 850 245 605 Walking, 3 mph Factory 1000 1000 300 700 Moderately-heavy work Bowling ± Bowling Alley 1500 1450 465 985 Heavy Work Factory *Adjusted total heat gain is based on normal percentage of men, women, and children for the application listed and is based on the gain from an adult female being 85% of the value for an adult male and the gain from a child being 75% of the value for an adult male.

† The adjusted total heat value for sedentary work, restaurant includes 60 Btu per hour for food per individual (30 Btu/hr sensible and 30 Btu/hr latent).

± For bowling, figure one person per alley actually bowling and all others as sitting (400 Btu per hour) or standing (500 Btu per hour).

The above values are based on 80° F room dry-blub temperature. For 78° F room dry-blub temperature, the total heat gain remains the same, but the sensible heat values should be increased by approximately 10% and the latent heat values decreased accordingly.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -13- Recommended values of heat gain, in Btu per hour, for a number of commonly used appliances are shown in Table 51T15. For appliances other than those listed, consult the more extensive table in the current issue of the ASHRAE GUIDE.

HEAT GAIN FROM APPLIANCES Recommended Btu/Hr for Average Use ELECTRICAL* GAS* STEAM* APPLIANCE Sensible Latent Total Sensible Latent Total Sensible Latent Total Coffee Brewer, ½ gallon… 900 220 1120 1350 350 1700 — — — Coffee Brewer Unit, 4½ gallon… 4800 1200 6000 7200 1800 9000 — — — Coffee Urn, 3 gallon… 2200 1500 3700 2500 2500 5000 2400 1600 4000 Coffee Urn, 5 gallon 3400 2300 5700 3900 3900 7800 3400 2300 5700 Coffee Warmer, ½ gallon 230 60 290 400 100 500 — — — Food and Plate Warmer, per sq ft of top surface… 350 350 700 — — — — — — Food Warmer (only), per sq ft of top surface… — — — 850 450 1300 400 500 900 Fry Kettle, per sq ft of fry area… 3500 5000 8500 6000 4000 10000 — — — Griddle, per sq ft of fry area… 3000 1600 4600 — — — — — — Grille, Meat, per sq ft of fry area… 4700 2500 7200 10000 2500 12500 — — — Grille, Sandwich, per sq ft of fry area… 2700 700 3400 — — — — — — Hair Dryer, Blower… 2300 400 2700 — — — — — — Hair Dryer, Helmet… 1870 330 2200 — — — — — — Hair Dryer System, 5 Helmets… — — — 15000 4000 19000 — — — Hair Dryer System, 10 Helmets… — — — 21000 6000 27000 — — — Neon Sign (½” tubes), per ft of tube… 30 0 30 — — — — — — Permanent-Wave Machine… 850 150 1000 — — — — — — Sterilizer for Physicians’ Instruments… 650 1200 1850 — — — — — — Stoves Short-Order, Per sq ft of top: Closed-Top… — — — 3300 3300 6600 — — — Fry-Top… — — — 3600 3600 7200 — — — Open-Top… — — — 4200 4200 8400 — — — Toaster, Belt-Driven, 2 slices wide… 5100 1300 6400 7700 3300 11000 — — — Toaster, Belt-Driven, 4 slices wide… 6100 2600 8700 12000 5000 17000 — — — Toaster, Pop-Up, per slice… 37 8 45 — — — — — — Waffle Iron,20 Waffles per hour… 1100 750 1850 — — — — — — *When hooded and with adequate exhaust, use 50% of above values. Use Tables 51T16 & 51T17 to calculate the summer and winter requirements for both ventilation and infiltration. Use the larger of the ventilation or the infiltration quantities, but not both. The quantity of air should not be less than that required by applicable codes or ordinances, nor less than that drawn from the spaces by any exhaust fan which may be used. For normal ceiling heights, the quantity of ventilation air supplied should not be less than one-half air change per hour.

Table 51T16 VENTILATION OUTSIDE AIR Cfm per Sq Ft of APPLICATION SMOKING Cfm per Person Floor Recommended Minimum* Minimum* Apartment Some 20 15 .33 Banking Space Occasional 10 7.5 — Barber Shops Considerable 15 10 — Beauty Parlors Occasional 10 7.5 — Cocktail Bars Heavy 30 25 — Stores None 7.5 5 .05 Drug Stores Considerable 10 7.5 — Factories None 10 7.5 .10 Funeral Parlors None 10 7.5 — Hospitals—Private Rooms None 30 25 .33 Hospitals—Wards None 20 15 — Hotel Rooms Heavy 30 25 .33 Meeting Rooms Very Heavy 50 30 1.25 Offices—Private None 25 15 .25 Office—General Some 15 10 — Restaurants Considerable 15 12 — Cafeterias Considerable 12 10 — Theatres None 7.5 5 — The ventilation air should be calculated as follows:

...... cfm per person ×...... Persons=...... Cfm or

...... cfm per sq ft ×...... sq ft=.....Cfm

Table 51T17 INFILTRATION AIR CHANGES PER HOUR KIND OF ROOM OR Summer Winter BUILDING Weatherstripping or Storm Weatherstripping or Storm Ordinary Ordinary Sash Sash No Windows or Outside Doors 0.30 0.15 0.50 0.25 1.20 to 2.00 to Entrance Halls 1.80 0.60 to 0.90 3.00 1.00 to 1.50 Reception Halls 1.20 0.60 2.00 1.00 Bath Rooms 1.20 0.60 2.00 1.00 Infiltration through Windows: Rooms, 1 side exposed 0.60 0.30 1.00 0.50 Rooms, 2 sides exposed 0.90 0.45 1.50 0.75 Rooms, 3 sides exposed 1.20 0.60 2.00 1.00 Rooms, 4 sides exposed 1.20 0.60 2.00 1.00 The air quantity is computed as follows:

