In this article we’ll explain the differences between a Closed-Circuit and an Open-Circuit Cooling Tower and the advantages and disadvantage of each. How do you know which type to use for a project? They both provide heat rejection but in slightly different ways. These systems are often referred to as open and closed loop systems. These are the two commonly used HVAC cooling tower system designs.
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In a closed-circuit cooling tower, the process fluid, which could be water, or a water-glycol mixture is circulated within a closed loop piping system. There are two separate water sources, one external within a closed loop, and the second one that circulates water from the tower basin over the heat exchanger. There is never any contact between the water in the enclosed loop and the water circulated within the tower.
The water in the tower basin is pumped up and sprayed over the coil that is fed from the closed-loop piping while a fan forces air over the wet coil. This increases heat transfer through the coil while minimizing the evaporation of water from the basin. The cooled water in the closed-loop coil returns to the building to absorb more heat.
Return air brings heat from the space over the indoor coil. The refrigerant cycle moves that heat to the water-cooled condenser coil, where the water circulated from the cooling tower picks up that heat. The heat is pumped to the cooling towers closed-loop heat exchanger coil where water is sprayed over it as air is induced or forced over the coil.
In an open circuit cooling tower, the water is directly exposed to the outside air. Water enters the top of the tower and is sprayed over the fill or heat transfer media. The exchange of heat occurs directly between the water and the entering ambient air. The water that is circulated to the chiller’s condenser and the air within the tower touch each other. This increases the chance of contaminants being scrubbed out of the air and into the cooling tower basin.
The water in the basin is then returned to the facility or the condenser side of a water-cooled chiller, which could foul the chillers condenser coil if the water is not properly treated. The makeup water to the tower can also introduce contaminants to the process water.
There are several tower configurations including induced draft and forced draft. The fans for an Induced draft tower are located at the top of the tower and induces air into the tower. The fans for a forced-draft tower are located at the bottom of the tower and forces air into the tower and over the coil.
Closed circuit cooling towers also known as Evaporative Fluid Coolers play a crucial role in the operation of water source heat pump systems in the HVAC industry. It’s important to use a closed loop system because the water is circulated into the coils of all the water-source heat pumps scattered throughout the building. Sending water from an open tower into all these remote coils could create a maintenance nightmare along with a reduction in efficiency for these units.
Closed circuit cooling towers are often preferred in applications where water quality is critical, and there’s a need to minimize the risk of contamination. This makes them suitable for water-cooled heat pump applications and those industries with strict quality standards, such as laboratories or data centers.
Open circuit cooling towers are commonly used in the HVAC industry where water quality is less critical, and the focus is on cost-effective cooling. They are found in all types of buildings spanning the HVAC industry. The addition of a heat exchanger between the cooling tower and the chiller can add a layer of protection against the fouling of the chiller’s condenser tubes, but at an added cost. This doesn’t eliminate the need for water treatment of the tower, it just shifts the tower basin water from contacting the chillers coil to the heat exchanger.
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To determine the perfect cooling tower design and size for your needs, Delta makes it easy with our downloadable sizing program. To discover Delta’s cooling tower sizing & selection program, simply click here.
If you are interested in learning the methods of determining the proper size cooling tower, rest assured that Delta is here with guidance. Explore our handy information. Click here to learn about sizing & selecting.
Whether your application is for industrial process cooling or HVAC condenser cooling, the data required is the same. The following design data is required for cooling tower sizing to properly select the appropriate model:
The Design Heat Load is determined by the Flow Rate, and the Range of cooling, and is calculated using the following formula:
Heat Load (BTU/Hr) = GPM X 500 X Range (T1 – T2) °F
If the range of cooling, Heat Load, and one of the other two factors are known (either the GPM or the ° Range of cooling), the other can be calculated using this formula.
The range of cooling is directly proportional to the Heat Load.
How comfortable are you working up a cooling tower selection?
The cooling tower selection table may look confusing, but after you have made a few selections, the process is straightforward. If you need a refresher, this may help. The following design data is required to select cooling towers:
Flow Rate in GPM
Range of cooling in °F (T1 - T2)
Area Wet Bulb Temperature in °F (Twb)
The Design Heat Load is determined by the Flow Rate, and the Range of cooling, and is calculated using the following formula: Heat Load (BTU/Hr) = GPM X 500 X ° Range of cooling.
More importantly, if the Heat Load and one of the other two factors are known, either the GPM or the ° Range of cooling, the other can be calculated using this formula.
