Monthly Archives: July 2017

Here’s another example of excellent information that is available to students and technicians in the HVACR industry, just for the asking.

 

Why Worry about Airflow

By

Randy F. Petit, Sr. CMHE
Regional Vice President
HVAC Excellence Office of Program Development

The air surrounding us can be measured, filtered, circulated, cooled, heated, de-humidified, and humidified; but first, HVAC professionals should understand it.

Airflow is one of the most over-looked functions of heating and cooling systems. System efficiency and comfort is compromised without the proper indoor airflow. A technician cannot properly charge an air conditioning system using the system superheat or subcooling methods when there is improper airflow. A heating system without proper airflow may operate at an unsafe temperature or short cycle, reducing the efficiency and life of the furnace.

The quantity of air flowing across the evaporator changes the sensible to latent heat ratio of the air conditioning system, in turn changing the amount of moisture the system can remove. Nominal airflow for a central air conditioning system is 400 cubic feet per minute (CFM) per ton of cooling capacity. Depending on ambient air conditions for a location, the quantity of air required across the evaporator for moisture removal could be as low as 325 CFM per ton. The airflow may be high as 450 CFM for environments with low humidity.

System efficiency, air filtering, sound levels, and most important of all, human comfort, are all influenced by system airflow. The airflow required for each room in a building is determined by doing a room-by-room load calculation. If a load calculation is not preformed then it’s all guesswork.

To understand how and why airflow is based on load calculations, we must understand air properties. The properties of air are constantly changing with any change in temperature, humidity level, or altitude.

Common properties of air are normally listed at sea level conditions, 1 atmosphere of barometric pressure and 68°F. The following are two air properties used in general calculations made by HVAC technicians:

Density = 0.075 lbs. per cu. Ft.

Specific heat = 0.24 Btu

Using these two properties of air (density, specific heat) and time, we can derive a factor used to calculate airflow. The factor, Btuh, and temperature difference are used to calculate the airflow volume required for heating and cooling.

This is a sensible heat factor used to calculate the volume of air based on the Btuh heating or cooling required for each room and the total Btuh for the structure. A room-by-room load calculation is the only acceptable method for determining required airflow for heating and cooling in each room. Some contractors guess by using the square footage or volume of a room. That does not work! Let say we have two rooms 12’ x 12’ x 8’. The rooms are side by side with one on the corner of the house. The loads for the floor and ceiling should be the same, but the load on the walls will be different. Room (a) has 96 Sq. Ft. of exposed wall and Room (b) has 192 sq. ft. The room (b) on the corner has twice the load on the wall and with a window in each wall the total can be a lot more. This means it must have more air entering for heat and cooling.

Remember I said the Sensible Heat Factor for air is used in the calculation. This is how numbers work. The properties for air at standard conditions are; Specific heat 0.24 Btu, Density 0.75 lbs. per cu. ft., and 60 minutes per hour. When multiplied we obtain a sensible heat factor of 1.08 (Some books round up to 1.1).

0.24 x 0.075 x 60 = 1.08

Let’s say room (a) has a load of 3,144 Btu and room (b) has a load of 3,288 Btu without adding Btuh loss for windows. Using the formula: CFM = Btuh ÷ (1.08 x ΔT), we can figure the amount of air required for each room. The temperature difference is determined using the sensible and latent loads calculated for the building. The sensible heat ratio is the sensible load divided by the total load of the building. Based on the sensible heat ratio (SHR) these are the recommended temperature difference across the evaporator.

SHR                                   Air ΔT

0.75 To 0.79                        21°F

0.80 To 0.84                        19°F

0.85 To 0.90                        17°F

If we use for our cooling load, a sensible load of 29,479 Btuh and latent load of 6,471 Btuh we will have a total load of 35,950 Btu.  

The sensible heat ratio will be 29,479 Btuh ÷ 35,950 = 0.82, which gives us a 19°F ΔT.

Now we plug the numbers into the formula for CFM.

Room (a) 3,144Btuh ÷ (1.08 x 19°F) = 153 CFM

Room (b) 3,288Btuh ÷ (1.08 x 19°F) = 160 CFM

When we look at the total airflow for the system, three scenarios can occur. For the same system, one contractor may use the 400 CFM per ton rule and adjust the blower for 1,200 CFM. The second contractor may use the proper method and adjust the blower for 1,437 CFM and the next contractor may base the airflow on the total Btuh for the system and adjust the blower for 1,754 CFM.

In our example for the rooms, there is only a 7 CFM difference in airflow. It doesn’t seem like it would make that much difference but it does. The temperature split though the evaporator, and how long the system operates determines the amount of moisture that will be removed from the air, making the home comfortable, dry, or humid.

With the nominal airflow of 400 CFM per ton which is the lower amount from our example. The air conditioning system will need to move tons of air a day in order to maintain set conditions. At standard conditions, air weighs 0.075 pounds per cubic foot. A three-ton air conditioning system, moving 400 CFM per ton, will have 1,200 CFM of air flowing through the system. 1,200 CFM multiplied by 0.075 lbs. per cubic feet equals 90 lbs. delivered each minute. Ninety pounds times 60 minutes per hour equal 5,400 pounds of air. If the system runs for 20 hours per day, we are looking at 108,000 lbs. or 54 tons of air moving through the system by a ⅛ to ¼ hp blower motor. We usually do not think about how much work the blower motor has to perform.

