A theoretical question about condensers

[gateeight]

Hi all,

I am new to this site, so thank you for taking the time to read my post. I have a question on the fundamentals of the vapour compression refrigeration system that hopefully someone can help me with. I'm sure the answer is straightforward but unfortunately all the literature I have found on the internet doesn't seem to answer my question.

In a typical one stage compression cycle, the expansion valve is controlled to regulate the low pressure in the evaporator, whether its a thermal expansion valve or electrically controlled valve it will act on the superheat somehow to optimise the evaporating temperature dependent on the duty (or load). Following the evaporator the vapour is compressed but what I don't understand is what sets the pressure/condensing temperature in the high pressure side?

Is it determined by the temperature of the cooling medium, i.e. ambient air or water? Or is it determined by the compressor?

And finally, if I am using two cooling mediums at different temperatures to provide all the necessary condensation (i.e. some of the refrigerant is condensed in an air cooler and some in a water cooled condenser - either before or after the air cooler), what sets the condensing temperature then? The pressure will be equal throughout the high pressure side (neglecting pressure drops), so the condensing temperature will also be equal throughout.

Many thanks again.

[jpsmith1cm]

Well, since it sounds like you're a student, we'll start with an ideal or a perfectly operating system.

Nothing is really determined by ONE factor in isolation, but rather by a confluence of multiple factors acting in concert.

Of course, temperature of the condensing medium is a factor, but size of the condenser is also a factor. This factor is typically fixed by the design of the equipment, but it's a factor nonetheless. You'll see this in working on older equipment (with typically smaller condensers) vs newer equipment (with typically larger condensers)

System load is also a factor (higher load, higher pressures)


I'll caution you that you're incorrect on this as well:

the expansion valve is controlled to regulate the low pressure in the evaporator, whether its a thermal expansion valve or electrically controlled valve it will act on the superheat somehow to optimise the evaporating temperature dependent on the duty (or load)

While you're correct that the metering device controls superheat, that is ALL it controls. Evaporating temperature is also 'set' similarly to the condensing temperature.

[gateeight]

Thanks jpsmith1cm, some good pointers. Whilst I am not a student, I am interested in an ideal cycle yes.

I'm really interested in what happens if I replace some of the condenser load, that is already satisfied with an air fan, with a water cooled condenser (i.e. turn the fan speed down on the air cooled condenser and install a water cooled condenser after it). The water is cooler than the air, but will this affect the condensing temperature? The load remains unchanged.

[ryan1088]

Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?

[2sac]

Thanks jpsmith1cm, some good pointers. Whilst I am not a student, I am interested in an ideal cycle yes.

I'm really interested in what happens if I replace some of the condenser load, that is already satisfied with an air fan, with a water cooled condenser (i.e. turn the fan speed down on the air cooled condenser and install a water cooled condenser after it). The water is cooler than the air, but will this affect the condensing temperature? The load remains unchanged.

If it's done properly, it doesn't matter if you use water, air, or both. In Chicago there are still refrigeration condensing units that utiize both air and water.

[Tommy knocker]

You see this from time to time as a heat reclaim set up.

[Freightshaker]

Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?

I can think of a lot of large package units near dirty processes that we have considered this. Add a small water cooler after the cond. that starts flowing at x head pressure.

[icemeister]

Combination air & water cooled condensing units were fairly common about 30-40 years ago. They were often used in areas where the ambient air temperatures could get rather high and water availability was limited. They also used them as "water savers" in areas where the cost of city water became expensive.

They were typically arranged so the compressor discharge gas first went to the air-cooled condenser and then through a water-cooled condenser piped in series with a pressure actuated water regulating valve on the water leaving of the condenser. The regulating valve was set to limit the condensing pressure, so during cooler weather very little water was needed.

[icemeister]

Hi all,

...Following the evaporator the vapour is compressed but what I don't understand is what sets the pressure/condensing temperature in the high pressure side?

Is it determined by the temperature of the cooling medium, i.e. ambient air or water? Or is it determined by the compressor?

And finally, if I am using two cooling mediums at different temperatures to provide all the necessary condensation (i.e. some of the refrigerant is condensed in an air cooler and some in a water cooled condenser - either before or after the air cooler), what sets the condensing temperature then? The pressure will be equal throughout the high pressure side (neglecting pressure drops), so the condensing temperature will also be equal throughout...

The temperature of the cooling medium is only one factor. The others would be the condenser's capacity (ability to reject heat) and the amount of heat which must be rejected.

