The Condenser

The Condenser

Once the refrigerant leaves the compressor, it flows through the hot gas line into the top of the condenser. The condenser is an important part of the refrigeration cycle where heat is rejected from the refrigerant. Three things happen to the refrigerant in the condenser. First, its cooled to its saturation temperature, the condensing temperature. Second, the refrigerant vapor is converted into its liquid form. And then, the liquid refrigerant is cooled more.

In an air cooled, comfort cooling system, the condenser is also the repository for the varying amounts of liquid refrigerant on the high hide of the system. It plays the part of the receiver.

Before we can think about the condenser in detail we must think about the importance of heat transfer from the condenser. Ask yourself do you accept that any system that is running is rejecting all the heat it is absorbing? This means all the heat that is absorbed in the evaporator, in the suction line, cooling the compressor and the heat of compression must be rejected, mostly by the condenser.

           

Do you accept the idea that any unit that is running, is rejecting all the heat that it is absorbing?  And that any unit that can’t reject all the heat it’s absorbing will shut down? When I present this in some groups this is accepted without further discussion and in others this seems to be controversial. It is certainly true; there is no heat warehouse available to the refrigeration cycle. If you were to try to test the theory, you might cover the condenser or shut off the condenser fan and watch what happens.

Isn't it true that if all the heat that is absorbed from all sources is not rejected for whatever reason, the head pressure will continue to rise until the compressor shuts off due to the tripped high-pressure switch?  And if there is no high pressure switch, at some point an internal bypass built into compressors will have to open and bypass the hot gas into the compressor crankcase. This will continue until it opens the internal over-temperature switch and the compressor shuts off, or the compressor motor burns out.

Assuming we have reached agreement on the fact that all the heat absorbed into the refrigerant must be rejected, we can move on to discuss the condenser itself.

The condenser has three distinct regions; the top of the condenser is called the de-superheating section. This is where the hot discharge gas is cooled to the condensing temperature. The next region is where the refrigerant is changed from a vapor at condensing temperature to a liquid at condensing temperature. That region, the central part of the condenser is called the condensing section. The bottom of the condenser is the subcooling section where the liquid refrigerant is cooled further.

Subcooling

Subcooling is the measure of the heat rejected from a substance below its saturation temperature. Another way to think about subcooling is that it’s the difference between the temperature we estimate by looking up the liquid pressure on the T-P chart, and the actual temperature of the liquid line leaving the condenser. It is how much cooler the liquid refrigerant leaving the condenser is than when it condensed to a liquid.

To measure subcooling, the liquid pressure and the liquid temperature must be measured in the same place. That’s because the alternative, measuring hot gas pressure to estimate the condensing temperature of the liquid in the liquid line adds an error to the subcooling calculation. This error is the unknown pressure drop through the condenser. Condenser pressure drops may be small in smaller equipment and it may be large in some larger rooftop package units.

When calculating subcooling, estimate the saturation temperature of the refrigerant and calculate how much warmer it is than the temperature of the liquid line. An inaccuracy in the estimate of the saturation temperature caused by measuring hot gas pressure in a R22 system with a 50 psi pressure dropthrough the condenser, which is high but not unheard of, will contribute an error in the subcooling calculation of 14-16°, depending upon the head pressure. We are very often looking for only 10°-12° of subcooling.  This means using discharge pressure when trying to estimate subcooling may introduce a large error, sometimes larger than the quantity we are looking for.

There are rooftop package units that don’t have a convenient liquid pressure port. When that is the case, we will have to assume a pressure drop in the condenser when discharge pressure is the source of the high-side pressure information.  The pressure drop could be more than or less than our assumption. So how accurate is the subcooling calculation when using hot gas pressure?

Since you cannot measure the pressure drop when there is no convenient liquid port, the pressure drop is, by definition, unknown. It’s not clear on any given system when discharge pressure is used, how accurate the saturation temperature estimate might be, it might be a little off or it might be very wrong and the correction you make may make the estimate better or it may make it worse.

