How to setup condition and dimensioning

[NoJosh]

Hi, im new to refrigeration systems, and need some help on where to start when you have to put an AC system together.
If i have these given information
Ambient temp: 60deg
Supply temp: 40deg
Cooling capacity: 500W

I know the basics of how refrigeration systems work, but have never set one up myself and therefore unfamiliar with where i have to start when i have to dimension the system (setting up the right parameters and finding and calculating the needed component). If someone has any examples or can help me in the right direction i would appreciate it alot.

Thanks in advance

[Brian_UK]

Firstly, are your temperatures Celsius or Fahrenheit?

[NoJosh]

It's Celsius

[Rob White]

Hi, im new to refrigeration systems, and need some help on where to start when you have to put an AC system together.
If i have these given information
Ambient temp: 60deg
Supply temp: 40deg
Cooling capacity: 500W

I know the basics of how refrigeration systems work, but have never set one up myself and therefore unfamiliar with where i have to start when i have to dimension the system (setting up the right parameters and finding and calculating the needed component). If someone has any examples or can help me in the right direction i would appreciate it alot.

Thanks in advance

Are you sure those numbers are correct?
500 watts will barely cool an egg at those temps.

Is this a homework project?

Regards

Rob

.

[NoJosh]

Yes it is a homework project and the temperatures should be right

[Rob White]

.

Your going to be in trouble with an AC system in those conditions.

60 degC ambient means the condensing temp will be at about 70 - 75 degsC.
a common garden R410 system is off the scale at that temperature.
At a condensing temp of 60 degC it would be at a pressure of 38 bar.

You are asking the system to work at pressures that are above the maximum
recommended safe limit for the system.

Recheck your figures and then post your opinions?

Is this a test from your tutor to see if you understand the
relationship between pressure and temperature?

Regards

Rob

.

[NoJosh]

The assignment is not about understanding relationship between pressure and temp. Could be that he made error in the assignment, i will ask him.
Nonetheless any example with any given temperatures will help, i want to learn where to start when calculating such refrigeration systems, at which component do you start, and how to set up the right parameters.

[Rob White]

.

AC for the most part is designed as comfort cooling.

Comfort cooling is approximately 21 degC, so to keep
a room at 21 degC you then have to calculate how
much heat you need to move.

If the room is at 21 degC and the ambient is 30 degC, then
the temperature difference is 9 degsC. You also need
to take into account what is inside the room you are treating.

People generate heat so you have to calculate for that, PC's,
monitors, printers, TV's, lights and all the other things in the
room all add to the heat.

The location of the room and its orientation (facing North, South and such)
also need to be factored in and then you have to allow for the
amount of window coverage.

The positioning of the condenser needs to be considered because it
requires good ventilation and needs to be able to reject all the heat
that is removed from your room.

That's enough to start with, you have to do some work yourself :p
but in AC they work at pressures and temperatures of about:-

Condensing temperatures about 15 degs above ambient
Evaporating temperatures about 10 degs lower than room temp (air off evaporator)

Regards

Rob

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[NoJosh]

Thanks you for your explanation it really helped :)
It may seem like a stupid question, but i just can't grasp how exactly does the system keep the temp difference between the condensing temperature and ambient, if for example ambient temp changes. And same for evaporator, does it always have a constant temperature difference with the room temperature.

[Rob White]

.

Heat (energy) flows from high to low so to move heat you always
need a temperature difference. The greater the difference the the
quicker and the greater the heat flow.

In refrigeration we need to mechanically create that difference and
that costs money. Experience and design have found that a temp
difference of about 10 - 15 degs is all we require to achieve the system
to work, any more is just expensive and unnecessary.

When the ambient changes the condensing temp / pressure change also
and as the room temperature changes the evaporating temperature changes
also. There are very few constants in fridge because the variables change so
much and that is where the controls come in.

Running the system just hard enough to do the bare minimum work and
keeping the operating costs down and sizing it so it is just big enough to
move the heat required is where the skill comes in.

It all varies dependent on how big the compressor is and how much air
you flow over the heat exchangers. If you have a big compressor and run
it hard it will create a large difference between the low and high side and
if you flow air over the condenser and evaporator heat exchangers, you
will give them more or less work to do which affects the pressures (temps).

Balancing the low side pressure (temp) with the high side, maintaining just enough
difference to move the required heat all depends on sizing and controlling the system
as a whole.

Regards

Rob

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[Rob White]

.

The system must be designed to be balanced,

Whatever heat the evaporator generates the compressor adds
about another third so the condenser must be big enough to
reject the heat from both the evaporator and compressor combined.

