Chemical-Free Cooling Tower Treatment
Pure water is a rare commodity. Water, as we know it, contains many dissolved minerals. When evaporation occurs in a cooling tower, only the water evaporates; it exits the cooling tower as water vapor, but leaves the minerals behind to concentrate in the cooling tower’s water system.
The concentrations of these dissolved minerals gradually increase until a process called precipitation occurs. Precipitation happens when dissolved minerals such as calcium carbonate (limestone) reach a certain concentration and become solid, usually clinging to equipment and piping surfaces in the cooling tower. HVACR personnel refer to these solids as scale.
Tiny suspended particles exist in large quantities in all city water, or well water that is used for cooling tower or boiler makeup water. Once in the cooling tower water system, these suspended particles neither sink nor float because of their small size. They are transported by the flowing water.
The particles will concentrate during the evaporation process and be attracted to the equipment surfaces in the cooling tower. When the concentration is so great that the water can hold no more minerals, they are forced to find surfaces to precipitate to as a solid, scaling the equipment. This concentration and attraction of particles eventually becomes hard, equipment-damaging scale.
Now, a proprietary and patented technology has been developed by an engineering and research team. This chemical-free technology eliminates scale, inhibits bacterial growth, and inhibits corrosion in water purification (Figure 1).
How it Works
When the cooling tower water holding these small suspended particles passes through a water treatment module and is activated by a high-frequency electrical pulse field, the natural electrical static charge on the particle’s surface is removed.
In removing this surface charge on the suspended particles, they are now the preferred site for precipitation of minerals to occur, instead of the equipment surfaces. The suspended particles now act as seeds for precipitation of dissolved minerals. Thus, the hard scale is prevented from forming on the equipment’s surfaces and instead bonds to the tiny suspended particles in the water.
The minerals in the water now adhere to and coat the suspended particles. As more and more minerals bond to the suspended particles, they become heavier and can no longer suspend themselves in the water stream. They eventually make their way to the cooling tower’s basin as a harmless fluffy powder or tiny coated particle.
This powder or coated particle can easily be removed from the cooling tower’s basin by manual means, filtration, or centrifugal separation. The quantity of powder is typically about 15 percent of normal blow-in dirt in a cooling tower.
Particle Separator
Particles can be removed from the bottom of the cooling tower’s basin using a centrifugal separator (Figure 2). Water from the basin is pumped to a centrifugal separator, where it enters the separator tangentially. This gives the water the proper inlet velocity and causes a constant change of direction to generate an initial vortexing action.
Internal tangential slots located on the inner separation barrel causes the water to accelerate further and magnify the vortex strength. Particles in the water are now separated through centrifugal action caused by the vortex. The particles spiral downward along the perimeter of the inner separation barrel and are deposited in a collection chamber below the vortex deflector plate, where they can be automatically purged.
Free of separable particles, the water spirals up the center vortex in the separation barrel and upward to the outlet. A vortex-driven pressure relief line draws fluid from the separator’s solids-collection chamber and returns it to the center of the separation barrel at the vortex deflector plate. This allows even finer solids to be drawn into the solids-collection chamber that would otherwise be re-entrained in the vortex.
Figure 2 (left). The removal of particles at the bottom of the cooling tower’s basin can be accomplished using a centrifugal separator. (Courtesy of Lakos Separators and Filtration Solutions)
Bacterial Control
There are two methods of controlling bacteria or microbial population in cooling tower systems: encapsulation and electroporation.
Normally, bacteria form a biofilm or slime layer on equipment surfaces. The slimy bacterial secretion forms a protective canopy to protect the bacteria beneath it from chemical biocides. It is very slimy to the touch, four times more insulating to heat transfer than mineral scale, and is the primary cause of microbial-influenced corrosion on equipment.
The bacteria that live in a biofilm and adhere to the equipment surfaces are called Sessile bacteria; they represent 99 percent of the total bacteria in a system. However, this slime layer can be eliminated through a process of nutrient limitation.
The suspended particles in the water of a cooling tower incorporate most of the free-floating planktonic bacteria. Normally, since like charges repel one another, the bacteria are repelled by the suspended particles in cooling tower water due to the fact that nearly all tiny particles have similar negative static electrical charges on their surfaces. However, after being activated by the high-frequency electrical pulse field at the water treatment module by the signal generator, the natural electrical static charge on the particle’s surface is removed.
The repulsion to the bacteria is eliminated; therefore, the bacteria are attracted to the powder and become entrapped in it. The powder, in effect, sweeps the water clean of planktonic bacteria and renders them incapable of reproducing. This process is referred to as encapsulation.
The high-frequency, pulsing action of the signal generator also damages the membrane of the planktonic bacteria by creating small pores in their outer membrane. The condition weakens the bacteria and inhibits their capabilities to reproduce. This process is referred to as electroporation. Microbial life has a 24- to 48-hour life span. Any microbe not captured in the forming powder are zapped by the secondary pulse of the signal generator, forcing them to spend their lives repairing cell wall damage rather than reproducing.
All of the living organisms in a cooling tower system depend on one another for their food supply. Thus, when the nutrients from the planktonic bacteria are diluted by both encapsulation and electroporation, the biofilm cannot be sustained and it will disintegrate.
The biofilm will never be created if the cooling tower system is installed using a high-frequency electrical pulse field and creating encapsulation and electroporation processes. The combined effects of encapsulation and electroporation result in exceptionally low total bacterial counts (TBC) in cooling tower water.
Corrosion Control
Most corrosion in cooling tower systems or boilers comes from:
• Chemical additives;
• Softened water;
• Biofilm; and
• Mineral scale.
