SUPERHEAT CONTINUED

Air conditioning technicians must learn how to measure superheat for two big reasons:
1. To be able to prove coil performance.
2. To be sure to protect the compressor.

Here is an example of a air conditioning line that had frozen up and some explanations about it.

First, turn the thermostat from the fan in the "Auto" poisition (where it should be normally set) to the "Fan-On" position. That turns the compressor off and the indoor fan on to melt the ice off of the coil as soon as possible. Of course, the filter should be checked to make sure it is not dirty and restricting airflow and the supply vents should be check to make sure they are all open. That is about all that can be done while the coil is thawing.

As a coil freezes the airflow becomes less and less until it is a solid block of ice and the only air that flows is around the coil ends. Eventually the ice pattern will travel down the suction line and on to the compressor. As air flows around the coil ends, it will start to melt. In this case, the ice quality is aerated ice. It has a lot of air in it because of the way it was formed. It will melt fairly quickly and when air is flowing, then a good look can be taken at the coil inlet to be sure the coil is not dirty. Then the unit will be started and the coil checked for performance. In the meanwhile, a look around to see if any oil can be found on the surface of the piping, a sure sign of a leak. There was oil around the liquid line service port. Checking with an electronic leak detector proved there was R-22 refrigerant leaking. Apparently the valve stem was not tightened. Tighten the valve stem and the refrigerant is no longer leaking.

Starting the unit and looking at the gauges it appeared the suction pressure had dropped to 30 psig. This is R-22, so the refrigerant is boiling in the coil at 7°F°, the superheat is 68° (75° - 7° = 68°).

The compressor does not have cool enough return suction gas to keep it cool and the coil cannot be performing well with so little refrigerant in it. There is a fixed bore orifice on this system. What superheat should the system have?

On this system, the piping is very short. It is a split system and the suction line is only about 8 feet, so let’s say that the superheat should be 12° at the condensing unit. We are going to have to make some assumptions here. We cannot check the actual superheat at the coil, so we will assume that the tubing will gain about 2° along the way from the ambient air. The suction line is well insulated. If the suction line were longer, we would assume that it would gain more super heat due to conduction from the ambient air. You can use two rules of thumb for measuring the superheat at the condensing unit:

1. When the line set is 10 to 30 feet, we expect the superheat to be from 10° to 15° measured at the condensing unit.
2. When the line set is 30 to 50 feet, we expect the superheat to be from 15° to 18° measured at the condensing unit.

Please understand that there are some qualifications on these rules of thumb. If the unit has a charging chart furnished, you should use it. It is probably more accurate. The humidity in the conditioned space should not be excessively above or below 50 percent. Unless the unit has been off for a long time in a humid climate, you should be all right.

Most importantly, the condensing temperature or the head pressure should be close to a design day. The outside temperature on a design day is 95° and the condensing temperature should be about 30° higher than the design outdoor temperature, or 125°. The head pressure should be about 278 psig for R-22.

The reason for this is because the air conditioning system is designed by the manufacturer to have an exact operating charge in each coil. With a fixed bore metering device, to ensure the correct amount of refrigerant in the evaporator is to have the correct pressure difference across the expansion device. With a suction pressure of about 70 psig and a head pressure of about 278 psig, the correct flow will occur across the expansion device. The evaporator will have the correct charge and the condenser will have the correct level of liquid refrigerant in it.

What do you do if it is not a design temperature day?”

If it is not a design day, you can block the air to the condenser until you have the same head pressure as a design day and charge the unit until the evaporator has the correct superheat. The thermometer says that the outdoor temperature is 85° and we will want the head pressure to be about 275 psig to simulate a design day of 95°, so we will block some of the airflow to the condenser by either blocking the fan or putting some plastic around the condenser. At this time, there is not enough refrigerant in the unit to get the right head pressure. As we add refrigerant, watch the head pressure and adjust the airflow to maintain about 275 psig.

Don’t add refrigerant too fast. It is a lot easier to charge a system to the correct superheat while adding vapor than it is to get the superheat correct by removing refrigerant from the system. When it gets close, stop adding refrigerant and let the system run for a few minutes. Watch for the superheat to vary for a few minutes. With a fixed bore metering device the system will have too much refrigerant in the condenser for a few minutes, resulting in high superheat and then it will overfeed the evaporator for a few minutes. Until the system gets in balance, it will vary for a while. Sometimes referred to hunting equilibrium. Once it reaches equilibrium, the pressures will maintain a steady state.

