Condenser water temperature range in centrifugal water cooled chillers 3

[HowdyDowdy]

Consider the operation of a water cooled centrifugal compressor chiller. Increasing the condenser water temperature range reduces the condenser water flow, which requires smaller pumps and piping. It also increases the required condenser pressure while improving the LMTD for the cooling tower. Increasing the condensing pressure on the chiller will result in a combination of increased chiller cost and reduced performance.

Given this, how does reduced condenser water flow rate cause the required condenser pressure to increase in the chiller? I would like to understand this from a thermodynamic point of view. Can anyone explain this?

 

[mauner]

When there is a phase change the equation for the condenser's heat transfer can be described by

Q=UA(Tsat-Tave)

Where the condenser pressure is a function of saturation temperature.

For a given inlet temperature, Tin, the outlet temperature, Tout, will increase as you decrease condenser cooling water flow (m) based on Q = mc(Tout-Tin). Also, the average cooling water temperature (Tout+Tin)/2 increases.

Thus, assuming negligible changes in Q and U, the saturation temperature and condenser pressure will increase when the cooling water flow is decreased.

 

[Montemayor]

MCMortley:

What you have described is a conventional, mechanical refrigeration system driven by a centrifugal compressor. As such, you incorrectly call it “a water cooled centrifugal compressor chiller” when, in reality, it is the refrigerant condenser that is water cooled. The condenser, besides the compressor, is one of the main components in the refrigeration cycle that you are describing and its production of condensed refrigerant is what is the key element producing “cold” downstream in an evaporator that is presumably producing a “chilled” process fluid.

You have asserted various statements which I list and comment on:

1) “Increasing the condenser water temperature range reduces the condenser water flow, which requires smaller pumps and piping.” You are basically correct if you are maintaining the initial cooling water temperature entering the condenser constant year-round. If you are using a cooling tower to produce the cooling water (CW) temperature, I doubt you can maintain the product CW supply (CWS) temperature constant year-round, but I will assume that the variance is negligible.

2) “It also increases the required condenser pressure while improving the LMTD for the cooling tower.” Here is where you loose me and, I believe, you’ve lost your logic. By increasing the CW temperature range (with a constant inlet temperature) you can only mean that the return CW temperature is as high as you (actually, the CW tower) can accept it. Normally, this cooling water return (CWR) is maintained at a maximum of 120 oF on a practical basis. This is the maximum limit on most CW tower systems. If you increase the condenser water temperature range you can only mean that you are lowering the CWS temperature. With a properly designed and applied condenser, you can expect to achieve a 5 oF approach to the CWS temperature. This means that with an 80 oF CWS, you can expect an 85 oF refrigerant condensed product. We engineers know our Thermo and, consequently, we know that the condensed refrigerant product is a saturated fluid whose vapor pressure is directly related to its temperature. The lower the temperature, the lower the vapor pressure. The same vapor pressure is the system pressure in the condenser and receiver, prior to going to the refrigeration expansion valve and the downstream evaporator. Consequently, you are wrong in stating that the condenser pressure increases with an increased temperature range. Rather, the condenser pressure will decrease when you have a ceiling on the CWR and you increase the temperature range - because this means that the CWS temperature has to be lower. Additionally, there is NO LMTD involved in the design and operation of a cooling water tower. The Log Mean Temperature Difference (LMTD) is a pseudo-driving force derived for use in describing the overall temperature driving force in the Fourier equation as applied to the design of tubular heat exchangers – not simultaneous heat and mass transfer unit operations such as a cooling tower. Sorry, but I’m quoting basic engineering knowledge to your statement.

3) “Increasing the condensing pressure on the chiller will result in a combination of increased chiller cost and reduced performance.” Not necessarily true, but you fail to state that you are implying capital costs rather than operating costs. You also fail to identify the expected performance you foresee as “reduced”. An increased condenser pressure for most refrigerants identifies a lower Latent Heat of Vaporization (or, in your case, condensation). Consequently, you will require less water flow rate. The Latent Heat requirement is easily seen in the Mollier Diagrams for refrigerants as found in ASHRAE publication: “Thermodynamic Properties of Refrigerants” (ISBN 0-910110-47-6). The “Tit” formed by most refrigerants in the Mollier Diagram is more slender as the pressure increases – until the Critical Point is reached.

Now to address your specific question: “how does reduced condenser water flow rate cause the required condenser pressure to increase in the chiller?” Answer: If you reduce CWS to a refrigerant condenser, you will condense less refrigerant (rate wise) and your CWR temperature will increase. You will still be condensing, but at a lower rate and at a higher temperature. This will yield a higher operating pressure within the condenser and the receiver and, if the process persists, will cause an over-pressure situation on the condenser and receiver, activating their pressure safety relief valves. You must supply the correct and necessary rate of CWS to the condenser at all times while the unit is operating in order to condense all the refrigerant entering at a specified rate. The more CWS you can furnish and the colder that same CWS, the faster the refrigerant vapors will condense and the colder and the lower the vapor pressure inside the condenser-receiver assembly. The colder the refrigerant liquid in the receiver, the better and more efficient the chiller operation. That is my personal design and operational experience with Ammonia, CO2, Freons, and other refrigerants in industrial applications.

Summarizing: I don’t know where you gathered or got your assertions on the mechanical refrigeration cycle, but I would recommend you do some boning up on the basic and detailed engineering theory of mechanical refrigeration as found in good text books such as the ASHRAE Fundamentals Handbook or Ernest Ludwig’s classic “Applied Process Design for Chemical and Petrochemical Plants”, Volume 3.