What is the value of the subcooling number when no liquid pressure measurement is available? In that situation, we must treat the subcooling estimate as a soft number. Use it the same way you would if you had liquid pressure to work with but place more weight on the other three calculations, evaporator temperature, superheat and condensing temperature over ambient. If you can get those three correct and the subcooling is off a little, chalk it up to the pressure drop error.

The subcooling goal value

The subcooling goal changes from model number to model number. When we take measurements and do calculations like this, it’s because we want to know if the quantity we are concerned about, in this case subcooling, is what it should be or if it’s higher or lower than what you would expect. So, when working on air conditioners, what is the subcooling goal?

First let’s be clear that when we are talking about finding the subcooling goal, we are talking about equipment with a thermostatic expansion valve (TxV) as the metering device. We will examine metering devices in the next installment. This is because we charge TxV equipment using subcooling. However, when we are working on fixed orifice metering device equipment we charge by superheat.

When working on TxV equipment, it turns out that the manufacturers stated subcooling goal varies from model number to model number. Either you know the goal value for the machine you are working on or you don’t and you have to guess. A good guess would be 10°F. Most TxV units have subcooling goals between 5°F and 20°F.

A question might be how are you going to find out the subcooling goal for the TxV equipment you work on?  You may have the documentation that came with the unit, you may try to find it on the internet and you may call the OEM supply house. This is a business decision; the answer will need to be made by a supervisor for his crew. However you choose to go about getting the subcooling goal number, while you are looking up things, you might as well get the SEER or EER rating at the same time.

When working on TxV equipment we need to know the manufacturer’s subcooling goal to precisely charge the unit. Alternatively, you could guess. If you are measuring discharge pressure and guessing the condenser pressure drop or guessing at the subcooling goal value, remember that the subcooling goal value or calculated value is a guess when doing fault detection later. Under these conditions, the subcooling low/OK/high judgment isn't reliable. In that case, do the best you can and pay more attention to superheat, evaporating temperature and condensing temperature over ambient. Constantly remind yourself that the subcooling number may be inaccurate.

The liquid temperature should never be less than the ambient temperature. When the liquid temperature is less than ambient, it probably means that the condenser is wet and we are getting evaporative cooling, or that we are over-estimating the ambient temperature. This could be the case if the ambient temperature sensor you are using is lying on the roof or on the top of the unit instead of in the condenser airflow. But it clear, the thing we are cooling cannot be cooler than the thing we are cooling it with.

The liquid temperature should never be much warmer than the condensing temperature. If it is, it’s not liquid. If your assumption of the refrigerant type is wrong, none of the calculations will make sense. Whenever you have calculated values that just don’t make sense, figure out what’s wrong and fix it before doing diagnostics or recording data.

Heat transfer

We will consider three service-related factors that affect heat transfer through the air cooled condenser.

One factor that affects heat transfer is the amount of surface area available. We understand that there is more heat transfer when there is more contact between the thing you are cooling and the thing you are cooling it with. The aluminum fins in air-cooled condensers provide more surface area between the warm copper coils and the air used to cool it.

Another factor effecting heat transfer in an air-cooled condenser is the volume of air being pulled past the fins. The more air that moves through the condenser coil, the more heat is transferred to the environment.

Still another factor effecting heat transfer is the ΔT, or the difference in the temperature between thing being cooled and the thing we are cooling it with.

A good way to visualize that third factor is to consider an ice cube in a frying pan. The ice cube will melt at some rate because of the heat transfer between the ice and the pan. If we were to put a fire under the pan and increase the ΔT, the ice cube would of course melt at a different faster rate because of the additional heat transfer offered by the increased ΔT.

When some technicians hear ΔT they assume we are talking about the difference in the air temperature before and after it has been through the condenser. That is a useful calculation as we will see later, but that is not the subject here.

What are we cooling and what are we cooling it with, in the air-cooled condenser?  We are cooling refrigerant. The temperature of the refrigerant in the condenser, that we use in our ΔT calculation, is the condensing temperature. We are cooling the refrigerant it with air. The temperature of the air, that we use in our ΔT calculation, is the ambient temperature.