When the system stabilized and the suction pressure was 65 psig and the line temperature was 60°. Then what was the superheat?

The line temperature is 60° and the suction pressure is 65 psig. That means the liquid in the evaporator is boiling at 38° corresponding to 65 psig. So the superheat is now 22° (60° - 38° = 22°). More vapor refrigerant must be added carefully.

After a small amount of vapor was added, and after a few minutes, new readings of 70 psig and a line temperature of 52°. What is the superheat now?

A reading of 70 psig corresponds to 41° and the line temperature is reading 52° so the superheat is 11° (52° - 41° = 11°). That seems perfect.

Let me tell you the steps that we took:
1. Fastened gauges.
2. Fastened a temperature lead to the suction line close to where we took the suction line pressure.
3. Started the unit and observed the pressures.
4. Blocked the airflow to the condenser until we got the head pressure to correspond to the design conditions of 125° condensing temperature. For R-22 that was about 275 psig.
5. Then we added refrigerant until we had the desired superheat, which was supposed to be 10° to 15°, and we got 11°.

We were able to get the charge very close to the exact charge the manufacturer wanted by:
1. Creating the correct pressure drop across the orifice metering device so the correct refrigerant charge was in the evaporator and the condenser.
2. Verifying that the evaporator charge was correct.
3. Protecting the compressor.

The next step that will be covered is subcooling.

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about Indoor Air Quality and other HVAC topics, click here to visit our website.

HVAC SUPERHEAT & SUBCOOLING

Superheat and subcooling are not easy topics and it will be explained in a couple of different blogs. What it is. Why it is important. How it is used. How it is monitored.

Let’s take a look at water, a substance that everyone has some familiarity with. Water is a refrigerant just like R-22 or R-410A. It just operates at different pressures and temperatures. Most people know that water boils at 212°F. What most people don’t know about water is that it boils at 212° only at standard conditions — a barometric pressure of 29.92 in. If you change the barometric pressure, you change the boiling temperature. For example, if you take a pan of water high in the mountains, the barometric pressure is less, because the column of air above it is not as high, so the water will boil at a lower temperature. Air is matter; it has weight and takes up space, the definition of matter. The weight of the air above is what creates atmospheric pressure. When the water boils up on the mountain, it boils at a lower temperature because it is at a lower pressure.

A very important point: The boiling temperature of a liquid can be controlled by controlling the pressure above the liquid. Once you understand that statement, you can better understand what the evaporator portion of the refrigeration system is all about.

Since water boils at a lower temperature up on the mountain, it is very hard to cook certain foods. Beans and potatoes don’t get up to the required temperature. Your grandmother solved this problem with a pressure cooker. By increasing the pressure in the pan with a lock-on lid, she could raise the boiling temperature by containing the steam on top. She typically raised the pressure to about 15 psig above atmospheric pressure and the beans would cook while at high altitude up on the mountain. Pressure cookers are also used at sea level to elevate the temperature and shorten the cooking time.

This boiling temperature that we are talking about is also known as saturation temperature. The saturation temperature for water at standard conditions is 212°. It is called saturation temperature because at standard conditions, 29.92 in. Hg, the water is saturated with heat; you cannot heat water above 212°. If you add more heat, the water will just boil faster.

While water is boiling, it is changing its state from a liquid to a vapor. Heat it more and you just make more vapor, or steam.

Another very important fact: The steam that is leaving the water is saturated with heat. If you add more heat to it, it will be superheat. Heat that is higher than the saturated temperature is superheat.

If you take any heat out of the steam above the water, it will condense back into water and fall back into the pan.

That is a good explanation of boiling of water and turning it to steam. How does it apply to air conditioning?

The evaporator in a refrigeration system is just like a pot of water on the stove. The only difference is the air conditioning evaporator (or refrigeration evaporator) is like the pressure cooker, except at different temperatures and pressures. The liquid refrigerant boils and changes to a vapor and the vapor is superheated to a temperature of about 10° above the boiling or saturation temperature. Actually, water can be used as a refrigerant in an air conditioning system but it is hard to work with.

The design temperature for an air conditioning system evaporator is 40°. When you reduce the pressure low enough on a container of water, it will boil at 40°. The reason water is not used as a refrigerant is because the amount of vapor generated is so large that a compressor is not feasible.

Modern refrigerants do not require large piston or chamber displacements because they do not have to boil large volumes of vapor.