For example: GPM = Heat Load (BTU/Hr), or 500 X ° Range of cooling ° Range of cooling = Heat Load (BTU/Hr) 500 X GPM
So, as you can see, the Design GPM and the ° Range of cooling, are directly proportional to the Heat Load.
And, 500 is the “fluid factor” which is based on water as the heat transfer fluid. The fluid factor is obtained by using the weight of a gallon of water (8.33 lbs.) multiplied by the specific heat of the water (1.0) multiplied by 60 (minutes/hour).
The first step in selecting a cooling tower is to determine the Nominal cooling tower load. Since a cooling tower ton is based on 15,000 BTU/Hr, the formula is:
Nominal Load = GPM X 500 (Constant) X ° Range of cooling, 15,000 BTU/Hr/Ton or, the more simplified version of the same formula, Nominal Load = GPM X ° Range of cooling 30
Once the Nominal cooling load has been calculated, a Correction Factor must be determined to calculate the Actual Rated cooling tower tons required for the specific conditions of service. The correction factor adjusts for the ease or difficulty of cooling based on the Theoretical Design of all cooling towers.
The Nominal Ton Correction Factor is determined by using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed. Note that the curves are shown as three separate sections. The WET BULB CORRECTION SECTION, the APPROACH SECTION, and the CAPACITY MULTIPLIER FACTOR SECTION. First, find the Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the Range line over to the intersection of the Wet Bulb line.
Now move down along the Wet Bulb line to the APPROACH SECTION, in the lower left-hand section of the chart, and stop at the intersection of the Approach line. Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curves and stop at the intersection of the Range line and read the CAPACITY MULTIPLIER FACTOR.
The Actual Rated cooling tower tons can now be calculated by multiplying the Nominal cooling tons, which was previously calculated, by the CAPACITY MULTIPLIER FACTOR. The Actual Rated cooling tower tons is the capacity required for the specific conditions of service, and the next largest size cooling tower should be selected for the application.
Following are selection examples for three different applications. One example is based on conditions that are identified as "Theoretical Design," for reasons which will become apparent.
The second example, entitled "Actual Design" is a selection based on adjusting from Theoretical to Actual design.
The third example, "Modified Application", converts an actual once-through well water system to a cooling tower recirculation system.
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The following conditions are provided for selection purposes:
The operating water flow rate is 600 GPM.
Hot water temperature (T1) to the cooling tower is 95° F.
Cold water temperature (T2) desired from the cooling tower is 85° F.
The installation location's wet bulb temperature (Twb) is 78° F.
You can now make a cooling tower selection with this information:
The water flow is 600 GPM. The Range of cooling is 10° - (T1 - T2). The Approach to the wet bulb temperature is 7° - (T2 - Twb).
First the cooling tower NOMINAL load has to be determined:
Nominal Load = GPM x 500 x ° Range, = GPM x ° Range, therefore, 15,000 BTU/Hr 30
Nominal Load = 600 gpm x 10° Range = 200 tons of cooling required.
30 Since the Heat Load = Flow (gpm) x 500 x °Range of cooling= 600 gpm x 500 x 10° = 3,000,000 BTU/Hr and a cooling tower nominal ton = 15,000 BTU/Hr, the nominal cooling tower ton is derived from the actual heat load. Therefore, a heat load of 3,000,000 BTU/Hr = 200 nominal cooling tower tons.
Now the Nominal Ton Correction Factor has to be determined for the conditions established:
A 10° Range of cooling, and a 7° Approach to the design wet bulb temperature of 78°F, using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed.
Find the 10° Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the 10° Range line over to the intersection of the 78° Wet Bulb line.
Move down along the 78° Wet Bulb line to the APPROACH SECTION, (the lower left-hand section), and stop at the intersection of the 7° Approach line.
Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curve and stop at the intersection of the 10°Range line, and read the CAPACITY MULTIPLIER FACTOR, which is 1.0.
To select the proper cooling tower for this application, multiply the 200 Nominal tons calculated, by the 1.0 CAPACITY FACTOR. As previously stated, the correction factor adjusts for the ease or difficulty of cooling in relation to the Theoretical Design. So in this case, since the CAPACITY CORRECTION FACTOR is 1.0, the Nominal and Actual Rated tons are the same as the Theoretical Design, and a Model DT-200I cooling tower can be quoted.
Now we will select a cooling tower for the same 200-ton Nominal Load as Example #1 but is different from the Theoretical Design. The operating water flow rate is 300 GPM.
Hot water temperature (T1) to the cooling tower is 105° F.