We express this relationship using the term, “condensing temperature over ambient,” or COA. Condensing temperature over ambient may be a new concept to some technicians. This is an important concept that we will be coming back to several times. When the condensing temperature is very warm compared to the ambient temperature, it means that for some reason the condenser is having a hard time moving heat from the refrigerant to the outside air.

Some problems that can reduce the heat transfer capacity of the condenser include condensers coils covered with, and even in more extreme cases, impacted with dirt or other substances. Condenser coil fouling negatively impacts heat transfer in two ways; fouling is an insulator and it is an obstruction to air flow. Additionally condenser coils can be degraded by salt air or harsh chemical cleaners that aren't rinsed off thoroughly. And they can be physically damaged by hail or vandalism. When servicing air cooled HVAC equipment, be aware of the condition of the condenser.

We agreed earlier, that all the heat that is absorbed by a refrigeration cycle must be rejected in order for it to continue running. And most technicians have seen units running with dirty or damaged condensers. How can the refrigeration cycle reject its heat load with reduced surface area perhaps reduced air flow?

When this happens, the system needs to raise the ΔT in order to transfer out the heat. The ambient temperature is whatever it is. It isn't something that is controlled. The condensing temperature is what needs to rise to increase the ΔT. What does that look like to a technician on a job?  The head pressure rises when rejecting heat through the condenser is impeded.

Refrigerant Charge

           

How do we know the system is properly charged?  Starting to answer that question requires a correct understanding of which metering device is being used in the system. Fixed orifice units are charged to a superheat specification, TxV units are charged to a subcooling goal. There are factors other than the amount of refrigerant in the system that affects the superheat and subcooling calculations and they must be considered when charging a system. One important factor is the mass flow rate.  We will look at that closely when discussing metering devices. The thing to remember now is that not all refrigeration cycle problems are charge problems,

When we add refrigerant to a unit that is already sufficiently charged, where does the additional refrigerant go?  When we over-charge a system the additional refrigerant backs up behind the metering device and starts to over-fill the condenser. Refrigerant overcharge increases the amount of the condenser coil used for subcooling and reduces the area used for de-superheating and condensing. The effect is to make the condenser de-superheating and condensing regions smaller. For that reason, an overcharged unit may be running with higher than expected head pressure.

Another interesting effect of the condenser being over-filled with liquid refrigerant is that when more of the coil is used for subcooling, we see more subcooling when doing our calculations. Subcooling is the best indicator of how much refrigerant is on the high side of the refrigeration cycle and subcooling is the best measure of the amount of refrigerant charge in units that have expansion valves.

Now that we are able to be more precise in our charging methods because of the use of subcooling, measuring liquid pressure and knowing the correct subcooling goal with TxV equipment is required when charging TxV equipment more precisely is a goal.

Always charge air conditioners with a fixed-orifice metering device using superheat. The superheat expectation changes with the driving conditions, that is the return are wet-bulb temperature, that can be thought of as the load on the evaporator and the ambient temperature. The higher the wet bulb temperature, the higher the load on the evaporator. We then use the charging chart to arrive at the correct superheat expectation. In fixed orifice equipment, high superheat usually indicates an under charged system. Low superheat may mean the system is over charged.  However, low superheat could also indicate many other problems on the low side.  These include dirty filters or evaporator coil, restricted duct work or the indoor fan is running too slow, or even in the wrong direction.  Inspect the condition of the low side of the system prior to charging fixed orifice equipment using superheat.

Head Pressure

           

When is the head pressure high? 

It depends on the ambient temperature.

When analyzing head pressure, what we are looking at is the condensing temperature and comparing it to the ambient temperature. When the condensing temperature over the ambient, or COA is high, it means the condenser is warmer than it should be at the current ambient temperature. Since head pressure and condensing temperature are converted from one another on the T-P chart, high COA means that the system is running with high head pressure.

When the condensing temperature is more than 30°F over the ambient temperature, the condensing temperature may be too high. Higher efficiency equipment will have lower COA expectations and older, low efficiency equipment may have slightly higher COA expectations.

