All About Process Cooling

Chillers are often categorized by size (compact or central) and by the type of condenser (air or water). The classification by condensers will also be discussed here.

Air Cooled Chillers
As the name suggests, chillers having air-cooled condensers use air to remove heat from the refrigerant. A fan forces air across small tubes containing the hot refrigerant and discharges that heat into the ambient air. Compared to water, air is a poor conductor of heat and therefore air-cooled chillers are larger and less efficient. The typical condensing temperature for an air-cooled chiller is 120°F as opposed to a 105°F in a comparable water condensed chiller. Air-cooled chillers also operate at higher compressor ratios – which means less cooling per watt energy consumption.

For the smaller, portable chillers, the air-cooled condenser is integrated in the chiller cabinet and so it’s heat is ejected into the area around the chiller. This heat can be reclaimed to supplement a building’s heating system in the winter.

However, in the summer, this will cause an additional load on the building’s air conditioning system unless the chiller and/or condenser is located outdoors. There two types of air-cooled chillers- split or integrated systems. With a split system or “remote condenser” the chiller is indoors and the condenser outdoors. For the integrated unit, you can put a small unit indoors since the load on the air-conditioner will not be much. Or you can use the PCA indoors that allows the heated air to be ducted outdoors. You can always put the entire chiller outdoors (if it is designed that way).

Water Cooled Chillers
Water-cooled condensers are of three basic designs; tube in tube, shell in tube or brazed plate. The tube in tube design has one tube inside another and the tubes are coiled into a “donut” shape to minimize space requirements. The heat transfer from the refrigerant to the water takes place when the refrigerant flows through one tube while water flows in the opposite direction through the other tube. This counter flow enhances the transfer of heat.

The shell in tube design is very similar except that there is a bundle of tubes contained in a shell. The refrigerant in the shell is around the water flowing through the tubes. This arrangement allows the tubes to be cleaned out in case of fouling.

The brazed plate design is a highly efficient and compact design. This heat exchanger has stainless steel plates that are embossed with small channels to provide multiple contact points and increased fluid turbulence thus providing excellent heat transfer while lowering the potential for fouling. The plates are stacked and brazed together to form two independent circuits running in alternate layers.

Since this type of unit is constructed with stainless steel plates and copper alloy brazing, it provides a superb corrosion free environment. Not only does the brazed plate technology significantly reduce floor space requirements over conventional shell-and-tube or tube-in-tube evaporators, brazed plates are also far more efficient. As with the shell and tube design, the refrigerant and water have counter flow to further enhance the transfer of heat.

As mentioned, water condensed units are more efficient than air condensed, often operating in the range of 15 EER or better (EER: energy efficiency ratio or BTU per hour per Watt energy consumption). Water cooled chillers require a source of cooling water, such as cooling tower water, to extract heat from the refrigerant at the condenser and reject it to the ambient environment. The typical condensing temperature in a water-condensed chiller is 105°F.

Evaporative Condensers
Another alternative to the air or water-cooled condensers described above is the evaporative condenser. Evaporative condensers are like cooling towers with built in heat exchangers. Refrigerant passes through a copper tube bundle in the evaporative cell. Water cascades over its outer surface and airflow counter to the flow of water causes some of the water to evaporate. This results in the efficient cooling of the refrigerant.

There is a sump in the bottom of the condenser to store water and a pump draws the water to spray over the coils. In the winter, the pump is de-energized and only the air flowing across the coils is sufficient to cool the refrigerant. The chiller thus becomes aircooled.

At the heart of any chiller is the compressor. As the name suggests, compressors are used to increase the pressure of the refrigerant. There are a variety of compressors available on the market, the most common are the reciprocating, scroll and screw compressors.

Reciprocating Compressors
Reciprocating compressors are driven by a motor and use pistons, cylinders and valves to compress the refrigerant. These compressors are available in hermetic, semi-hermetic or externally driven versions.

