7 replies to this topic

[choychoy]

Hi,

The chiller in the chemical plant I am working for has it evaporator outlet process chilled liquid temperature controlled by suction throttling of a centrifugal compressor. The control temperature of the outlet chilled liquid is 2degC while the control temperature of the inlet chilled liquid is 20degC. Freon is being compressed in this chiller.

Thus, in the event the chilled liquid temperature increases above 2degC as a result of inlet chilled liquid temperature increasing to 21degC, the suction valve will open up more to allow more freon being sucked into the compressor.

What I would like to know is that if the amount of freon being compressed and send to the condensor is more than previous due to opening of the suction valve, does it mean that the pressure of the condensor will increase? It is assumed here that cooling water flowrate and temperature, which is used for cooling in the condensor is constant.

Since the cooling water flowrate and temperature is constant, the heat removal by the condensor should be still the same. Does it mean that freon exiting the condensor and moving to the evaporator will be hotter since more freon is being compressed as explained above?

However, since the flowrate of freon had increased but with hotter freon, does it mean that the ability to remove heat by the freon is still the same?

I know by experience that the above is not true as when outlet chilled liquid temperature increases, the suction valve opens to compress more freon which eventually lowers down the outlet chilled temperature back to the control point of 2 degC.
However, I cannot find a good reason to explain the above.

Pls correct me if I am wrong.

[djack77494]

We really cannot fully address your inquiry without knowing more. Assuming a fairly simple and straightforward refrigeration loop, as you open the suction throttle valve, you will tend to reduce the pressure on the suction side. This includes the evaporator, where the pressure reduction = a temperature reduction. There will be more refrigerant circulating in the loop, and the JT valve (or turbine) will take an additional pressure drop. Hope I've pictured your system correctly; if not, please provide more details and/or a sketch.

[choychoy]

Hi,

Below is a rough description.

1) Evaporator is connected to the compressor suction via a suction throttling valve. Water is run throught the evaporator to be cooled down as "chilled water".
The suction throttling valve is controlled by the chilled water outlet temperature of 2degC.
Inlet chilled liquid temperature is controlled at 20degC.
Flowrate is constant.

2) The compressor outlet is to the condensor where it is cooled by fixed plant cooling water flowrate and temperature.

3) The condensor outlet is then linked back to the evaporator via RO for the autorefrigeration of freon.
Freon is then used to cooled the chilled liquid.

The inquiry is that if for some reason, inlet chilled liquid temperature increased to 21degC and flowrate is constant, outlet temperature also increased to 2.5degC.
Thus, the suction throttling valve should open up to suck in more freon into the compressor.

In the condensor side, since I did not alter the cooling water flowrate and temperature, should the temperature of freon in the condensor increase?
Thus, in the condensor side, the amount of freon entering the condensor increased while the temperature of freon increased?

With a higher freon temperature and flowrate leaving the condensor, how does it help to reduce the outlet chilled water temperature back to 2degC? I always thought that the increase flowrate of freon cancels out the increase in temperature of freon, thus resulting in no additional cooling effect?

My whole question basically lies with energy balance. Pls assist more.

[Art Montemayor]

Choychoy:

We need a drawing or sketch of your refrigeration cycle. I have put together the attached flow diagram as fast as I can. You should correct it or modify it to suit what you have and to better explain what you are trying to state. My comments to your first post are as follows:

The chiller in the chemical plant I am working for has it evaporator outlet process chilled liquid temperature controlled by suction throttling of a centrifugal compressor.
(I don’t understand this sentence)

The control temperature of the outlet chilled liquid is 2 degC while the control temperature of the inlet chilled liquid is 20 degC. Freon is being compressed in this chiller.
(I don’t understand all these “control” temperatures – use a sketch or drawing to explain)

Thus, in the event the chilled liquid temperature increases above 2 degC as a result of inlet chilled liquid temperature increasing to 21 degC, the suction valve will open up more to allow more freon being sucked into the compressor.
(How is the compressor’s potential surge characteristics being controlled?)

What I would like to know is that if the amount of freon being compressed and send to the condenser is more than previous due to opening of the suction valve, does it mean that the pressure of the condensor will increase? It is assumed here that cooling water flowrate and temperature, which is used for cooling in the condensor is constant.
(You must ensure a constant cooling water flow with regards to the maximum refrigerant being compressed. The pressure in the condenser – and receiver – will remain constant regardless of the amount of refrigerant compressed.)

