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Water-Cooled Condensers & Chiller Barrel Fundamentals
Water-Cooled Condensers & Chiller Barrel Fundamentals
Water-cooled condensers and chiller barrels are specialized heat exchangers. They exchange heat by removing heat from one fluid and transferring it to another fluid.
A water-cooled condenser is a heat exchanger that removes heat from refrigerant vapor and transfers it to the water running through it. Having the refrigerant vapor condensed on the outside of a tube does this. In doing so, the vapor condenses and gives up heat to the water running inside the tube.
A chiller barrel works just the opposite. A chiller barrel is actually a direct expansion evaporator. Chiller barrels evaporate the refrigerant inside the tube. Heat is removed from the water running through the outside shell of the tubes.
The water-cooled condenser is an important component on the high side of an air-conditioning/refrigeration system. The chiller barrel is an important component on the low side of a system.
It is necessary to know four basic things in order to predict how a heat exchanger will perform:
- Temperature differential (DT)
- Velocity and pressure drop (V and DP)
- Fluid type
For condensers, DT is the condensing temperature minus the incoming water temperature. For chillers, DT is the incoming water temperature minus the suction temperature. The greater the DT, the greater the rate of heat exchange in a given time period, usually expressed in BTU’s/Hr.
Velocity is the speed at which a fluid flows. There is an ideal rate of flow through a heat exchanger for any fluid. At this ideal flow rate, the fluid mixes with itself in such a way that it produces maximum heat pick-up. Turbulent flow causes cooler fluid to be constantly moved into contact with the heat surface. If the flow is too slow, a laminar condition may develop. That is a condition in which only the fluid right next to the heat exchange wall is being heated, but beyond this very thin layer, the heat can’t penetrate to the rest of the fluid. But — the velocity must be limited by another condition, pressure drop (DP). DP increases with velocity. After a certain point, the amount of energy expended to overcome DP will be more than any efficiency picked up by increased velocity. High DP and high velocity also produces problems that greatly shorten the life of a heat exchanger. Impingement corrosion and erosion will shorten the life to just a few months, if bad enough.
Fouling occurs because most water is not pure. There are many materials dissolved or suspended in water. These materials coat the surface of the tubes and inhibit heat transfer. Even on the refrigerant side, oil can coat the surfaces and act as an insulator between the refrigerant and the water. It is impossible to prevent all fouling, so a fouling factor must be included when sizing a heat exchanger.
The fourth factor, fluid type, has to be taken into consideration. As an example in many chillers, glycols or brine solutions are used in low temperature applications. Due to a decrease in heat transfer coefficients, ratings based on water are no longer valid.
Sizing Water-Cooled Condensers
To size a water-cooled condenser, we must first find the total heat of rejection for the system. For an air-conditioning or high backpressure system, it is safe to size the condenser by nominal horsepower, or tons of refrigeration load. 12,000 BTU/hr is the normal figure for one ton or horsepower. To this, add 3,000 BTU/hr heat of compression for a total of 15,000 BTU/hr per ton.
For medium and low temperature systems, take the actual load and add 3,000 BTU/hr per horsepower.
For instance, a low temperature three-ton load with a 10 horsepower compressor would be calculated:
3 tons = 36,000 BTU
3,000 x 10 = 30,000 BTU
total = 66,000 BTU’s.
Note: In medium and low temperature applications, it is prudent to add 10% to the calculated load for pull-down conditions. This results in 6,600 BTU’s. Therefore, the condenser requirement is for 72,600 BTU’s/hr.
Now, a specific condenser can be selected from a manufacturer’s catalog. Many catalogs will be based upon the ARI standard of 20° DT between the incoming water temperature and the condensing temperature. This is because 105°F is used for water-cooled condenser condensing temperature. The cooling tower water temperature into the condenser is assumed to be 85°F. If the DT is significantly greater than 20°F, a smaller, less costly unit may work. We can apply the 8/10 rule. For every 10% increase in DT, there will be an 8% increase in capacity.
Example: If condensing temperature rises to 109°F, a 20% greater DT has been obtained (4°F is 20% of 20°F). This will be a 16% increase, at a given water flow of capacity (.8 x 20% = 16%).
