head pressure

[Bachuss]

Ola is me again, thought Id jus ask another dumb question.

In words only can you describe how to find head pressure using design condenser temperature, ambient air temp and a comparitor.

The handout i have says to:

Find design condenser temp
add 30F (17C) to the ambient air temp
read across comparitor to apporpriate refrigerant to find pressure. (but where did the 30F (17C) from? was it the condenser temp?)

[Bachuss]

could some one also tell me if this is right:

compresor temp 55C
ambient temp 32C
R134a test pressure 13.7bar

Because this is the reason I am questioning my teachers handout.

[Brian_UK]

Ola is me again, thought Id jus ask another dumb question.

In words only can you describe how to find head pressure using design condenser temperature, ambient air temp and a comparitor.

The handout i have says to:

Find design condenser temp
add 30F (17C) to the ambient air temp
read across comparitor to apporpriate refrigerant to find pressure. (but where did the 30F (17C) from? was it the condenser temp?)Answer: There in words is the process to find your head pressure.

The 17°C is the air temperature rise across the condenser coil due to the hot refrigerant within it.

[Brian_UK]

could some one also tell me if this is right:

compresor temp 55C
ambient temp 32C
R134a test pressure 13.7bar

Because this is the reason I am questioning my teachers handout.Sorry, don't quite understand here.

'Test' ? pressure.

[Gary]

Ola is me again, thought Id jus ask another dumb question.

In words only can you describe how to find head pressure using design condenser temperature, ambient air temp and a comparitor.

The handout i have says to:

Find design condenser temp
add 30F (17C) to the ambient air temp
read across comparitor to apporpriate refrigerant to find pressure. (but where did the 30F (17C) from? was it the condenser temp?)

What this is telling you is that under ideal design conditions the condenser TD (difference between condenser air in temp and saturated condensing temp) should be about 30F/17K.

In the real world, where conditions are rarely ideal, the condenser TD can run anywhere from 20-35F/11-19.5K, depending primarily upon the heat load.

[Gary]

Answer: There in words is the process to find your head pressure.

The 17°C is the air temperature rise across the condenser coil due to the hot refrigerant within it.


If I understand correctly, the handout is describing how to find design TD, not design dT.

[Brian_UK]

If I understand correctly, the handout is describing how to find design TD, not design dT.
True, I stand corrected; thanks Gary.

[bangoman]

hie guys ! r we saying the temperature of the air out of the condensor converted to the pressure of the refrigerant at that temperature is my head pressure ???

[Gary]

hie guys ! r we saying the temperature of the air out of the condensor converted to the pressure of the refrigerant at that temperature is my head pressure ???

No... we are saying that under design conditions the head pressure should convert to a temperature that is about 30F/17K above the ambient temperature. The ambient temperature would be the air entering the condenser, not the air leaving the condenser.

[US Iceman]

It is interesting to see old numbers thrown around for teaching purposes though. The 30° TD (F) is based on the way it used to be done. In today's market of higher energy costs this number should be about 15-20°.

In the lesson posted as an example the condensing temperature would drop with a corresponding decrease in condensing pressure.

[Gary]

That's true... today's higher efficiency systems use larger condenser coils resulting in lower TD's.

[Tony]

Hi Bachus,


compresor temp 55C
ambient temp 32C
R134a test pressure 13.7bar


These figures come from BS EN 378,

What is should say is if you have an air cooled condenser, that is on a system designed to work in temperate climate (e.g. UK), where the maximum temperature is not expected to rise above 32C.

You should obtain the saturated refrigerant pressure relating to 55C for R134a.

This pressure will then give the the minimum allowable pressure (AP) for the high side of a R134a system in accordance with BS EN 378.

[Bachuss]

thank you all very much, still I am confused. If the 17C is the condenser TD and the 32C is the ambient air temp, why do I need to know the design comdenser temp for to find my pressure?

[US Iceman]

If the 17C is the condenser TD and the 32C is the ambient air temp, why do I need to know the design condenser temp for to find my pressure?


Because, you need these values to find the design condensing temperature (17 + 32=49). 49C is your condensing temperature and you use the refrigerant tables to find the pressure for that saturation temperature.;)

That is one way of finding what the normal condensing pressure should be on a day where the ambient dry bulb temperature is 32C and the condenser is clean and operating as it should be. However, be warned this the condensing pressure is dependent on the condenser TD. If the condenser was selected with a different TD, the condensing pressure could be higher or lower.

It is important you recognize this is an example. Not everything you may find we have the same exact numbers.

[Bachuss]

so with these numbers I would find what pressure to pressure test at? and if so should i be adding 0.3 of my answer to my answer to stay in line with the BS EN 378 part 2 directive thingy??

[US Iceman]

I can't comment on the referenced standards. The only thing the calculation procedure provides you is what the design, normal operating condensing pressure should be.

If you need to find the test pressure, then it is usually based off of the normal design pressure + some margin. The referenced standards should provide what the exact safety margin is, or should be.

[Gary]

so with these numbers I would find what pressure to pressure test at?

No. Test pressure is an entirely different subject.


thank you all very much, still I am confused. If the 17C is the condenser TD and the 32C is the ambient air temp, why do I need to know the design comdenser temp for to find my pressure?

You don't. You find what the pressure is by looking at the high side gauge.

They are trying to give you a rough idea of what the pressure should be under ideal conditions.