Over the years, manufactures have changed their design parameters in order increase equipment efficiency to meet the Seasonal Energy Efficiency Standards (SEER).  In the 1970s the evaporator design temperature started to climb from 40°F to 45°F and now as high as 50°F. With larger condensers, larger evaporators and smaller compressors the compression ratio is reduced requiring less electrical energy to do the same work in Btuh as the older units. The problem is the older systems with a 40°F evaporator had close to a 30% latent capacity enabling high humidity removal and the newer systems average 20% latent capacity. This makes it more important than ever to have the airflow right. This means when installing or servicing a system, matching the airflow with the sensible and latent loads is vital with high efficiency systems to get the proper moisture removal.

An analogy I used when teaching technicians pertains to the automotive industry. When you bring your car or truck in for service, you expect the job to be done right. Spark plugs gaped to specs, right air pressure in tires, proper oil level etc. If your gas mileage drops after the vehicle is serviced, you would complain, bring it back to dealer, and demand it be serviced at no cost to you. It is a lot harder for our customers to tell if their air conditioning system is operating at peak efficiency. They must depend on us to perform equipment service and installations, as they should be, to render efficiency and the comfort in their home you would expect.

If you would like to get in touch with Randy, his email address is rpetit@hvacexcellence.org

He can be reached by regular mail or phone:

HVAC Excellence Office of Program Development
292 Alice Street – Ama, LA 70031
Local: (847) 483-8781(Toll Free (877) 394-5253

 

Learn From Yesterday…..Live For Today……Look Forward To Tomorrow

Jim

I came across an article on the subject of superheat and subcooling and decided to post it here as an example of just how much valuable information is available today….all you have to do is search for it.

 

Subcooling and Superheat: Superheroes of System Charging

Don’t always assume you have to “add refrigerant.” Consider the three main causes of low suction pressure, and check superheat and subcooling to make the correct diagnosis

By Skip Egner | Aug 24, 2016

 

Here’s a common scenario. You go on a service call, put your gauges on a condensing unit, and find that the suction pressure is low. What do you do?

In too many cases, the answer is “add refrigerant.” But doesn’t it seem like a good idea to confirm that low refrigerant is the problem before you start adding refrigerant? That’s why checking superheat and subcooling is so important.

Let’s go back to the beginning. You go on a service call and find low suction pressure. However, this time you consider the three main causes of low suction pressure, and check superheat and subcooling to make the correct diagnosis.

 

CAUSE #1: Insufficient heat getting to evaporator.

This can be caused by low air flow (dirty filter, slipping belt, undersized or restricted ductwork, or dust and dirt buildup on blower wheel) or a dirty or plugged evaporator coil.

Checking superheat will indicate if the low suction is caused by insufficient heat getting to the evaporator. To check superheat, attach a thermometer designed to take pipe temperature to the suction line. Don’t use an infrared thermometer for this task. Then take the suction pressure and convert it to temperature on a temperature/pressure chart. Subtract the two numbers to get superheat.

For example, 68 psi suction pressure on a R-22 system converts to 40F. Let’s say the suction line temperature is 50F. Subtracting the two numbers gives us 10F of superheat. Superheat for most systems should be approximately 10F measured at the evaporator; 20F to 25F near the compressor.

If the suction pressure is 45 psi, (which converts to 22F) and the suction temp is 32F, the system still has 10F of superheat. The fact that these readings are normal indicates the low suction pressure is not caused by low refrigerant, but insufficient heat getting to the evaporator.

 

CAUSE #2: Defective, plugged, or undersized metering device.

Let’s say a system has 45 psi suction pressure (converts to 22F) and 68F suction line temperature, the superheat is 46F (68 minus 22). This indicates low refrigerant in the evaporator. However, before adding refrigerant, check the subcooling to be sure the problem isn’t caused by a defective, plugged, or undersized metering device.

While superheat indicates how much refrigerant is in the evaporator (high superheat indicates not enough, low superheat indicates too much), subcooling gives an indication of how much refrigerant is in the condenser.

Subcooling on systems that use a thermostatic expansion valve (TXV) should be approximately 10F to 18F. Higher subcooling indicates excess refrigerant backing up in the condenser. On TXV systems with high superheat, be sure to check the subcooling as refrigerant is added. If the superheat doesn’t change, and the subcooling increases, the problem is with the metering device. In the case of a TXV, it’s likely that the powerhead needs to be replaced.

To check subcooling, attach a thermometer to the liquid line near the condenser. Take the head pressure and convert it to temperature on a temperature/pressure chart. Subtract the two numbers to get the subcooling.

For example, 275 psi head pressure on an R-22 system converts to 124F. The liquid line temperature is 88F. Subtracting the two numbers gives 36F. High superheat and high subcooling indicates a problem with the metering device.

Keep in mind that subcooling won’t increase on systems with a liquid line receiver, as extra liquid will fill the receiver instead of backing up in the condenser. Receivers are rare on air conditioning systems, but very common on small refrigeration systems such as walk-in coolers and freezers. If a system with a receiver has high superheat and the liquid line sight glass is full of liquid (no bubbles), check the metering device. If the sight glass has bubbles, the system could be low on refrigerant, or the liquid line filter/dryer could be plugged. Your clue here is that a noticeable temperature drop across a liquid line filter/dryer indicates it’s plugged.

 

CAUSE #3: Low refrigerant.

Yes, it’s true! There are indeed some cases where low suction pressure is going to be caused by low refrigerant. If the superheat is high and the subcooling is low, the refrigerant charge is probably low. Just keep in mind two things here: first, find and fix the leak. Second, monitor both superheat and subcooling as you add the refrigerant, to prevent overcharging.

 

Skip Egner is a technician with CS Service Experts, Ft. Myers, FL. He has been in the HVAC industry for 30 years, and in 2006 won the North American Technician Excellence (NATE) Certified Technician Competition-at HVAC Comfortech. He can be reached at 239/768-2665.

 

Learn From Yesterday…..Live For Today…..Look Forward To Tomorrow

Jim

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