If the initial DT is less than 20°F, the 9/10 rule can be used. For every 10% decrease in initial temperature, there will be a 9% decrease in capacity.
For instance, the condensing temperature drops to 101°F. We now have a 20% decrease in DT. This is an 18% decrease in capacity, again at the same flow rate. This could result in the selection of a larger, more costly condenser.
There are limitations in applying these rules, and one of them has to do with water velocity. The water velocity in shell and tube condensers (the most popular design) should be kept less than 8 feet per second. The velocity is a function of design and water flow rate. Manufacturer’s catalogs may show performance graphs that list the maximum GPM a particular condenser can handle. If not otherwise stated, that rating should be at less than 8 feet per second. If there is any doubt, call the manufacturer. In fact, most condenser manufacturers, such as Standard Refrigeration Co., have computer programs to properly size a water-cooled condenser from your specifications.
It’s also very important to consider the fluids the condenser is going to handle. To handle non-corrosive refrigerants and water, condensers are made of steel and copper. If ammonia, brine, or other odd solutions are going to be used, consult the manufacturer. While a water-cooled condenser is primarily a heat exchanger, the shell and tube condenser is also a receiver. Pump down requirements should be considered in its selection. If a system needs greater receiver capacity than a particular condenser selected can supply, receivers can be put in series behind the condenser.
Most condenser ratings printed in manufacturer’s catalogs take into consideration a “fouling factor”. It is usually expressed as a “.005 fouling factor”. “Fouling” is the coating of tubing walls with scale and dirt. This increases heat transfer resistance and reduces the condenser’s efficiency. It is impossible to completely prevent fouling so an allowance is made in condenser ratings for some fouling. Water conditions vary widely, so it is the user’s responsibility to keep the condenser clean. Generally speaking, if the water leaving the condenser is more than 10° greater than the refrigerant condensing temperature, the condenser needs to be cleaned.
Sizing Chiller Barrels
Chiller barrels function just the opposite of a condenser. Instead of using liquid to cool refrigerant, a chiller barrel uses refrigerant to cool a liquid. It is an evaporator. Refrigerant evaporates inside tubes as water flows through a baffled course on the outside of the tubes.
Sizing a chiller barrel depends on the same basic factors as for condensers: DT, velocity DP, fouling, and fluid types, plus range, approach, and superheat. Range is the difference between the incoming water temperature and the outgoing water temperature. Approach is the temperature difference between the outgoing water temperature and the refrigerant temperature. Superheat is the difference between actual saturated refrigerant temperature and suction gauge temperature. The best way to size a chiller barrel is by temperature range and GPM flow rate. GPM should be converted to pounds of water per hour by multiplying GPM by 500 (1 gallon of water is 8.3 pounds. 8.3 x 60 = 498, rounded off to 500.)
Example: Incoming water temperature 55°F, outgoing water temperature wanted 45°F. Therefore, range is 10°F. Flow rate, 20 GPM. 20 x 500 = 10,000 pounds per hour. 10,000 x 10 = 100,000 BTU’s per hour.
If the liquid isn’t water, the BTU figure has to be adjusted for the heat values of the fluid to find the true BTU load. A common “fluid” is a mix of glycol and water. The capacity correction factors for glycol solutions are shown in Figure 1.
If we picked a chiller barrel rated at 100,000 BTU/hr but our fluid is a 50/50 mix of glycol and water, the barrel will be only rated at 60,000 BTU/hr (100,000 x .60). We will need to pick a larger chiller barrel to get back to the required 100,000 BTU/hr with the 50/50 glycol water mix.
Another way to size a chiller barrel is by compressor capacity. A chiller barrel can only do what a compressor can pump. If the compressor is rated 200,000 BTU/hr at a certain suction and condensing temperature, the chiller barrel must have the capacity to handle that.
Sizing for air-conditioning is easiest of all. The barrel can be sized by nominal tons. ARI ratings for air-conditioning are based on a 10° range, 9° approach, 7° superheat, and a .005 fouling factor.
