CFM Details

7/27/2019 CFM Details

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Tape measure

Ductulator or computer with Internet access

Calculator

Instructions

1. Instructionso 1

Measure the length and width of a room where you need to calculate the requiredairflow. Multiply these together to get the square footage of the room, i.e., if theroom is 10 ft x 10 ft, the square footage would be 100. sq. ft. Airflow is measuredin CFM, or cubic feet per minute. It is a measurement that shows how much airflows into a space per minute. You want 1 CFM per square foot, so the above

room's requirement would be 100 CFM. For rooms with a lot of windows or havelong periods of direct sunlight, you want 2 CFM per square foot.

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Continue measuring each room and calculating the CFM requirement for each.

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Add up each room's required CFM. This total is the amount of CFM your HVACunit needs to produce in order to properly heat or cool the space.

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If you don't already have a HVAC unit for the space, purchase one at your localHVAC retailer that matches your CFM requirement. Install the HVAC unit permanufacturer's instructions.

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Using your Ductulator, match each room's CFM requirement to your plannedfriction rate. Most HVAC systems use a friction rate of 0.1" water column per 100
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feet of duct. As you match each room's CFM to the Ductulator, it will give youthe size of duct you need to run to the room.

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Work back from the farthest room. As you add a room, make sure you add theCFM and increase the duct size accordingly. Install the HVAC duct from eachroom to the HVAC unit. A professional HVAC installer may need to be contactedfor proper duct installation.

Read more: How to Calculate CFM in HVAC | eHow.comhttp://www.ehow.com/how_5677999_calculate-cfm-hvac.html#ixzz2FP4psQj0

ules of thumb are abbreviated short cuts, time savers, and approximations. Each is a quickestimation of what most systems should do most of the time, but remember that none of thesenumbers are infallible.

The rules are not cast in concrete. They do not always apply to every system, nor do they work inevery circumstance. It is expected that the technician using them is a professional who can usereason and exercise caution in their application.

Rules of thumb usually assume certain normal design and operating criteria, which are not

always present.

The rules are not to be used for system design purposes. Rather, they are the result of carefullystudying good system designs. Technicians and engineers, however, can benefit from using themas tools for estimating and solving operational problems.

Here are some common hvacr rules of thumb.

AIRFLOW

Air quantities (cfm):

400 cfm per ton of cooling is needed for normal comfort applications, 500 cfm per ton ofcooling for heat pump and high sensible heat applications, and 350 cfm per ton of cooling forhigh latent heat applications.

These are the approximate amounts of air that should be moving through evaporators for threecommon applications. Say, for example, you have a comfort cooling application at a savings and
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loan office that has a 10-ton system. Therefore, you multiply 10 tons by 400 cfm per ton toreveal the total system airflow requirement of 4,000 cfm under normal operating conditions.

One cfm is needed per square foot (1 cfm/sq ft) of floor area.

This is the average air quantity required for a room or an entire building. This number is basedupon an averaged heat load calculation for comfort cooling. There is an assumption of an 8-ftceiling, no unusual window areas, and average insulation. This rule of thumb provides about 7.5air changes per hour.

This rule is a quick way to approximate the cooling load for a room or building and may behelpful in estimating room air quantities. For example, to estimate the number of tons required tocool a 1,600-sq-ft home, multiply 1,600 by 1 cfm/sq ft to get 1,600 cfm of air. Using the 400-cfm/ton rule, divide 1,600 cfm by 400 cfm/ton to get 4 tons of required cooling.

Six to 10 air changes are needed per hour.

This is the average number of times in each hour that the air in the building or room (assuming7- to 8-ft ceilings) is removed and replaced by circulating the air.

Air velocities (fpm):

700 to 750 fpm are needed for disposable filters; 250 fpm for HEPA filters; and 500 fpm forelectronic air cleaners.

These are the recommended air speeds for filters. Speeds higher than these will decrease filteringefficiency.

550 to 600 fpm maximum and 400 fpm minimum are needed for evaporators; 1,000 fpm forcondensers; and 700 fpm for hot water coils.

These are the recommended velocities for coils. Speeds faster than these may cause condensationto be blown off of the evaporator fins and down the duct. Speeds slower than 400 fpm may causethe evaporator to freeze over.

Terminal velocities are the velocities at the end of the run, such as registers or diffusers, wherethe air enters a room.

Bypass air should be at a rate of 0.1 to 0.35, or 10% to 35%.

This is the average percentage of air that bypasses or fails to come into contact with theevaporator as it passes through the evaporator. The higher the air speed in fpm, the higher thebypass factor. High bypass factors are normally found on heat pumps in the heating cycle, and onhigh sensible heat applications like computer room systems. Low bypass factors are found onhigh latent heat applications, such as those found in restaurants (especially cooking areas) andbowling alleys.