Since the cooling water flowrate and temperature is constant, the heat removal by the condensor should be still the same. Does it mean that freon exiting the condensor and moving to the evaporator will be hotter since more freon is being compressed as explained above?
(This statement makes no sense. You must have sufficient cooling water for the maximum refrigerant flow rate. The refrigerant temperature will be the same in all cases.)

However, since the flowrate of freon had increased but with hotter freon, does it mean that the ability to remove heat by the freon is still the same?
(See above statements)

I know by experience that the above is not true as when outlet chilled liquid temperature increases, the suction valve opens to compress more freon which eventually lowers down the outlet chilled temperature back to the control point of 2 degC.

However, I cannot find a good reason to explain the above.
(It is all about vapor pressure. The refrigerant is saturated and its vapor pressure corresponds to the temperature.)
 Centrifugal_Chiller.xls   30.5KB   54 downloads

[choychoy]

Hi,

attached is the schematic of the chiller.

1) Warm Process Fluid temperature control is 20degC
2) Chilled Process Fluid temperature control is 2degC which then controls the suction of the compressor.
3) There is no receiver, only a economizer between the condensor and evaporator.
4) There is a RO between the condensor and economizer and another RO between the economizer and evaporator.
5) Cooling water temperature and flowrate for condensor is always fixed.

Thus, the question is that when for some reason warm process fluid temperature raises above 20degC to say 21degC,
compressor suction valve should open in order to reduce the chilled process fluid temperature back to 2degC.

This will result in more freon being compressed in the compressor. If more freon is sent to the condensor, how does the condensor maintained its pressure if the amount of cooling water flowrate and temperature is the same.
I believe you replied me that the vapour pressure, hence temperature of freon in the condensor is the same.

How does more freon being compressed help to reduce the chilled process fluid temperature back to 2degC.

I am not too knowledgable in this area, perhaps a step by step explaination?

Pls assist more.

Attached Files

•  Centrifugal_Chiller.xls   30.5KB   39 downloads

[djack77494]

The suction throttling valve is controlled by the chilled water outlet temperature of 2degC.
Inlet chilled liquid temperature is controlled at 20degC.

I don't believe the latter statement. My guess and your schematic suggest that the inlet chilled water temperature is not controlled at all. Why would it be? You don't care what the return temperature is, right?

The inquiry is that if for some reason, inlet chilled liquid temperature increased to 21degC and flowrate is constant, outlet temperature also increased to 2.5degC.
Thus, the suction throttling valve should open up to suck in more freon into the compressor.

Follow this a step further. As the suction throttling valve opens further, the refrigerant pressure in the evaporator drops resulting in a decrease of temperature. In a refrigerant system like this, a change in pressure has a corresponding change in temperature. There is a 1:1 link between the two, so if you're not changing the pressure then you're not changing the temperature.

In the condensor side, since I did not alter the cooling water flowrate and temperature, should the temperature of freon in the condensor increase?

Refer to previous comment. There probably was an increase in condensor refrigerant pressure due to increased mass flowrate of refrigerant = higher dP across orifice.

With a higher freon temperature and flowrate leaving the condensor, how does it help to reduce the outlet chilled water temperature back to 2degC? I always thought that the increase flowrate of freon cancels out the increase in temperature of freon, thus resulting in no additional cooling effect?

My whole question basically lies with energy balance. Pls assist more.

I don't know if you're thinking too hard or not enough, but you've gotten off the track needed to properly understand what is happening. Try again considering the above.

[Art Montemayor]

ChoyChoy:

What you are now revealing to us tells me this could be a very lengthy and confusing thread if left under these conditions. Therefore, I’ve spent some time in creating a reasonable and detailed, normal 2-stage centrifugal compressor chiller PFD. I’ve done this to clear up a host of issues that are starting to come up in your explanation and request for help. I know that we can be of great help to you, but I would ask you for your patience and your application to learn just how a chiller package is supposed to work and how you can best understand what makes it work that way.

Please dedicate some serious time to studying the PFD that I am attaching in the Rev2 version of the Workbook we are using. This is a very important thing for you in order to fully understand what we are stating and recommending to you. You should have some thermodynamic text books at hand or fully studied already. You have to fully understand how a mechanical refrigeration cycle works – thermodynamically and mechanically. If you don’t, then let us know. We don’t want to waste your time with terms and engineering principles that you won’t be able to understand.