Sizing chiller barrels for other than standard air conditioning requires careful selection.
The temperature differential is made up of two different components:
Range on the DT between incoming and outgoing water.
Approach the DT between outgoing water and refrigerant temperature.
These are critical. Changing approach temperature can result in dramatic results. One degree change in approach means a 15% change in chiller barrel capacity. Five degrees difference can amount to a 300% change! A chiller barrel at a 10°F range and a 4°F approach rated 36,000 BTU will be 164,000 BTU at a 12°F approach. But one never gets something for nothing. Approach does have limits. The capacity of the compressor will drop by lowering the suction evaporator temperature. Another risk in wide approach temperatures is freezing. Any evaporator temperature below freezing is in danger of freeze-up, which destroys the chiller barrel. Systems that do run below freezing with glycol mixes should be hand fed to prevent dilution of the mix if there are any leaks, so the freezing point remains low.
Also, there is simply a limit on the amount of heat transfer that can occur in any heat exchanger. There is only so much surface area to work with.
Most manufacturers size their units for 7 or 8°F superheat, although some use 0° superheat. The catalog literature should specify this. It there is any doubt call the manufacturer.
3°F superheat equals about one degree of approach and although this means a 15% increase in capacity, too low a superheat may damage the compressor. It is poor practice to go below 5°F superheat, and then only if an accumulator is used. (Accumulators benefit all systems.)
Flow velocity in a chiller barrel must be less than 4.5 feet per second. Excessive velocity will damage the chiller barrel. Most manufacturers’ catalogs will list GPM flow rates using 4.5 F.P.S.
Pressure drops of 8 psig or under are acceptable. If the DP is higher than 8 psig, choose a different model chiller barrel where the DP will be 8 or less than 8 psig.
Proper freeze protection should be used on all water chillers. A freeze stat set at 34°F on the outlet of the chiller barrel should always be used. Freeze-up is the primary cause of unit failure. Of course, if the chiller barrel is located outside, where ambient freezing temperatures may be encountered, some type of heat must be applied, like heat tape, to keep from freezing the barrel.
Chiller barrel selection is now a simple matter. (For the following example, use Standard Refrigeration Company’s “Evaporator Catalog, Chiller Barrels, Subcoolers, 1994-1995”.)
A chiller barrel has the following specifications:
Must handle 900,000 BTU/hr load
- Has a 10 psig pressure drop available
- No suction accumulator
- Incoming water temperature: 55°F
- Outgoing water temperature: 45°F
The compressor will operate at a 34°F suction temperature
We have: Range = 10°F (55°F Water In - 45°F Water Out)
Approach = 11°F (45°F Water Out - 34° Suction Temperature)
Chiller barrels are usually used where downtime is very costly. Quick serviceability is a plus factor.
Chiller barrel construction is important. Chiller barrels can be more than one circuit barrels. A dual circuit barrel has two refrigeration inlets and outlets. Each circuit can be used for separate, but similar loads. Quad circuits are designed for four separate and similar loads.
Chiller barrels will need to be cleaned periodically for proper operation. If chiller barrel price is not the only consideration, consider purchasing a cleanable barrel with removable heads. If price is the only major parameter in selecting a chiller barrel, “sealed” or non-serviceable chiller barrels are available in smaller sizes, up to about 25 tons capacity.
With this in mind, we will use the standard FSX serviceable chiller barrel.
First find the chart for the FSX 10° range. Under the approach column of 11°F, go down until the appropriate tons is encountered. 900,000 ÷ 12,000 = 75 tons. In this case, it will be an FSX 60, rated at 77.2 tons. The pressure drop (DP) shown is 9.90 psig at 185 GPM. (See note on page 12 on determining flow rate.) See Figure 2. The FSX60 is the choice since it meets all the specifications.
First, determine the load as accurately as possible. Second, get the operating conditions -- fluids, pump capacities, system accessories, ambient temperatures, etc., and judge how they may affect performance. Then pick a condenser or chiller barrel that assures good results. If in doubt, call the manufacturer for help. Now, they all have computer programs to help pick the proper product.