The condensing pressure that results is a balance of all three factors. The condenser capacity is typically expressed in terms of heat rejection over a temperature difference...ie, Btuh/ºF TD.

The heat is called the total heat of rejection (THR) and is calculated by adding the refrigerating capacity (Btuh) and the power input (Watts expressed as Btuh) to the compressor.

The TD is the condensing temperature minus the ambient temperature, which for air cooled condensers is usually between 10ºF-30ºF where the niminal ambient temperature is 90ºF and is defined at the time of equipment is selected. For water-cooled, the standard is 85ºF for cooling tower water and 75ºF for city water...giving you either a 10ºF TD or a 20ºF TD respectively.

Let's say you have a system with a one ton load (12,000 Btuh) and the compressor's power input is 880 watts (3000 Btuh). The THR would then be 15,000 Btuh. Your condenser's capacity is 500 Btuh/ºF TD, so the TD would be 15,000/500 = 30ºF. If the ambient is 90ºF, then the condensing temperature would be 90+30 = 120ºF. You would do a similar calculation for a water-cooled condenser.

If you were to set up a air & water cooled system, then the one with the lower calculated condensing temperature would determine the condensing pressure...assuming that condenser were capable of handling the load.

[gateeight]

Let's say you have a system with a one ton load (12,000 Btuh) and the compressor's power input is 880 watts (3000 Btuh). The THR would then be 15,000 Btuh. Your condenser's capacity is 500 Btuh/ºF TD, so the TD would be 15,000/500 = 30ºF. If the ambient is 90ºF, then the condensing temperature would be 90+30 = 120ºF. You would do a similar calculation for a water-cooled condenser.

If you were to set up a air & water cooled system, then the one with the lower calculated condensing temperature would determine the condensing pressure...assuming that condenser were capable of handling the load.

icemeister, thank you very much for your detailed response. Your final statement is exactly the answer I was after, and I hadn't realised the importance of the condenser capacity. Why the lower of the two calculated temperatures defines the condensing temperature I cannot understand, but still, I have my answer.

[hvacnut]

The condensing temperature determines the head pressure. The hotter the cooling medium the higher the condensing temperature and therefore head pressure and vice versa. The compressor's job is to raise the low temp low pressure gas from the evaporator to a high temp high pressure gas in the condenser.

[icemeister]

icemeister, thank you very much for your detailed response. Your final statement is exactly the answer I was after, and I hadn't realised the importance of the condenser capacity. Why the lower of the two calculated temperatures defines the condensing temperature I cannot understand, but still, I have my answer.

Heat flows from hot to cold...and it also tends toward the lowest temperature.

If you were to consider an air-cooled condenser with a TD of 30ºF TD and let's say 100ºF ambient with the same 15,000 Btuh THR, the expected condensing temperature would be 130ºF. Now if you add the water-cooled condenser with its 20ºF TD and have 70ºF water entering, you would see a 90ºF condensing temperature. Since this is lower than the ambient air, there would be no condensing at all in the air-cooled condenser. Actually, It would only be doing some desuperheating, but no condensing.

Clear as mud?

[jpsmith1cm]

Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?

I'm a bit puzzled, too.

It would depend on the sizing of the coils and the temperature of the mediums involved.

Now, if I'm reading between the lines well enough, I'm envisioning a water heater type setup and, as was previously mentioned, this isn't uncommon.

You'll want the water portion first, though, to take advantage of the superheated discharge gas rather than wasting that heat to the air.

[gateeight]

Now, if I'm reading between the lines well enough, I'm envisioning a water heater type setup and, as was previously mentioned, this isn't uncommon.

Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.

Thanks everyone for your replies.

[barbar]

The refrigeration is loop, so any change in the loop will effect the whole loop, any change a new equilibrium point is reached!
So a very basic example.
If you have a starting point, you change the refrigeration cond temp by 1C, you will get a change in power consumption of approx 3.5%, and change in refrigeration capacity "how much cooling"
Again from this starting point, if you change the evap temp by 1C, you will get a change in cooling capacity of about 2.5%.
But remember this is a loop, so any change with one of the above will still effect the other.
And of course there are limits on an existing system
Using the other fine explanations, you may get a feel, or not.

[gateeight]

Using the other fine explanations, you may get a feel, or not.

Thank you, yes I think I'm getting there. I'm using CoolPack to give a (theoretical) feel for what should happen when the condensing temperature changes.