In a hermetic unit, the motor and compressor are enclosed in a common housing, which is sealed. Because the components are not accessible for repair, the entire compressor unit must be replaced if it fails.

In the semi-hermetic unit the motor is also part of the unit, however it is not sealed so it is serviceable.

In a direct drive unit the motor and compressor are separated by a flexible coupling. These types of units utilize older technology and are not commonly used today.

Scroll Compressors
Scroll compressors perform at higher efficiency levels than reciprocating compressors. The compressors operate without cylinders, pistons or valves so it offers:

• Low maintenance and high reliability
• Low noise and vibration levels
• Low space requirements
• Relatively low weight

Inside the scroll compressor, two spiral-shaped members fit together forming crescent shaped gas pockets. One member remains stationary while the other orbits relative to first. This movement draws gas into the outer pocket and seals off an open passage. As the spiral movement continues, gas is forced toward the center of the scroll design, creating increasingly higher gas pressures.

With several pockets of gas simultaneously compressing, you receive a nearly continuous compression cycle. Gas discharges from a port at the center of the fixed scroll member. With both radial and axial compliance, the scroll members wear in rather than wear out.

Screw Compressor
A screw compressor’s moving parts include a main and secondary rotor. It also has significant benefits:

• Dramatic reduction of compressor parts
• Low maintenance and high reliability
• Low noise and vibration levels
• Low space requirements
• Relatively low weight

The screw compressor’s suction, compression and discharge all occur in one direction. Suction gas is pressed into one grooved rotor by the second similar rotor. The screw-like rotor motion continues toward the end of the compressor’s working space. In this way, refrigerant volume steadily reduces or compresses until it reaches the stationary end of the compressor. It is here that the vapor discharges to the condenser.

Due to this design, the need for compressor parts is low. Therefore, familiar components, such as oil, assume new roles. Instead of lubrication, oil now performs as a dynamic sealer. By design, motor cooling occurs with refrigerant gas or vapor passing through holes in the rotor – which also functions as a “built-in” liquid separator. The scroll design also enables higher evaporating temperatures, beneficial to all applications excluding those with low temperature conditions. The screw compressor is also more forgiving to liquid slugging.

The most fundamental principle in process cooling is heat balance. This means that the amount of heat generated by the process is to be removed by the chiller: no more, no less. Frequently implemented methods for capacity control and heat balance are:

• Compressor cycling
• Hot gas bypass
• Cylinder unloading
• Multiple compressors
• Condenser water or air regulation

Compressor Cycling
Compressor cycling is one method to control the level of cooling in a chiller. It involves using a thermostat and a predetermined refrigeration temperature called the “set point”. When cooling reaches the set point, the thermostat stops the compressor from circulating refrigerant through the system. Since the coolant circulation pump continues to operate, the process gradually raises the temperature of the process water. The thermostat detects this rise and turns the compressor back on.

The drawback of compressor cycling is the additional wear placed on the compressor motor windings when the compressor is started. With the current spiking up to 600% of its normal operating level, compressor cycling over an extended period could cause premature failure.

Hot Gas Bypass
Hot gas bypass is used to supply the compressor with a continuous full load while the chiller is catering to partial load conditions. Normally, the compressor adds energy to the refrigerant by increasing its pressure and temperature. The resulting hot gas goes to the condenser. The condenser removes heat from the gas and allows it to pass to the thermal expansion valve (TXV) as a liquid. This flow path changes with a hot gas bypass. As the compressor satisfies the process load, water begins to over-cool. A thermostat senses this drop in temperature. At a preset temperature condition, the thermostat opens an electric solenoid hot gas bypass valve-allowing refrigerant to take the path of “least resistance.”

