Outlines for Designing a Chiller Plant

Hello Viewers.....!! Am Back after many days, and today i've selected a topic of Designing a Chiller Plant, This Post may include many sub topics, which i think have covered so far, 

A chiller unit is nothing but just a cooling unit, which can be used to maintain some high temperature variation from room temperatures, Basically a Chiller Plant works on the Principle of Carnot Cycle, and Heat Transfer. Many of you think that the chiller plant as a big mystery to solve, but trust me by the end of this post, i'll make you feel better when you again think about Chiller Plant.

As mentioned this works on Carnot Cycle, lets have a look 

Now compare the 4 major units shown in the above pic with those in the Chiller unit,

The Piston can be compared to a Compressor, Condenser will remain the same as Evaporative Condenser, Pump can be compared to as Expansion Valves, and finally the boiler to be Chiller.

A typical Chiller unit will seems like, 

Credits: Pharmacalc.blogspot.com

So, now i'll Explain you the major units of chiller plants and also how they have impact on the cooling of the utility,

1. Compressor: A Compressor is the major part of the chiller plant as the process will start from here, Compressor will be used to compress the chilling agent, usually a chilling agent will be Ammonia, as ammonia will have somewhat higher heat capacity. Immediately you may have a question that when a compressor was made running for many hours, wont it have any wear-tear problem, and for avoiding this problem an oil is circulated to cool the overheated compressor screw, Usually the Ammonia Pressure that is developed in a compressor will be very high, and of the the orders 12-15 Kg/cm2, And for this we need to select pipes having very high Design Pressure, the Red line represnt the high pressure Ammonia.

2.  Evaporative Condenser: This Evaporative Condenser is usually used to cool the high Pressure Ammonia that is forwarded from the compressor, usually water is used as the cooling in the cooling towers as coolant for cooling ammonia from gaseous state to liquid state, as shown in the figure the Water is sprayed over the tube bundle [Not Shown in pic] in which the Ammonia flows, Usually the water after getting into contact with the tube bundle will get vaporized and these vapors are immediately sucked off by the blower, and the water which is not vaporized get collected in the below bed, the water which is collected get re-circulated by the pump, and based on the water level that is being vaporized, a makeup water quantity is added in to the collection tub. 

Makeup water = Evaporation + Blowdown + Windage + Drift

These Windage and Drift will be negligible in large scale,

Makeup water * Cds1 = ( Evaporation + Blowdown ) * Cds2.

Cds1 = Dissolved solids concentration in Makeupwater,

Cds2 = Dissolved solids concentration in Evaporation + Blowdown. 

Consider the Ammonia quantity to be cooled is 800 L per 4 hrs from 20°C to 0°C, and the Heat removed per hr is 800 W, So now i gonna calculate the makeup water here,

Qamm = 0.800 * 0.73 *  0.52 * (20-0) = 6.0637 KCal

Makeup water = Qamm / ( 1 * 45 ) = 6.0637/45 = 0.1347 L 

[ ** here Water is considered to be vaporized at 45°C due to the blower effect as the blower Creates some suction ]

Cooled Ammonia is collected in the Receiver, the Receiver Capacity should be considered as 40% excess to the volume of ammonia processed. 

Basically the ammonia shouldn't contain any oil, but in practical cases the ammonia will carry oil also with it, so after collecting it in receiver, the oil should be removed/separated, The Liquid ammonia is then taken back to a shell and tube heat exchanger which is used to cool the oil from compressor, thos job is done inside the Oil Cooler chamber as given in the above pic, the cooled oil will be transferred again to the compressor.

So, Now lets consider the compressor is  creating a pressure of 15 Kg/cm2 [ 20°C], and in this process 70 L compressor oil [rho = 1300 Kg/Cu.m, heat capacity = 0.8 KCal/kg.°K] is getting heated upto 70°C, and i'm having ammonia cooled to 0°C, so now i need to cool the oil back to 30°C to reprocess it, then we need to calculate the Heat Transfer area of the Required heat exchanger, shown below,

Qoil = 700*1.3*0.8*(70-30) = 29120 KCal,

A = Qoil/U*LMTD,

LMTD = ((70-0)-(30-20))/Ln((70-0)/(30-20)) = 30.8°C

U = 350 KCal/sq.m. hr. °C,

A = 29120 / (350*30.8) = 3 Sq.m

So my required Heat exchanger capacity is 3 Sq.m,

3. Economizer: An Economizer is something like a vessel, but actually its not a vessel, but which helps to stabilize the flow of ammonia towards the expansion valves, and also reduce the load on the compressors, Cold, low-pressure chiller vapor is used to subcool the saturated liquid refrigerant. This decreases the refrigerant circulation rate, and may reduce compressor power. 

Credits: Jmcampbell.com

Let’s consider removing 1.0×10⁷kJ/h which is equal to 2778 kW (9.479 MMbtu/hr) from a process gas at -35°C (-31°F) and rejecting it to the environment by the condenser at a condensing  temperature of 35°C (95°F). Assuming 5 kPa (0.7 psi) pressure drop in the chiller and 5 kPa in the suction line pressure drop, the compressor suction pressure is 132.4 kPa (19.1 psi). The condenser pressure drop plus the pressure drop in the line from the compressor discharge to the condenser was assumed to be 50 kPa (7.3 psi); therefore, compressor discharge pressure is 1270 kPaa (184.2 psia). The compressor discharge temperature is 66°C (150.8°F). At these conditions, the condenser duty is 4434 kW (15.13 MMbtu/hr). Pure propane is used as the working fluid.

4.  Expansion Valves : The Expansion Valves plays a major role on the degree of cooling in the chiller as the extent of opening of these expansion valves defines the amount/flowrate of chiling agent into the chiller, so as the flowrate into chiller increases the temperature of the process fluid out will be reduced, Usually these are not regularly operated valves, they were kept consistent.

5.  Chiller : This is just a Shell and Tube Heat Exchanger used to Cool the Process fluids that were incoming with the available chilling agent, usually the process fluid will enters the tube side, and the chilling agent will enters shell side, the required capacity of the chiller can be calculated as follows,

Let the flowrate of the water inside is 50 cu.m /hr, and the overall heat transfer coefficient be 450KCal/hr.Sq.m.°C, LMTD be 50°C, and the incoming water to be cooled from 30°C to 5°C, then

Qwater = 50 * 1000 * 1* 25 = 1250000KCal

Req. Heat Transfer Area, A = Qwater / U * LMTD = 1250000 / (450*50) = 55.6 Sq.m

 6.  Surge Tank : Surge tank is like a storage tank which allows to store the ammonia vapors that were produced after heat transfer in the chiller with the process fluid, the size of the surge tank should be 30% less when compared to that off ammonia collection receiver from the evaporative condenser, the residence time of the ammonia vapors in the surge tank depends on the Compressor suction pressure.

So, Now we came to an end and i'll discuss you the Elements that effect the Cooling Rate of the Process fluids in a chiller,

1. Expansion Valves Opening - if the opening is high, then the flowrate will be high into chiller, ultimately leading for high temperature gain from process fluids,

2. Compressor Suction Pressure - if the suction pressure is low then the chilling agent will have some high retention time in chiller which leads for high heat transfer rate.

So, Now if you understand the principle of working of Chiller Plant just say Cheers....!!

If any queries, then feel free to ask us, Comments are most appreciated.........................!!!

About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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