Let’s get back to using water as an example for a refrigerant. If you reduce the vapor pressure above the water to .248 in. Hg of vacuum, the water will boil at 40°, the actual design temperature for an air conditioning coil. This would work, but the vacuum pump would have to be enormous. And operating a system at those low vacuums would require that it be perfectly leak-free. These design conditions prevent water from being used in the compression type of refrigeration. Water is used in another type of system, called an absorption system.

Up to now, we have only been talking about boiling or evaporating the refrigerant. After the refrigerant is boiled to a vapor, the vapor has to be removed by a pump, a vapor pump. That is what the compressor is. The vapor is removed in the suction line to the compressor. A compressor is a vapor-only pump. It takes the vapor and compresses it to a very small volume and pushes it into the condenser. When we say that a compressor is a vapor pump that is exactly what we mean. It will not tolerate liquid. If liquid gets into the cylinder of a piston compressor, it will not compress. The piston may push some liquid through the discharge valve, but the liquid will scrub the lubrication from surfaces that need to be lubricated. If the compressor cannot push the liquid through the valves, it will stall, and break the valves, shaft, piston or rod. The one thing it will not do is compress. Some compressors are more tolerant to liquid, such as the scroll or rotary compressors.

Once the vapor is removed from the evaporator, we must be sure that it is vapor, not liquid. Remember, the liquid refrigerant will have a saturation temperature corresponding to the pressure. In a typical evaporator that is made into a coil of tubing, when the liquid all boils to a vapor, the vapor temperature will be saturated with heat at the same temperature as the liquid refrigerant. In simple terms, suppose the boiling saturation temperature is 40°; the vapor rising off of the liquid will be 40°. Remember it is saturated with heat; if we remove any heat, it will turn back to a liquid. If we raise the temperature, the vapor will take on heat, called superheat. When the vapor is superheated, there is no liquid present.

There are two things that superheat tells the manufacturer, and the manufacturer expects the service personnel to understand these two things:

1. Superheat proves the efficiency of the coil. The manufacturers’ designers want the coil to be as full of liquid refrigerant as possible because it is a more efficient heat exchange.

2. The correct superheat reading assures protection of the compressor from liquid damage.

Most manufacturers use 10° (+ or - 2°) of superheat, or somewhere between 8° and 12° superheat. We have to be able to accurately take superheat readings to know how a coil is performing and that the compressor is protected. It is vital that we learn how to take superheat readings.

The most accurate way to measure superheat is to take the pressure reading and the temperature reading at the same location. You really want to know what the pressure and temperature readings are at the coil outlet (Figure 4), but you usually do not have a pressure port at the evaporator, so you may take the temperature and pressure at the suction line at the condensing unit and make some assumptions.

TO BE CONTINUED

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about Indoor Air Quality and other HVAC topics, click here to visit our website.

HEAT PUMPS

When you think about cooling a hot building, you probably don't think of heat pumps. In fact, the words "air conditioner" are likely the first things that come to your head unless you're tight with your pennies. Then you might go with "window fans." As it turns out, a heat pump can both heat and cool, and in some applications, it's preferred to separate heating and cooling systems.

Simply put, a heat pump is a device that uses a small amount of energy to move heat from one location to another. Not too difficult, right? Heat pumps are typically used to pull heat out of the air or ground to heat a home or office building, but they can be reversed to cool a building. In a way, if you know how an air conditioner works, then you already know a lot about how a heat pump works. This is because heat pumps and air conditioners operate in a very similar way.

One of the biggest advantages of a heat pump over a standard heating ventilating and air conditioning (HVAC) unit is that there's no need to install separate systems to heat and cool your home. Heat pumps also work extremely efficiently, because they simply transfer heat, rather than burn fuel to create it. This makes them a little more green than a gas-burning furnace. And they don't just heat and cool buildings. If you've ever enjoyed a hot tub or heated swimming pool, then you probably have a heat pump to thank. They work best in moderate climates, so if you don't experience extreme heat and cold in your neck of the woods, then using a heat pump instead of a furnace and air conditioner could help you save a little money each month.

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about Indoor Air Quality and other HVAC topics, click here to visit our website.

Converting Constant to Variable Volume

Variable-Volume Conversion Can Offer Significant Energy Savings


Some interesting information from "the Air Conditioning|Heating|Refrigeration News".

Converting constant-volume package rooftop units to variable-volume operations can have a significant impact on energy consumption. Since their creation, package rooftop units (RTUs) have been designed to operate at either zero or 100 percent.