I hope I’ve been of some help in orienting you on refrigeration thermodynamics.

(As an aside, how does your basic query rate a "Star"?)


Art Montemayor
Spring, TX

 

[quark]

Excellent comments by Mr Montemayor as usual.

Theoretically, heat absorbed by cooling water is equal to the sum of heat addition to the evaporator and heat equivalent of compressor power. Once this is fixed, you can play with cooling water flowrates and temperature. For a fixed wet bulb temperature, lower cooling water leaving temperature(condenser) requires longer condensers. Further, as the convective coefficients highly depend upon the velocity of flowing fluids, your heat transfer area should be higher.

What I couldn't understand is that how can you increase the range of condenser cooling water temperature without increasing the sytem pressure?

One more good reference is Principles of Refrigeration by Roy J Dossat.

Regards,




I wish us all A Happy and Prosperous New Year

 

[HowdyDowdy]

Montemayor:

Having read your reply, I believe that there is some misunderstanding on your part. Perhaps I should have written a more detailed explanation on the issue at hand. Further, there are some capabilities of centrifugal water cooled chillers, as applied in the HVAC industry, that you may not realize. I will try and address some of the discrepancies we might have in the following.

When I refer to a "water cooled centrifugal compressor chiller" I am simply referring to a chiller that is both water cooled and uses centrifugal compression as a method of compressing refrigerant gas. I you wish, you could indicate the chiller type by using a comma as in "water cooled, centrifugal compressor chiller." I am simply using industry standard jargon that is well understood within the HVAC industry.

In response to your first point, it is quite possible to lower the CWS to the condenser, and maintain this temperature throughout the cooling season, with an adequate cooling tower controls strategy. Of course, maintaining this constant temperature off of your cooling tower throughout the season can be done within reason and depends on ambient wetbulb temperature. You simply use cooling tower by-pass and fan modulation in your cooling tower to maintain this reduced temperature. It is not practical to maintain this constant reduced CWS temperature, but for argument purposes it can be done.

With regard to your second point, nowhere in my question do I mention that I am operating my CWR temperature around the ceiling of my cooling tower water supply temperature. I am simply saying, consider a condenser approach operating in the middle of your min. allowable CWS and max. allowable CWS range. To increase your approach within this range, you can either decrease your CWS temperature through cooling tower relief or you can increase your CWR temperature by increasing your compressor lift--take your pick. So this business about the condenser pressure decreasing when you have a ceiling on the CWR doesn't apply. Furthermore, when you state that you can expect to see a 5 degree Fahrenheit approach with a properly designed and applied condenser is simply erroneous. The Air-Conditioning and Refrigeration Institute has established ARI Standard 550/590-98, which is a standard for water chilling packages using the vapour compression cycle. It states that the leaving chilled water temperature should be 44oF at 2.4 gpm/ton and the entering condenser water temperature should be 85oF at 3.0 gpm/ton. Recall that the heat that needs to be removed from the condenser is equal to the heat collected in the evaporator plus the work of compression. Assuming the work of compression is 25% of the heat collected in the evaporator, then the heat rejected in the condenser will be 125% of the evaporator heat. Calculating your resultant temperature change in the condenser for modern high efficiency chillers is found to be 9.4 degrees Fahrenheit at 3 gpm/ton. In short, by applying ARI conditions, the condenser approach should typically be ~10.0 degrees and not 5 degrees as you have indicated. ARI is a well established standard within the HVAC industry, and I would direct you to be conversant with its standards. I should state, however, that although ARI design conditions represent good "average" conditions to use, they may not represent the best design conditions to use for every project.

With regard to your mentioning that costs (either capital of operating) are not necessarily increased due to higher condenser pressure, this is entirely fallacious. Of course operating at an increased condenser pressure increases your capital costs, as you will have to rate your condensing vessel at a higher pressure! Moreover, consider the effect of increased condenser pressure on operating costs. As you increase your pressure, you increase the associated saturation temperature. This intern causes your chiller to operate between an increased difference in temperature sinks. A rule of thumb is that the COP improves by 2 to 4 percent for each degree Celsius the evaporator temperature is raised or the condensing temperature is lowered. Obviously, if you increase this difference you derate the COP of chiller. This results in increased operating costs over the life cycle of the chiller. I am only stating elementary thermodynamic principles here.

Further, it is not imperative that you maintain the correct and necessary supply of CWS to the condenser at all times. Chiller plants play around with these entering conditions all the time depending on operating conditions. I do agree with you that you must not exceed the maximum condenser pressure, but there is a range of pressures that the condenser can operate within.

I hope that I have given adequate treatment to the misconceptions you might have.

Wishing you and yours a happy new year.

 

[quark]

MCM,

There have been discussions on this issue but manufacturers are reluctant to reducing either cooling water or chilled water flowrates across the refrigeration system, or atleast they don't take the risk(?).

For a better performance, a cooling tower should always be operated at a fixed L/G ratio. You can have better savings when you switch off the fan as soon as you get required temperature.

Convective coefficients are better at higher velocities(turbulent) and reducing the flowrate may decrease the overall heat transfer rates.

You can't reduce the system pressure by just reducing the cooling water temperature. You have to remove the required amount of heat from refrigerant gas. If you are keeping heat rejection to cooling water constant, by reducing the flowrate and increasing the TD across condenser, then power consumption of compressor remains same.(PS: theoretical power consumed by compressor = mass flow times the enthalpy difference of gas)

Further, better savings are acheived from compressor side when we have the advantage lower than designed cooling water temperatures(without altering the flowrate). York did some good work in this regard and you may get some empirical data from them.

Good luck,