A portion of the refrigerant bypasses the condenser and the TXV valve. The un-condensed gas mixes with the refrigerant that has passed through the TXV. Since the mixture of liquid and gaseous refrigerant loses some capacity to remove heat, a “free wheeling” occurs: Without heat removal from the refrigerant, the water in the chiller begins to rise in temperature. Again, at a pre-established temperature, the thermostat detects the temperature rise in the chiller and closes the bypass valve when water reaches the set temperature. In this way, the hot gas bypass prevents the compressor from short cycling when the chiller operates under partial load conditions. This valve is particularly important when operating a semi-hermetic compressor since the compressor must receive a full amount of refrigerant for motor winding cooling.

Cylinder Unloading
Capacity control also occurs through compressor cylinder unloading. A thermostat energizes a solenoid (or solenoids if there are multiple cylinders in the compressor) that forces the discharge valve to stay open. Since the cylinder chamber is open to the discharge manifold, no refrigerant gas compression can take place. The result is a drop in refrigeration capacity that is in direct proportion to the number of cylinders being “unloaded.” The torque on the electric motor reduces and results in lower power consumption. Cylinder unloading is most desirable as a method of capacity control since it balances a chiller’s capacity to the process load and saves power consumption.

Two cylinder compressors’ capacity can be controlled only through cycling or hot gas bypass. The cylinders unload from six to four and then two. This capacity reduction is then followed by hot gas bypass. For the screw compressor, although there are no cylinders to “unload” to match capacity to process, they do have two stages

There are some applications where failure of refrigeration equipment could result in serious financial loss beyond the equipment repair expense. In such cases, it is advisable to consider a multiple compressor chiller system.

Under partial load conditions, the compressors may be cycled in and out of service as required as well as providing a level of redundancy in the event one of the compressors should fail. Although the system will operate at a lower capacity if a compressor fails, it should not be allowed to run in this condition for long periods of time to prevent possible damage to the other compressors in the chiller.

Condenser water regulation for water- cooled chillers, or condenser fan cycling for air-cooled chillers, are other methods of capacity control. Either method has limitations in its capability to reduce a chiller’s capacity.

If the incoming water (or air) temperature drops due to seasonal changes, the colder water (or air) will increase a chiller’s capacity. If the process load remains constant during this change, throttling the condenser flow (or cycling the condenser fan) only reduces the chiller’s capacity within a limited range. As such, neither method is truly successful as a means of capacity control.

Another consideration in selecting a chiller is its construction. A noncorrosive component that prevents water corrosion in process equipment is essential for long and maintenance free life of not just the chiller, but the equipment it is servicing.

For CTI products, all chiller circulating pumps, evaporators and reservoirs are constructed with stainless steel and pipes are made with copper. All CTI chillers are fitted with a Ystrainer at the water inlet to prevent any particles from clogging up the evaporators. As may be surmised, CTI’s selection of components provides efficient operation and reduces equipment maintenance.

Another special area of standard features includes safeties. Some of the safeties that should be considered include; high and low refrigerant pressure, low compressor oil pressure, low flow, freeze protection, high temperature alarm and low level alarm

There are three basic types of towers. The first, a forced draft tower, has a sensor to thermostatically control the cooling tower fan. The sensor monitors the process water temperature after it exits from the tower. The fan engages or disengages when the process water temperature rises either above or below the desired set point.

A second type of tower, induced draft, has a fan in the wet air stream to draw air through the fill. Cooling Technology generally recommends this type of tower cell for industrial processes.

A third type, ejector natural draft tower, has no mechanical means to create airflow. In this case, water pumps to the tower, enters a manifold with nozzles, and ejects under high pressure that induces a draft of air. The finely sprayed water contacts freeflowing air to perform the evaporation process.

In all types, towers use the force of gravity to drain water into an indoor pump and tank station. The pump delivers the water to process through piping where it picks up heat. The now-warmed water continues to flow back to the outdoor tower through return lines. The cycle continuously repeats.

Factory assembled cooling towers are available in numerous sizes starting at less than 5 tons and reaching several hundred tons. Larger capacities are designed by banking several units together and piping them to operate in tandem.