If you put too large an evaporator on a system, it will add too much
heat and both the compressor and condenser will struggle.

If you put too small an evaporator onto a system the not enough
heat will be generated and the compressor will seem over sized.

Think of a car designed to move 4 people on a flat road.
If you drive that car up a long hill it will do it but it will struggle,
now think of the car driving down hill, it will coast so the engine
seems over sized.

Refrigeration is the same, we move heat and it can be measured.
If you need to move large amounts of heat you need to capture
and transfer it with a large system but if you do smaller amounts,
a smaller system will manage (within the design parameters).

As ambients change the compressor seems either over sized or
undersized and if it was designed correctly it would cope with a wide(ish)
range of temperature differences.

Rob

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[Doug30293]

The assignment is not about understanding relationship between pressure and temp. Could be that he made error in the assignment, i will ask him.
Nonetheless any example with any given temperatures will help, i want to learn where to start when calculating such refrigeration systems, at which component do you start, and how to set up the right parameters.

The temperatures suggest the heat load is electronics. If this is a thermodynamics assignment there is also the possibility your instructor does not expect you to use a common refrigerant such as R410 or R404. You may not even need to operate your device in the saturated region. Water might work.

Lastly, the instructor may have deliberately given you inadequate information hoping you would ask for more data. Don't hesitate to ask.

[NoJosh]

Thanks for the information, it really helps understand how it works

[NoJosh]

Which components are the controling ones, and the ones that changes. Is it the compressor and TXV? And i guess Cap tube is just fixed.

[Rob White]

Which components are the controling ones, and the ones that changes. Is it the compressor and TXV? And i guess Cap tube is just fixed.

Yes for the most part.

Compressors that are fixed speed are either on or off so no real control
unless you have 5 or 6 of them running in parallel and then you turn them
off as required.

Compressors can also be inverter driven, digital scroll or capacity controlled,
they all have an ability to increase or decrease capacity.

Thermostatic expansion valves (TEV's), Electronic expansion valves (EEV's)
back pressure regulators and other such valve control pressure but
capillary tubes are fixed length and have no real potential to fluctuate.

Rob

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[NoJosh]

Thank you for the explanation, im learning quite alot :)

[NoJosh]

I understand that the condensing temperature and pressure change in accordance to the ambient temperature and that the evaporating temperature also change compared to the room temperature, as you mentioned. But i was wondering what theory that let's this happen? Is it all dependent on the superheat, so if superheat gets high it will enter the compressor and the discharge temperature will get higher making condensing temp and pressure higher and therefore also increasing the pressure of the low side?

[Rob White]

.

Roughly speaking the more work in the evaporator the
more work in the condenser, so if the low side is high
the high side will be even higher.

If the system has a expansion (or electronic) valve it
will open and close enough to maintain the correct level
of liquid in the evaporator, so the valve opens and closes
as required (within a limited range).

With a capillary tube you have no control so the system
only works correctly when down to temperature, at all
other times (until it achieves temp) it will appear to be
short of refrigerant because there is nothing to control
the flow of refrigerant into the evaporator.

That is why a system with an expansion (electronic) valve
on, almost always has a receiver to hold the liquid refrigerant,
until it is required in the evaporator.

With capillary tubes there is no need for a receiver because the
system is critically charged and is charged to be correct when
at the right temperature.

rob

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[Rob White]

.

Imagine a system running and you fit your gauges.

Your gauges show you what the pressures are on the low and high side,
we then convert those pressures into temperatures.

If the ambient was high the system would need to be higher to reject
the heat so the pressure will be higher also.
If the product is at a temperature then the evaporating temperature
will correspond to that and will show as a pressure, you know that.

The variables are trying to maintain the product temp even though
the product temperature does not stay constant (nothing in refrigeration is constant)

As the product cools down the system will become (in principle) over sized and
to prevent the system from cutting in and out we put a bit of flexibility
into the system in the form of a flow control on the liquid to the evaporator.

As we need more work the valve opens up and floods more liquid into the evap
and then there is more liquid boiling off, in turn this puts more refrigerant vapour
down your suction to the compressor and it will have to work harder to cope,
but it will show as a rise in pressure.

This rise in pressure will also be reflected in the discharge side because all that
work has to be rejected.

Rob

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[Rob White]

.

To explain this is quite hard and quite wordy
I wish I could stand you in front of a working fridge
and explain it to you while you are looking at it,
You would understand in 15 mins

Regards

Rob

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[NoJosh]

Thanks for trying to explain it to me. I just find the dynamics of refrigeration systems quite hard to understand