[(H)×(L)×(W)×(AC)]/60=...... Cfm

Where: H=room height, ft

L=room length, ft

W=room width, ft

AC=air changes per hour

The total simultaneous infiltration for an entire building will be approximately 50% of the sum of the infiltration allowances of individual rooms. Infiltration through Doors;*

For each person passing through a door leading to the outside or to an unconditioned space, add the following door infiltration or the summer infiltration through windows:

Usage Cubic Feet per Minute Infrequent 60 Ave rate 50 Heavy 40 36 Inch Swinging Door 100 *These figures are based on the assumption that there is no wind pressure and that swinging doors are in use in one wall only. Any swinging doors in other walls should be kept closed to insure air-conditioning in accordance with these recommended standards.

The total air circulation is not specified. It should be determined by the heating and cooling loads and the type and arrangement of supply openings. It should be adequate to meet the other requirements of the space.

Copyright © 1959, 2009, By Refrigeration Service Engineers Society. -15- The quantity and temperature of the treated air and the method of introducing it to the conditioned space should be designed to limit, to 3°F or less, the simultaneous variation in dry bulb temperature at the same level throughout that portion of a single room which is normally frequented by persons.

It is desirable to avoid air velocities exceeding 50 linear feet per minute in the zone between the floor and the five-foot level, in spaces normally frequented by persons who are not normally moving about.

Exception must be made of the vicinity of a supply or return grille when the construction requires it to be located below the five-foot level and in a space normally frequented by persons.

Cooling Load Calculations.

A Cooling Load Estimate Form, Figure 51F19, is included to facilitate the calculation of cooling loads for comfort air conditioning. It may be used for all normal applications. For special applications, or where extreme accuracy is believed desirable, refer to the current issue of the ASHRAE GUIDE.

Instructions.

The numbers of the following paragraphs refer to the item numbers on the Cooling Load Estimate Form. Separate air cycle calculations should be made for each zone.

1. All quantities should be inserted opposite the application conditions that exist when design conditions prevail.

2. Multiply the square feet of window sash area by the applicable factors given in Table 51T12C, to obtain the sensible heat load through glass.

3. Insert the square feet of net wall area (total wall area minus sash areas of windows), partition area (if applicable), roof or ceiling area, and floor area in the appropriate spaces on the form. Multiply these areas by the applicable factors given in Table 51T13, and by the difference between the design outside and inside dry bulb temperatures.

4. Multiply the number of occupants for each degree of activity by the applicable sensible and latent factors given in Table 51T14. Multiply the total wattage of lights in use at design conditions by the factor given on the form.

5. Multiply the total horsepower of motors in use at design conditions by the factors given on the form. Insert on the form the sensible and latent heat loads given in Table 51T15 for the appliances in use at the time of the design load.

6. Determine the cubic feet per minute of ventilation and infiltration air from Tables 51T16 & 51T17. Multiply the larger of the two cfm quantities by the design dry bulb temperature difference and by the factor given on the form to obtain the sensible load. Multiply the same cfm quantity used above by the difference in specific humidity (in grains per pound of dry air) between the design inside and outside air conditions and by the factor given on the form to obtain the latent heat load.

7. Total the individual sensible and latent loads.

8. Add the sensible and latent loads to determine the zone air-cycle load in Btu per hour. This value should equal the sum of the subtotals calculated for the various load components.

Use the same procedure as described for load calculations for cooling load, and calculate the air cycle loads as though the building were a single zone. Then multiply the various sub-totals of the load components by the factors in Table 51T18 to obtain the refrigeration load for a multi-zone building.

REFRIGERATION LOAD FACTORS FOR MULTI-ZONE BUILDINGS Type of Building Operation Glass Gain Transmission, Wall and Roof People & Lights Ventilation Office: Very Light Construction… 10 Hr 1.00 .90 .75 .98 24 Hr .80 .90 .60 .95 Average Construction… 10 Hr .80 .90 .75 .95 24 Hr .80 .90 .50 .91 Very Heavy Construction… 10 Hr .83 .90 .75 .92 24 Hr .83 .90 .45 .88 Apartments and Hotels… 24 Hr .75 .90 .45 .85 Equipment Capacity.

The total cooling load represents the capacity required of the equipment to produce the specified conditions inside when the specified conditions of temperature and humidity prevail outside and when all sources of load are taken at the maximum that will occur coincidentally. To assure that the proper equipment selection is made, the installation should also be analyzed for load conditions other than the maximum under which the air conditioning apparatus will be required to perform.

After the total heat load has been found, select or specify a condensing unit of the proper design (high suction pressure range), capacity and specifications (air-cooled, water-cooled, or combination air-water cooled, depending on geographical area and local conditions).

In order to have the proper ΔT between the coil temperature and the air to be conditioned, the condensing unit capacity is generally selected at approximately 40°F suction temperature.

Do not select a condensing unit designed for low suction temperature or medium temperature range of operations for any air conditioning application. To do so will quite likely overload the motor beyond its rated capacity, resulting in overheating and possible burnout of the motor.

The coil for an air conditioning system must be selected to balance with the condensing unit capacity, being sure to take into consideration the cfm air over the coil, the desired effective temperature in the conditioned space, and the relative humidity most suitable for the final results.