Understanding Aircon Heat Exchanger Capacity: Calculation Methods Explained"

[plucker_08]

is there such thing as condenser/evaporator capacity of an aircon?
how to compute for that?

thanks!

 

[ray b]

they are rated in BTU at a given temp and fan speed for air flow
but capacity can change at extream temps

 

[russ_watters]

There is a theoretical maximum capacity of any heat exchanger, based on the delta-T and flow rates of the fluids on each side of it.

 

[plucker_08]

any exact formula?

 

[machinest]

well,for the evaporator as a heat exchanger,the two fluids are the air and the refrigerant,which exchange the heat between each other,to calculate the evaporating capacity"cooling capacity":
Qref=c.c=Mref*delta h(enthalpy difference)
where:Mref:refrigerant mass flow rate,h:the enthalpy at two different points from the (p-h)chart.
for the air:
Q=(Mair*Cair*delta-T)
both Q are equaled.
for the condenser:
Qrej=(Mref*delta-h) which is equal=(Mair*Cair*delta-T) for air cooled condenser.
=(Mw*Cw*delta-T) for water cooled condenser.
also google the (p-h) chart,for a vapor compression cycle,to get the delta-h in each case for the evaporator capacity and condenser capacity.
and that's all about capacities for evap.and condenser,but for a simple vapor compression cycles,improved cycles may differe a bit,so if u told us more detail it would help us to help you if we can.
hope that helped.
thanks.
machinest.

 

[momentum_waves]

Heat transfer design of basic heat-exchangers, can be learned by reading a number of basic heat-transfer books.

Evaporator & condenser design will require you to read a fair amount on the physical mechanisms involved. Try Collier & Thome, 'Convection boiling & condensation'. You could also try Webb & Kim, "Principles of enhanced heat transfer".

There is no simple formula, but rather a mountain of information & plethora of formulae you will need to wade though.

mw...

 

[russ_watters]

The theoretical maximum performance of a non-phase change heat exchanger is the approach temperature times the flow rate times the specific heat capacity of the fluid with the lower specific heat*flow. The real performance is just that times an efficiency factor.

For one with a phase change, you just add a term for the heat of vaporization times flow rate.

To actually calculate the real performance of a heat exchanger completely from scratch is extremely complicated. In reality, it is done through testing.

 

[plucker_08]

my task is to design a heat exchanger testing facility (condenser and evaporator of an air con...

its objective is to test the capacity of both condenser and evaporator...

using simple tools like flow meter, thermocouples, etc...

 

[plucker_08]

up...

 

[plucker_08]

how can i measure the pressure inside the inlet of a tube?

 

[stewartcs]

how can i measure the pressure inside the inlet of a tube?

With a pressure transducer.

 

[plucker_08]

do i have to bore the tube or just place the transducer around the pipe.?

 

[plucker_08]

what values will i use for the effeciencies? should i assume values?

tnx!

 

[momentum_waves]

Plucker_08, if this is an industrial application, then I'd suggest that you employ the services of a professional consultant.

If this a student project, then you will have a lot of reading to do.

These are simple facts of life. Some consultants have spent years researching & applying the principles in practice. This comes at a price, unfortunately - time <=> money.

 

[russ_watters]

do i have to bore the tube or just place the transducer around the pipe.?

You solder a coupling with a pressure port onto the tube.

 

[plucker_08]

Plucker_08, if this is an industrial application, then I'd suggest that you employ the services of a professional consultant.

If this a student project, then you will have a lot of reading to do.

These are simple facts of life. Some consultants have spent years researching & applying the principles in practice. This comes at a price, unfortunately - time <=> money.

this is a student project sir.

 

[plucker_08]

You solder a coupling with a pressure port onto the tube.

the situation is this

for e.g. an owner wants to test the capacity of the condenser/evaporator inside the aircon she bought..

what if there is no pressure port? do i have to create one?

 

[momentum_waves]

Do you have a complete working HVAC system available for your test?

In other words, could you make a test chamber for the system & test it's capacity under controlled conditions? If that's the case, then you could read up on how companies test HVAC systems.

Testing each heat-transfer component separately can become rather complex, & subject to error.

 

[plucker_08]

does the maximum capacity of evaporator/condenser changes with different room volume or outside temperature?

 

[momentum_waves]

The maximum achievable capacity of the HVAC&R system as a whole, based on initial set-up conditions, charge volume & so forth - would be affected if the load & sink values were changed.

Most domestic hvac's have a rated capacity before they are installed. The technician then comes along, installs the system, charges it with refrigerant, runs a test & then leaves. Whether the system ever reaches the rated capacity would have to tested/simulated.

The operating capacity of each exchanger in the system will certainly be affected by the prevailing local conditions eg. room volume/temp, or outside temperature. This occurs via LMTD changes - affecting unit capacity.

Have a look at the Webb reference I mentioned earlier. He reviews this aspect fairly well.

 

[plucker_08]

ok.. thanks sir!

 

[plucker_08]

sir what are these load and sink values?

 

[momentum_waves]

The heat load is the amount of heat you want to extract from a particular space e.g. room, over a certain time. In other words rate of heat-extraction in Watts (typically kW, or Btu/h). This occurs on the evaporator in a domestic hvac system.

The sink is where the heat load, extracted from the cold space is discharged e.g. from the condenser to the outside air.

The hvac system (condenser, evaporator, compressor, piping, valve etc) is sized on the performance required to cool down a particular space.

Take a look at a book on Thermodynamics & understand the refrigeration cycle, before moving onto detailed heat-transfer books, & then onto specialist books like Webb.

 

[velz]

i want to calculate the mass flow rate of hot water in the heat exchanger?
my known datas are...
In the first heat exchanger exhaust gas flows with 650 c temperature inlet and mass flow rate is 18.7 m3/min
In the second heat exchanger hot water flows with 27 c temperature inlet and 90 c temperature outlet......
here i want to calculate mass flow rate of hot water in the second heat exchanger?
and how to calculate cp value for both exhaust gas and water?

 

[stewartcs]

i want to calculate the mass flow rate of hot water in the heat exchanger?
my known datas are...
In the first heat exchanger exhaust gas flows with 650 c temperature inlet and mass flow rate is 18.7 m3/min
In the second heat exchanger hot water flows with 27 c temperature inlet and 90 c temperature outlet......
here i want to calculate mass flow rate of hot water in the second heat exchanger?
and how to calculate cp value for both exhaust gas and water?

Apply the conservation of energy principle to a control volume of your system and balance it.

CS

 

[PREMNATH]

There is a theoretical maximum capacity of any heat exchanger, based on the delta-T and flow rates of the fluids on each side of it.

Can you share the formula to find out the maximum capacity of steel plate type heat exchanger without change in phase, between two liquids having delta T of say 10deg.C?
I have to transfer heat between two liquids having delta T of between 10 to 20 Deg C, using plate type heat exchanger. Can you provide me suitable formula to design the heat exchanger to meet this objective?