From what you describe, you have a chiller package that is fixed in operating capacity. You have Resistance Orifices (ROs) employed instead of control valves. This means your flow rates are, practically, fixed. Your throttling of the centrifugal suction in order to control the amount of chiller duty in the evaporator means that you are literally “starving” the compressor if it is running at a constant speed (which I assume). This feature will work – but for a very limited range; throttling the suction of a centrifugal compressor can take it into the surge region – which you don’t want to do. All centrifugal compressors have to be protected against surge by using instrumented controls.

Centrifugal compressor chiller packages are not designed to handle a varying chiller duty. They are meant for CONSTANT DUTY. If you have a varying chiller duty, then a reciprocating or screw type of compressor is the machine of choice because these positive type compressors can be unloaded with much more reliability and ease. My point here is that you have a basic, constant duty type of chiller package – especially if you have been supplied with ROs. Another feature that weakens your ability to vary loads and absorb duty variances is the fact that you have no refrigerant reservoir. I consider this a major weakness in any mechanical refrigeration package. I suspect that your unit was specified and purchased as a “low-cost, low-bid” package. That means that you probably have little – or no flexibility in the operation.

Now to answer your specific questions:
Regardless of the flow rate handled by the compressor, if the cooling water furnished to the condenser is sufficient, the pressure in the condenser and reservoir will be the constant. This is so because of the refrigerant’s vapor pressure. If you condense the refrigerant at a constant temperature – regardless of the refrigerant flow rate – the resulting vapor pressure will be constant. And refrigerant vapor pressure is the pressure that you have within the closed system.

If your chiller duty increases (because the inlet process fluid temperature increases), then you require more liquid refrigerant (Freon) to be expanded into the evaporator to furnish more cooling RATE. This is merely common sense.

I hope the above responds to your specific need. Using the supplied PFD you may now raise more questions and queries about your process. If so, please mark up the PFD with “callouts” and indicate where you have concerns or questions. When you modify the Workbook be sure to increase the Rev number in order to accurately identify what we are referring to.

Also, tell us if you are a student or if this is a real, industrial application. If it is a real application, then it belongs in the Industrial Professional Forum.

 Centrifugal_ChillerRev2.xls   395KB   66 downloads

[choychoy]

Hi Art,

I felt you misunderstood me of ignoring this post and posting another. If I really wanted to do that, I could just create another userID.

Also, there was a lengthly holiday on my side and thus I apologize for not replying. Also, I was doing my own studying and reading.

Also, I believe I can answer some of my own questions after I went back to my old schoolbooks and some reading on the internet.

1) I suggested that the chiller water supply temperture is controlled but I did not indicate on my excel diagram. The chilled water temperature is controlled by bypassing a heat exchanger before entering the evaporator. This heat exchanger is external to the chiller unit. Thus. the chiller inlet temperture is indeed controlled at a fixed temperature.

2) Also, previously I asked if the cooling water flowrate and temperature to the condenser is fixed, how does the hotter freon contribute to more cooling in the evaporator.
I think I am still correct to say that with more freon compressed by the compressor and sent to the condenser, if there is nothing done to the cooling water flowrate and temperature of the condenser, the condenser temperature(and pressure) will definitely increase. It has to condense at a higher temperature(pressure).

However, even with the increase in pressure of the condenser, the amount of freon circulated in the system in increased, this will lead to the increased cooling of the evaporator. Since the freon that is used for cooling is the unflashed freon, a quick look at % of freon flashed vs initial pressure suggested that the % of freon flashed increased only a few %. Thus, the % of unflashed freon still increased?
However, I am not too confident of this explanation. Maybe someone can assist?

3) With the above, I can roughly explained my own questions. With the increase in chilled water temperature, this is what happens

A) chilled water temperature increases to 21degC from 20degC. Chilled water outlet temperature is meant to be controlled at 2degC.
B) Assuming at this point in time, the amount of freon is still constant. Thus. more latent heat of vaporisation for the fixed amount of freon will be required, temperature(pessure) of freon will drop.
C) At the next moment, the compressor suction valve opens to compress more freon, sending more freon to the condenser.
D) With more freon to the condenser, more cooling water should be supplied to the condenser to maintain same condenser pressure.
E) More freon is circulated to the evaporator which leads to more cooling, bringing down the chilled water outlet temperature to 2degC. This only leds to evaporator pressure resuming back to normal values.

I am just a junior engineer learning the ropes. Any correction is welcomed. Also, perhaps I should post in the industrial profession forum as this is a real plant experience.