[icemeister]

Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.

Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.

So is this system solely for water heating...like a dedicated heat pump water heater...or are you simply capturing "free" heat from the cooling process? Since you have an air-cooled condenser, I would assume the latter.

I have a water cooled "condenser" for domestic water heating in the discharge line of my home A/C (before the condenser) which I figure has saved me about $400/yr over the past 10 years or so. In my case, it's primarily a desuperheater, and can maintain 130ºF+ in the tank even though the condensing temp never get much above 110ºF.

[Tecman1]

Sounds like the air cooled chillers with chiller barrels. Utilize water source loop then heat exchangers to transfer to the refrigerant side. But not both at once which would work but with todays controls it probably would make no sense. I have never worked on a water and refrigerant system. It sounds like it was before my time.

[Tecman1]

Ok now I see what you are thinking. (Heat Recovery) there are many companies offering refrigeration energy retrofits utilizing the waste condensor heat in a heat recovery strategy. In essence utilizing a water based system to reclaim waste heat at the condensor and then transfering through a heat exchanger to a hydronic system or inline coils in duct to pre-condition supply air. These are mostly utilized in institutional settings with large walk-in in coolers or freezers during energy retrofit projects and typically this strategy offers exceptional payback.

[derekkite]

Remember head pressure equals condensing temperature. The compressor adds energy to the refrigerant vapor. The temperature will rise to the point where the gas will condense. That temperature is a factor of the ambient temperature if air cooled, and the size and efficiency of your heat exchanger. Heat exchange is a factor of surface area and temperature difference. Your condensing temperature needs to be higher than the medium you are transferring to, ambient temperature + temperature difference required for your heat exchanger.

Load matters as well. A given heat exchanger will transfer X energy at Y TD. If X goes higher due to high load, Y goes up as well, and visa versa. A dirty condenser coil is less efficient a heat exchanger, so the head pressure rises to compensate, put another way, your TD of the heat exchanger rises.

A tx valve doesn't control temperature, so let's back up a bit. You have an evaporator of some sort, lets say a coil with air flowing across it. The amount of air, say X lbs of air per minute, at Y temperature crosses the coil. The size of the coil, the design, fins, etc. will determine how much energy can be transferred to the air at Z TD. So you have refrigerant entering the coil after your metering device. It boils, absorbing energy at the rate your heat exchanger allows. The air is cooled. What the tx valve does is senses the temperature of the pipe leaving the evaporator. If the coil design and the air flow is engineered to give you 20 degrees F saturated suction temperature, the tx valve will meter refrigerant so that the leaving pipe is at 20 degrees saturated suction plus superheat, say 7 degrees. What that means is that your serpentine coil with liquid refrigerant entering on one end, flowing through the pipes, evaporating absorbing heat from the air. The liquid refrigerant will all be evaporated before it reaches the end of the pipe to leave the evaporator, long enough for the vapor to absorb another 7 degrees of temperature.

The pumping capacity of the compressor determines how many pounds of refrigerant vapor are pumped. Pounds of refrigerant pumped is your capacity of the system. Assuming the capacity of the pump is constant at all pressure differentials (they are not, but leave that for now), the pounds of refrigerant evaporated determines the capacity of the evaporator. The condenser needs to reject that much energy plus the heat of compression from the compressor.

For most systems, and there are exceptions, the suction pressure and the head pressure are determined by the heat exchangers and the medium to which they are transferring energy to or from. If your condenser is in a cool spot, the head pressure will be lower. If the evaporator is loaded, warm, the suction pressure will be higher.

There are ways to actually control the pressures. To control head pressure you can shut the fan off, which changes the heat exchanger characteristics. Or you can install a device that backs up liquid refrigerant into the condenser coil, decreasing the size of available heat exchange surface area, forcing the temperature higher to exchange the same amount of energy.

On the suction side it is a bit more complicated, there are valves that hold back the suction pressure, but typically you cycle the compressor to prevent the pressure from going too low.

Most systems the evaporator, compressor capacity and condenser coil are matched so that the operating pressures are within the normal range for the application. A change in the sizes of any of these three will change the pressures and temperatures in all parts of the system.

All the rest of the devices in a refrigeration system are about controlling liquid refrigerant, starting and stopping the process, moving the air around, and other bits and pieces to protect the main components from harm. It's all quite simple. The key to remember is that you are moving energy around, so the laws of thermodynamics apply. Surface area, temperature difference, amount of energy.