In fixed orifice equipment, the load on the evaporator has a large impact on the head pressure expectation.  High humidity is a big load.  The more heat absorbed by a system, the more heat there is to reject.  If a fixed orifice unit is working under high loads, higher condensing temperature over the ambient would be expected.

The causes for high head pressure

There are three causes for high head pressure.

High side heat transfer problem

This means the condenser is hot with no sign of over charge. The COA calculation cannot tell you if the condenser fans are running correctly, if they are running in the right direction or if it has the right prop or if it is positioned properly in the cowl. The technician needs to satisfy themselves that the condenser fans are running correctly before thinking about a dirty condenser coil. If we assume that the condenser fans are operating correctly, a high side heat transfer problem means that there is a dirty condenser coil. A high condenser air ΔT (or ΔTca), with properly running fans and no indication of overcharge proves a dirty condenser.

Most technicians understand that you can’t tell if a condenser coil is clean just by looking at it. Even if it was just cleaned, it doesn't necessarily mean its effectively transferring heat. Condensing temperature over ambient and condenser air ΔT are used to determine if the condenser is clean. When the warmest air coming out of the condenser fan, as measured with a thermometer, is more than 30° warmer than the air entering the condenser, and the condenser fans are judged to be operating correctly and there is no indication of over charge, you probably have a dirty condenser coil. The coil is not transferring its heat to the air effectively.

Overcharge

           

When the subcooling is high it means there is a lot of refrigerant in the condenser.  If the evaporator temperature (suction pressure) is normal to high and the superheat is normal to low, the system is probably overcharged.

Non-condensables

Non-condensables are contaminants in the refrigerant. The contaminants that technicians see most commonly in refrigerants are air, moisture and nitrogen. A standing pressure test will prove non-condensables. 

The standing pressure test for non-condensables

A standing pressure test is performed in the following way.  The system must be at ambient temperature.  This is best done when the unit hadn't been running for a while.  It can be forced by running the indoor and condenser fans with the compressor off.  The pressure of the refrigerant is most influenced by the coldest temperature it is exposed to.  For that reason measure the suction line pressure and suction line temperature when performing a standing pressure test.  If the system is at ambient temperature and in steady state, the refrigerant should be at saturation temperature.  If the measured pressure is higher than what the T-P chart says it should be at that temperature, the best explanation would be non-condensables in the refrigerant.

What about liquid line restrictions?

A common misconception in the air conditioning industry is that restrictions cause high head pressure. Research has proven conclusively that refrigerant flow restrictions result in lower head pressures. This is true because the restriction is causing too little refrigerant to enter the evaporator. That results in the system picking up less heat than it would have without a refrigerant flow restriction. With less heat to reject the condenser looks relatively large compared to its heat load and the condensing temperature drops. Restrictions cause high subcooling but not high head pressure. Test this by closing a liquid line valve and pumping a system down.  The high-side pressure will drop to a point where the condensing temperature is very near the ambient temperature.

If that is the case, why do many technicians think that restrictions cause high head pressure?  It’s because they see systems with refrigerant flow restrictions and with high head pressure all the time.  How can that be?  It’s because refrigerant flow restrictions cause the suction pressure to be lower than normal and the superheat to be higher than normal.  Many technicians, those that do not check subcooling, will assume it is because of a lack of refrigerant charge.  If they act on that assumption and add charge, eventually the condenser will be over-filled with liquid refrigerant.  When that happens, the head pressure is very likely to be high.  The high head pressure is caused by the overcharge, and not the restriction.  If the system had the correct amount of refrigerant, the head pressure would be low when there is a restriction to refrigerant flow.

Review

High Head Pressure is one problem that has three possible causes, how do we know which one is causing the problem at any given job?

• We can know about the amount of charge on the high side by the subcooling calculation compared to the subcooling goal.
• Condenser air ΔT measures high side heat transfer capability
• A standing pressure test proves the existence of non-condensables

The goal of this method is to eliminate guess-work. When armed with well thought-out diagnostic tests based on taking measurements, doing calculations, and applying valid diagnostics rules, you can speak with confidence to your customer or your employer about specific diagnostic concerns.