An RTU is defined as an air handler that is designed for outdoor use, typically on roofs. The science of these units has remained basically unchanged since the invention of electromechanical cooling by Willis Carrier back in 1902. Although the idea isn’t much different than having forced air blow over coils to condition the indoor air while removing humidity, the design of the air handler has changed over time. Most of the units now include steel framing, insulated panels and filters, heating/cooling elements, a mixing chamber, and a supply fan. Almost all units incorporate fresh air to combat sick building syndrome, and high carbon dioxide levels indoors. This is typically accomplished using a basic damper or an economizer controlled with air sensors.

Although the types of refrigerants have changed (a lot) since the inception of what we call air conditioning, the actual refrigeration cycle is arguably the same as it was in the beginning. Consisting of a condensing coil, metering device, evaporator coil, and compressor, the refrigeration cycle is a little more than a mechanical device of energy transfer.

The process of energy transfer is a combination of all of the components of both the air handler itself and the refrigeration cycle. This harmony of mechanically operating devices and the physics of the refrigerant is what keeps us comfortable every day. The
last part is the most important. Without that reliable level of comfort, our modern world would be thrown into disarray. Air conditioning was once considered a luxury, but the days of when we worked in hot buildings all year long are over. It is now a necessity, especially in commercial buildings without operable windows. Without it, people would be passing out daily from heat exhaustion and a lack of fresh air. It would be absolute chaos. So if the process of air conditioning is so great, and if it has remained virtually unchanged since it was first created by Carrier, than why change it at all?

One could start by making the old argument, “If it ain’t broke then don’t try to fix it.” This argument was once a crowd pleaser and an easy out for anyone who just didn’t care. It was the best way to tell others that what they have is good enough so just accept it and move on. Well, those days are gone, too. We have a great and many things to worry about now beyond what were considered problems in the days of our parents. One thing, for instance, is energy consumption or the reduction thereof.

Global warming and climate change are becoming household terms. The science of each has been proven by more than 98 percent of all who have researched the topics. Is it us causing these problems or are they natural occurrences? I could make arguments for both, but the fact is that globalization and over population is speeding up the process. So I ask you, what’s the solution? If the problem is bigger than you and I, what can we do to help? Actually, in the HVACR industry, we can do a lot. We can do more than most, actually.

Mechanical cooling units consume up to 50 percent of the total energy used by commercial buildings. More than two-thirds of these buildings are conditioned with RTUs. The vast majority of these buildings with RTUs were built more than 30 years ago during a time when the informal norm was, “hey, if we need one, let’s throw in two.” This may sound anecdotal, but studies have shown that some buildings from those days have RTUs that are nearly 75 percent oversized, and waste far more energy than I’d like to admit. People have been literally throwing money out the window for generations, and we are finally making a move to put a stop to it.

It all starts when we consider how much mechanical cooling is needed to condition a space. We can figure this out based on occupancy and temperature. When it’s determined how much cooling is needed to satisfy the space temperature during the peak occupied schedule, we can compare those values to the existing design.

One important factor that we need to remember before getting into energy reduction is how much fresh air is needed at any given time. Local codes always overrule any suggested design standards, but for argument’s sake, minimum settings of 10 percent outside air with 30 percent supply fan speed is a safe bet.

Taking into consideration the desired space temperature, occupancy levels, outside air, and static pressure minimums, we can now begin to consider the option of cycling down the RTU to save energy without sacrificing the personal climate of the occupants.

So where do we begin? How do we go about creating savings when taking all this into consideration? The most tested and proven method is to convert a constant-volume unit into a variable-volume unit. This can be done to virtually any RTU that isn’t on the verge of falling apart. By installing a variable-frequency drive (VFD) on the blower motor, you can safely slow down the fan speed. Although this sounds relatively easy to do, it isn’t. There are companies out there who have devoted countless engineering hours and research dollars to design retrofit devices that accomplish this goal, but these devices include far more than just a VFD.

Slowing down the fan is just a single variable in the equation. You also have to take into account the amount of static pressure it takes to move the air to the end of the ducts, supply/return/mixed air temperatures, and compressor staging. Some companies out there are building RTUs with compressors that can be converted to variable-speed operation. Most of the older RTUs use one or more compressor(s) and stage them accordingly. Controlling the compressor staging can be a highly difficult task, but the science is there, and it’s been heavily tested.