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BUILDING AIR PRESSURE

Building air pressure is normally between 0.03 to 0.05 in. of static pressure. Infiltration air leaks

into a building from the outside, such as through doors and cracks around windows. Thisinfiltrating air is unfiltered and untreated.

To prevent infiltration, a building is kept at a slightly positive air pressure. This is accomplishedby adjusting the outside air dampers so they are open slightly more than the exhaust air dampers.Building air pressure measurements should be taken whenever an adjustment is made.

The thermoststic expansion valve controls evaporator superheat, which means it controls theamount of liquid refrigerant boiling in the evaporator.

REFRIGERATION AND A/C

There is one horsepower (1 hp) for each ton of cooling. It takes a l-hp compressor to provide1 ton of comfort cooling. Therefore, a 25-ton system has a 25-hp compressor motor. Note: Thisapplies to comfort cooling only. A lower-suction-pressure application such as refrigeration willhave a higher hp/ton ratio. The lower the suction pressure, the higher the hp/ton required.

Thermostatic expansion valve superheat should be 8 to 12F.

The valve controls evaporator superheat, which means it controls the amount of liquid refrigerantboiling in the evaporator. The lower the superheat, the more liquid in the evaporator.

However, superheat that is too low can cause the valve to lose control of the superheataltogether. The valve only controls evaporator superheat. If the valve is properly controlling

superheat, do not adjust it to change evaporator temperature or pressure. The valve is not anevaporator temperature or pressure control, so do not attempt an adjustment without measuringthe superheat. The 8 to 12 superheat is normal for air conditioning systems. Low-temperature systems often use different valves and lower superheat settings.

Remember, superheat is a temperature differential, not a single temperature measurement.


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Normal system superheat is 20 to 30. System superheat is not the same as thermostaticexpansion valve superheat. System superheat consists of the temperature differential from thepoint in the evaporator where all the refrigerant has changed to a gas to the suction line about6 in. from the compressor service valve.

Normally, charged and operating systems will have a system superheat of 20 to 30. Systemsuperheat greater than 30 may indicate that the low side of the system is starved forrefrigerant. System superheat less than 20 may indicate that the low side is overcharged.

Again, system superheat is a temperature differential, not a single temperature measurement. Thevalve superheat is part of the system superheat.

High-side subcooling is 5 to 15.

Refrigerant in the condenser changes from a gas to a liquid and then begins to subcool.Subcooling takes place in the bottom of the condenser and in the liquid line. The amount of

subcooling taking place in the condenser is 5 to 15. It is never greater than 15.Since only liquid subcools, the amount of subcooling is an indication of the amount of liquid inthe high side of the system. Overcharged systems have higher-than-normal amounts ofsubcooling. Undercharged systems have low subcooling.

Comparing high-side subcooling and low-side system superheat will usually solve mostrefrigeration cycle problems of overcharge, undercharge, and restrictions.

Normal evaporator air temperature drop is 18 to 20.

Measuring the temperature drop, or difference of the air as it moves through the evaporator, isone method of approximating correct airflow. Assuming 400 cfm per ton of cooling, when theairflow is correct there will be an 18 to 20 drop in air temperature.

Abnormally low airflow will remain in contact with the evaporator longer, and will be chilled toa lower temperature and greater temperature difference.

Normal condenser air temperature rise is not to exceed 30.

Lack of air over the condenser results in high head pressure, lower system capacity, andincreased power consumption. Air is heated as it passes through the condenser.

Low air across a condenser is indicated by an air temperature rise greater than 30. The smallerquantity of air over the condenser must absorb the same amount of heat; therefore, thetemperature rise is greater.

WATER


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For chilled and condenser water, the expected temperature drop or rise is normally 8 to10. Just like air, water has a temperature drop or rise as it is cooled or heated. As a chillercools water, the number of gallons per minute (gpm) circulated determines the number ofdegrees of drop.

The same is true for a water-cooled condenser. An 8 temperature change is proportional toabout 3.9 gpm circulated per ton. A 10 change is proportional to about 2.4 gpm circulated perton.

For example, if a 25-ton system has a chilled water temperature differential of 8, then 25 tonstimes 3.9 gpm/ton gives a total estimate of the chilled water flow of 97.5 gpm.

Normal chilled water supply temperature is in the range of 45 to 55. This is the correctrange of chilled-water supply temperatures for comfort cooling.

Normal hot water supply temperature is in the range of 100 to 150. This is the correctrange of hot water supply temperatures for hot water comfort heating or reheat. Systems that

require hot water temperatures over 150 usually have a low flow rate problem.Christopherson is an hvac instructor at San Jose City College, San Jose, CA.

Publication date: 02/05/2001

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