Compressor pressure and ambient conditions

[anandnambiar]

hello,
I am a final semester mechanical engineering student. The project i am planning to undertake is based on the vapor compression refrigeration cycle.
basic idea of the project:
as the ambient conditions vary, the temperature and pressure required for condensation to take place also changes. thus, we are planning to device a system where the output of the compressor can be varied, based on the ambient temperature, to suit the required condensation pressure. such that, excess pressure is not produced unnecessarily.

so, usually, does the compressor deliver refrigerant at the same pressure throughout the year, even if the required pressure is less than that?
Also, we would like to know about the technologies in existence, which are used to regulate and vary the output of a compressor. (is the technology of variable velocity drive used for this purpose?)...
(assume the case of a reciprocation compressor)
please help us.

regards...

[Persi]

Hi,

I am new in refrigeration business, but I can help you by sending a file that prity good describes compressor regulation. If you want, post an e-mail, so I can send You.

Best regards

[pepsiqazi]

plz send i m waiting pepsiqazi@yahoo.com

[The Viking]

Lol...
Normally you would regulate the compressor according to the system's load, not the ambient conditions.

The variation in ambient conditions is taken care of by various forms of head pressure controls (Fan Speed Controllers if air cooled, Flow regulators if water-cooled)

:cool:

.

[infoseaker]

this problem is mainly solved by expansion valve ... i am sure, you can find a lot of information about... eg: http://en.wikipedia.org/wiki/Thermal_expansion_valve

...also in combination of inverter technology...

but maybe you can search for entirely new solution

[Zeppers75]

Hi there,

As Viking says, the compressor output is dependant on cooling/heating demand and condensing temperature is regulated by the condenser fan. Most modern air con systems will have a fan speed controller to help keep the condensing temperature on target. This will usually take ambient conditions into consideration.
As for varying compressor speed and therefore output most modern air con manufacturers use inverter technology which basically ramps the speed up and down dependant on demand.

More basic systems would use a fixed speed compressor and regulate the flow of refrigerant through the LEV.

[anandnambiar]

my email id is anandnambiar_1618@yahoo.com :) thanks

[anandnambiar]

but then, assuming that the load remains the same, and the only thing that changes is the outside environment conditions that the condenser is exposed to, wont the conditions just necessary for condensation to take place vary accordingly?
for example, lets say, when the outside(ambient) temperature is 35 celcius, the pressure of the refrigerant just necessary for condensation to occur properly is 2 bar.... when the outside environnment temperature changes to 15 Celsius in another season, we would not require the same pressure of 2 bar for condensation to take place right? condensation could take place at pressures lower than 2 bar ryt?

that is what we are looking at. instead of the compressor continuing to deliver at 2 bar, which is more than required, if the output is only what is necessary and sufficient.

but in order to go ahead with this, we need to confirm that in a basic vapour compression system, the compressor always delivers at the same fixed pressure, which is maintained a constant, even if it is more than what is needed. :)

[infoseaker]

Seems, you are still talking only about cappilary systems. I am sure, you did not read anything about expansion valves. Get some information about TEV and AEV. This problem was solved many many years ago.

It is not about condenser fan (you can use bigger condenser without fan). I agree that regulated fan will help, but most of the job is done by expansion valves.

Your assumption "Compressor always delivers same fixed pressure" is not exactly correct. Compressor has volume capacity and delivery capacity, but question of pressure is combination of volume capacity, delivery capacity and expansion system.

Theese are fundametals.

[anandnambiar]

i was able to find out that the problem was solved. we are looking at the possibility of compression regulation using a temperature sensor based system, which is connected to the outlet valve of the compressor alone, without making any changes to any other components of the system,(eg-varying the valve timing of compressor etc.)
its the possibility of this particular thing that we are looking at. for that, we need info on the existing technologies used for this (ones that are based on the compressor alone, and are mounted or connected to it). and also whether variable-speed drive motors are used to achieve this.

i agree we dont have much of an idea. and we are pretty desparate to do something. we arent getting much support or assistanc efrom the people here, which makes things wore for us!

[anandnambiar]

*worse.......

[The Viking]

Whenever I start solving problems and well before I start thinking about a solution, I always try to make sure there is an issue there to start with.

I am sorry, but I can not see any scenario whereas the solution will be to vary a compressors capacity/speed depending on any temperature around the compressor itself. (discharge or ambient)

Perhaps this is why you find it hard to get support or assistance, you are looking for a solution to a problem that doesn't exist?

The basic refrigeration principles means that if we alter the compressors capacity, we will alter the duty the whole refrigeration cycle perform.
Whenever a system is designed the first question to be answered is what duty is required. Everything else, including compressor duty, is designed around this first question.
There is no benefits of having a system with all bells and whistles that are able to keep everything within a 0.1% tolerance delivering 1kW when the duty required is 5kW...

As far as I'm able to tell, your idea of compressor control could at best be described as the tail wagging the dog.

Sorry.
:cool:

.

[The Viking]

when the outside(ambient) temperature is 35 celcius, the pressure of the refrigerant just necessary for condensation to occur properly is 2 bar.... when the outside environnment temperature changes to 15 Celsius in another season, we would not require the same pressure of 2 bar for condensation to take place right?

Sorry Anandnambiar,

I have just reread your posts and now realise where the misunderstandings and/or confusion comes from.

The compressors task in the refrigeration cycle is not to ensure that condensation occurs.
If this is your belief, I am truly sorry but you are some heavy books away from starting to control compressors. You will have to learn to walk before you can run.

A shortcut might be some searches on the net, at youtube for example there are a lot of videos about the refrigeration cycle.
Example 1 (http://www.youtube.com/watch?v=b527al9D_rY) Example 2
(http://www.youtube.com/watch?v=0gmwzHAz8rw&feature=related)But there several other sources of information out there, try searching for "refrigeration principle" or similar.

You stated in your initial post that you are a student, you should be able to find books in the library about refrigeration and I would be surprised if your tutors/lecturers wouldn't be able to steer you in the right direction.

Whichever way you go with this I wishes you the best of luck,

:cool:

.````````

[FeeJee Cool]

Hi Anand
You may be researching into the unknown but practically, I feel the replies above should direct you to understand the basic principles of refrigeration.
Refrigeration plants are designed based on the refrigerated load which considers all the parameters including ambient temepratures, however, other components of refrigeration would deal with that instead of using the compressor.
Wish you well in your research!
Thanks

[Magoo]

Hi FeeJee,
welcome to the forum, how are things in Fiji the tropical paradise. In the past have done work in Suva