Cold water temperature (T2) desired from the cooling tower is 85° F. The installation location's wet bulb temperature (Twb) is 76° F.
You can now make a cooling tower selection with this information:
The water flow is 300 GPM. The Range of cooling is 20° - (T1 - T2). The Approach to the wet bulb temperature is 9° - (T2 - Twb).
First, the cooling tower NOMINAL load must be determined:
Nominal Load = GPM x 500 x ° Range, = GPM x ° Range; therefore, 15,000 BTU/Hr 30. Nominal Load = 300 gpm x 20° Range = 200 cooling tons required. 30 Since the Heat Load = Flow (gpm) x 500 x °Range of cooling= 300 gpm x 500 x 20° = 3,000,000 BTU/Hr and a cooling tower nominal ton = 15,000 BTU/Hr, the Nominal cooling tower ton is derived from the actual Heat Load. Again, a 3,000,000 BTU/Hr heat load = 200 Nominal cooling tower tons.
Now the Nominal Ton Correction Factor must be determined for the conditions established; a 20° Range of cooling, and a 9° Approach to the design wet bulb temperature of 76°F, using the COUNTERFLOW COOLING TOWER SELECTION AND PERFORMANCE CHART enclosed.
First, find the 20° Range line in the WET BULB CORRECTION SECTION in the upper left-hand section of the chart. Move along the 20° Range line over to the intersection of the 76° Wet Bulb line. Move down along the 76° Wet Bulb line to the APPROACH SECTION, in the lower left-hand section of the chart, and stop at the intersection of the 9° Approach line. Move across to the CAPACITY MULTIPLIER FACTOR SECTION to the right-hand curves and stop at the intersection of the 20° Range line, and read the CAPACITY MULTIPLIER FACTOR, which in this case is 0.62.
The final step to select the proper cooling tower for this application is to multiply the 200 nominal cooling tons required, which was calculated above, by the CAPACITY FACTOR, which in this case is 0.62. The cooling tower Actual Rated tons for the conditions given are therefore 124 tons, and a Model DT-125I cooling tower can be quoted. Since the correction factor adjusts for the ease or difficulty of cooling based on the Theoretical Design, in this case, the Actual Rated tower conditions are easier than Theoretical Design.
The following is an example of modifying a "once through non-recirculating cooling application" to a recirculating cooling tower system. A cooling tower is required for heat exchanger process cooling, which is now being cooled using 55°F well water at a flow rate of (1 Million gallons/day - 300,000 sanitary = 700,000 gal per day).
Approximately 500 GPM, and discharging to a lake at 80°F. With this information we can establish the Heat Load, which is 500 GPM x 500 x 25° R (80°F - 55°F) = 6,250,000 Btu/Hr.
We can establish the cooling tower design for a 6,250,000 Btu/Hr Heat Load based on the installation location design Twb, which, for this example, we'll say is determined to be 76°F, and by establishing a reasonable cold water temperature at a 7° Approach to the Twb, at 83°F.
What we have to determine now is either the design range of cooling or the appropriate design flow rate based on the established Heat Load. Let’s select the appropriate design flow rate by using a reasonable 15° Range of cooling; 83°F cold water + 15° = 98°F hot water.
Use the Cooling Tower Heat Load Calculation to find the design flow rate as follows:
Heat Load (BTU/Hr) = GPM X 500 X ° Range of cooling, or rearranged to determine the design flow rate. GPM = Heat Load (BTU/Hr) = 6,250,000 Btu/Hr = 835 gpm 500 X ° Range of cooling 500 x 15° R Now you can make your cooling tower selection based on 835 gpm, cooling from 98°F to 83°F @ a design 76°F Twb. The cooling tower selection is = 418 Nominal Tons x .83 DCF = 347 Rated cooling tower tons, or a 350-ton cooling tower selection.
Alternate #1:
A commercial cooling tower can also be selected for this heat load based on a 25° Range of cooling. The conditions for selection would be 500 GPM, cooling from 108°F to 83°F @ 76°F Twb, which is equal to 418 Nominal tons x .62 DCF = 259 Rated cooling tower tons, for a 260 ton cooling tower requirement.
Alternate #2:
Or select for a design to cool 110°F to 83°F = 27° R of cooling, the design flow would be 6,250,000 Btu/Hr = 465 GPM. 27° R x 500
The selection for 465 GPM cooling from 110°F to 83°F @ 76°F Twb = 418 Nominal tons x .58 DCF = 242 Rated tons; so you can recommend a single Model DT-250I cooling tower.
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