Help needed

[mazamm]

Hi everyone !!
I have just started my professional career in a chemical plant. I am facing problems in understanding the chiller operation.
We have a chiller equipped with a screw compressor of HANBELL company. The compressor is a 4 step capacity control system with a modulation slide valve installed over the screws.
When the compressor load increases the slide valve slides and change the capacity. But I am unable to predict what actually is being done in the whole cycle. As far as I have understood, as the load increases the evaporator temperature increases (increase of super-heat). Now the expansion valve opens and allows more refrigerant to flow which increases the suction/evaporator pressure. The increased flow then enters the compressor and capacity control system allows this increased flow rate to be compressed by screws.
If all this explained above is true, then what will be the effect on discharge pressure ?
Kindly correct me if I have got anything wrong


Regards

 

[dcasto]

nothing. Discharge pressure is set by the condenser.

 

[mazamm]

And who tells the condensor to set that discharge pressure :/

 

[LittleInch]

If this is a closed system, then it all depends on how and where the minimum system pressure is controlled or maintained either via an accumulator system or via a relief valve /make up pump and tank arrangement.

If the point is at the discharge of the compressor then you won't see much pressure rise. If it's at the inlet to the compressor then the pressure will rise to achieve the higher flow rate through all the components.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[petrochemical87]

the compressor discharge pressure depends only on the temperature of cooling media available in the condensor. The refrigerant leaves as saturated liquid in the condensor. check out this thread to learn about slide valve operation in an oil flooded screw.

http://www.eng-tips.com/viewthread.cfm?qid=302526

 

[dcasto]

so,to be clear, the condenser outlet temperature sets the outlet pressure. I've worked at places where the design called for a fixed outlet pressure and the condenser was controlled by adjusting the cooling system. In the dead of winter the condenser was running its 100 F maxing the compressor work harder than it should. It was a sad site. It told the engineers to let mother nature cool of the discharge and save some energy on the 30,000 HP refrigeration system.. NOPE, the level valve to the chiller wasn't designed for a lower DP. I shook my head and cried.

 

[Compositepro]

The condenser does control the compressor outlet temperature. But the statement "the condenser outlet temperature sets the outlet pressure" is not quite accurate, in that the outlet is usually sub-cooled liquid, and can be sub-cooled by a lot. So the pressure actually depends on the cooling capacity of the condenser. This will vary with air temperature, air flow rate and amount of the condenser that is flooded with liquid, condensing pressure, and other factors.

 

[dcasto]

how do you sub cool a condenser, if the fluid is say propane and the air condenser can cool the propane to 100 F, then the propane surge tank is 186 psia and the compressor puts up 186 plus pressure drop through system.. If it cold outside and the air condenser can cool to 80, the compressor will only run at 142 psia plus delta P

 

[Compositepro]

It all depends on how the equipment is designed. Most small refrigeration and AC equipment are designed so that the bottom part of the condenser is flooded so that only liquid comes out. The only way to achieve this is to subcool the liquid. One reason to do this is to avoid hot piping and vessels downstream from the condenser, which could be a burn hazard.

 

[petrochemical87]

I dont see the point of subcooling refrigerant condensate coming out of the condensor. I have seen economizers being used in refrigerant circuits to enhance the efficiency of the system, but using a flooded condensor(with a loop seal setup etc) seems like an awful waste of heat transfer area. I tend to agree with dcasto on this one.

 

[25362]

Visit the following threads to get some more ideas:

thread403-279014
thread403-155434
thread403-144104

 

[Montemayor]

dcasto is correct in asserting that the condenser of a pure fluid cannot be subcooled. By definition a subcooled fluid is one that is condensed and subsequently cooled to a temperature BELOW its saturation temperature. This cannot happen to a pure fluid that is condensed in a closed system where it is in equilibrium with it own vapaor - such as a refrigerant. I know that this is applying semantics, but the point here is understanding what we mean by the term "subcooling" as it pertains to a condenser.

Steam, when condensed in a closed steam condenser can only condense as a saturated liquid. However, when exposed to the atmosphere and cooled further, it results in subcooled water. The water is subcooled because it is not in equilibrium with a water vapor; it is in equilibrium with the atmosphere.

The principle still holds for the topic of this thread: the OP doesn't understand that the vapor pressure of the condensed refrigerant is SATURATED (not subcooled), and as such will exhibit a pressure that is equal to its vapor pressure at the condensing temperature. The colder that the cooling medium is (cooling water in this example), the lower that the saturated vapor pressure will be. This can be clearly appreciated in a Mollier (or Temperature-Entropy) diagram. Chemical engineers should easily understand this, considering all the Thermodynamics and Phase Equilibria they have to study. Mechanical Engineers don't take Phase Equilibria, so it's a little tougher for them.

 

[25362]

Gibbs' phase rule applies.