Converting a constant-volume RTU to variable volume can save anywhere between 20-50 percent in energy savings if done correctly. There have even been documented cases of up to 70 percent savings in some applications. The market seems to be primed and ready for these changes to take hold. The problem is most people don’t understand the benefits just yet, which means it’s up to us to raise awareness by educating our customers about the benefits of energy-saving techniques. Without this change awareness, the equipment we service will essentially be stuck in a short cycle.

Publication date: 11/4/2013

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about Indoor Air Quality and other HVAC topics, click here to visit our website.

Governmental Action on Refrigeration Safety, Regulations

I found this to be an interesting article from "the Air Conditioning|Heating|Refrigeration News". I hope our reader find it as interesting.

Refrigeration service technicians point out the need to keep up to date with what is happening elsewhere that could affect them. So, from time to time, we like to bring readers up to date on some news items related to refrigeration regulatory and safety issues.

Energy Reduction

First, attention needs to be paid to the changing landscape regarding refrigeration equipment energy efficiencies. The refrigeration sector continues to wend its way through new rules governing the efficiency of commercial refrigeration equipment including walk-in coolers and freezers. It is a complicated process because different regulations apply to different types of equipment and their respective sizes.

In that regard, the U.S. Department of Energy (DOE) has issued final guidance concerning automatic commercial ice makers and commercial and industrial equipment test procedures.

The fact that efforts are being made in this regard pleases the advocacy group Alliance to Save Energy.

In a statement issued in early September shortly before the final guidance document was announced, the alliance said it “applauds the Department of Energy’s issuance of the Notices of Proposed Rulemaking for commercial refrigeration equipment and walk-in coolers and freezers. This latest action demonstrates the DOE is serious about helping Americans save energy and money.”

Venting Issues

While reports of refrigerant venting violations are few and far between, when they are announced, they can be whoppers. In 2004, the Dominick’s supermarket chain in the Chicago area paid an $85,000 fine for venting HCFC-22 in six of its stores.

The ante has gone up quite a bit.

In a settlement agreement announced Sept. 4, 2013 by the U.S. Environmental Protection Agency (EPA), the supermarket chain Safeway, which owns Dominick’s, has agreed to pay a $600,000 civil penalty and implement a corporate-wide plan to significantly reduce its emissions of HCFCs from refrigeration equipment at 659 of its stores nationwide. That process is estimated to cost approximately $4.1 million. The settlement involves the largest number of facilities ever under the Clean Air Act’s (CAA’s) regulations governing refrigeration equipment, the EPA said.

The settlement resolves allegations that Safeway Inc. violated the federal CAA by failing to promptly repair leaks of R-22 and failed to keep adequate records of the servicing of its refrigeration equipment.

According to the EPA, “Safeway will now implement a corporate refrigerant compliance management system to comply with stratospheric ozone regulations. In addition, Safeway will reduce its corporate-wide average leak rate from 25 percent in 2012 to 18 percent or below in 2015. The company will also reduce the aggregate refrigerant emissions at its highest-emission stores by 10 percent each year for three years.”

The EPA announcement did not specify when the violations took place.

Safeway Inc., headquartered in Pleasanton, Calif., will be closing stores in the Dominck’s chain by next year due to underperforming sales, according to reports.

Safety Issues

Issues related to proper safety procedures takes on additional importance when the refrigerant used is ammonia.

According to an announcement made on Aug. 12, 2013 by the Occupational Safety and Health Administration (OSHA), a refrigeration warehouse in Honolulu faces $251,330 in fines after federal and state investigators discovered health and safety violations.

Many of the violations were related to the fact that the facility operates on ammonia refrigeration. Others were related to issues that might be found in any facility where workers are present.

Inspections at Unicold Corp. were conducted in February by the U.S. Department of Labor’s OSHA and Hawaii’s Department of Labor and Industrial Relations’ Occupational Safety and Health Division.

According to OSHA, 58 serious violations related to hazards associated with process safety management of highly hazardous chemicals in the ammonia refrigeration system; missing stair railings; unguarded floor openings on stairway platforms; deficiencies in the company’s plan for the response to workplace emergencies; and inadequate electrical equipment.

Other violations included locked and sealed exit doors, failure to keep exit routes free and unobstructed, and failure to label exit routes and post signs clearly indicating the route to the nearest exit.

Inspectors found 13 of the exit doors were locked from the outside and sealed shut, and that workers could not open or reach emergency exit doors because storage racks filled with pallets of products blocked the doors.

Publication date: 10/21/2013

Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about Indoor Air Quality and other HVAC topics, click here to visit our website.