So, by removing chemicals, avoiding the use of softened water, and using the chemical-free water treatment module and signal generator in cooling tower and boiler water applications, corrosion concerns can be eliminated.
The calcium carbonate that coats the suspended particles is in a state of saturation while it precipitates, and will act as a powerful cathodic corrosion inhibitor. It will greatly slow the corrosion process by blocking the reception of electrons that are thrown off by the corrosion process. With no place for the electrons to go, the corrosion process is physically, very effectively controlled.
Bad Compressor Valves
Many servicemen experience service calls where the compressor has both a low head pressure and a high suction
pressure. Often, the refrigeration equipment is still running, but the product temperature is suffering about 7 to 10°F. These calls are tough to handle because the compressor is still cooling, but not cooling to its rated capacity. The medium-temperature products will spoil quicker and the low-temperature products are not frozen as solid as they should be.
There are three main reasons why a compressor will simultaneously have
a low head pressure and a high suction pressure:
• Bad (leaky) compressor valves (Figure 1);
• Worn compressor rings (Figure 2); and
• Leaky oil separator.
Leaky Compressor Valves
Here are reasons why a compressor’s valves may become inefficient because of valve warpage from overheating or lack of lubrication, or from having carbon and/or sludge deposits on them preventing them from sealing properly.
• Slugging of refrigerant and/or oil;
• Moisture and heat causing sludging problems;
• Refrigerant migration problems;
• Refrigerant flooding problems;
• Overheating the compressor which may warp the valves;
• Acids and/or sludges in the system deteriorating parts;
• TXV set wrong — Too little superheat causing flooding or slugging;
• TXV set wrong — Too much superheat causing compressor overheating;
• Undercharge causing high superheat and compressor overheating; and
• Low load on the evaporator from a frozen coil or fan out causing slugging or flooding of the compressor.
Below is a service checklist for a compressor with valves that are not sealing.
Compressor With Leaky Valves
(Measured Values)
Compressor discharge temp …………………………….225?
Condenser outlet temp……………………………………..75?
Evaporator outlet temp…………………………………….25?
Compressor in temp…………………………………………55?
Ambient temperature ………………………………………75?
Box temperature …………………………………………….25?
Compressor volts……………………………………………230
Compressor amps………………………………………….. low
Lowside (evaporating) pressure (psig)….. 1.6 psig (10?)
Highside (condensing) pressure (psig) ……95 psig (85?)
(Calculated values ?F)
Condenser split………………………………………………..10
Condenser subcooling ……………………………………….10
Evaporator superheat ……………………………………….15
Compressor superheat ………………………………………45
Symptoms include:
• Higher than normal discharge temperatures;
• Low condensing (head) pressures and temperatures;
• Normal to high condenser subcooling;
• Normal to high superheats;
• High evaporator (suction) pressures; and
• Low amp draw.
Higher than normal discharge temperatures: A discharge valve that isn’t seating properly because it has been damaged will cause the head pressure to be low (Figure 1). Refrigerant vapor will be forced out of the cylinder and into the discharge line during the upstroke of the compressor. On the downstroke, this same refrigerant that is now in the discharge line and compressed will be drawn back into the cylinder because the discharge valve is not seating properly. This short cycling of refrigerant will cause heating of the discharge gases over and over again, causing higher than normal discharge temperatures. However, if the valve problem has progressed to where there is hardly any refrigerant flow rate through the system, there will be a lower discharge temperature from the low flow rate.
Low condensing (head) pressures: Because some of the discharge gases are being short cycled in and out of the compressor’s cylinder, there will be a low refrigerant flow rate to the condenser. This will make for a reduced heat load on the condenser thus reduced condensing (head) pressures and temperatures.
Normal to high condenser subcooling: There will be a reduced refrigerant flow through the condenser, thus through the entire system because components are in series. Most of the refrigerant will be in the condenser and receiver. This may give the condenser a bit higher subcooling.
Normal to high superheats: Because of the reduced refrigerant flow through the system, the TXV may not be getting the refrigerant flow rate it needs. High superheats may be the result. However, the superheats may be normal if the valve problem is not real severe.
High evaporator (suction) pressure: Refrigerant vapor will be drawn from the suction line into the compressor’s cylinder during the downstroke of the compressor. However, during the upstroke, this same refrigerant may sneak back into the suction line because the suction valve is not seating properly. The results are high suction pressures.
Low amp draw: Low amp draw is caused from the reduced refrigerant flow rate through the compressor. During the compression stroke, some of the refrigerant will leak through the suction valve
and back into the suction line reducing the refrigerant flow. During the suction stroke, some of the refrigerant will sneak through the discharge valve because it is not seating properly, and get back into the compressor’s cylinder. In both situations, there is a reduced refrigerant flow rate causing the amp draw to be lowered. The low head pressure that the compressor has to pump against will also reduce the amp draw.
Worn Compressor Rings
When the compressor rings are worn, high-side discharge gases will leak through them during the compression stroke, giving the system a lower head pressure (Figure 2). Because discharge gases have leaked through the rings and into the crankcase, the suction pressure will also be higher than normal. The resulting symptom will be a lower head pressure with a higher suction pressure. The symptoms for worn rings on a compressor are very similar to leaky valves.
Leaky Oil Separator
When the oil level in the oil separator becomes high enough to raise a float, an oil return needle is opened, and the oil is returned to the compressor crankcase through a small return line. The pressure difference between the high and low sides of the refrigeration system is the driving force for the oil to travel from the oil separator to the compressor’s crankcase.
The oil separator is in the high side of the system and the compressor crankcase in the low side. The float-operated oil return needle valve is located high enough in the oil sump to allow clean oil to automatically return to the compressor’s crankcase.
Only a small amount of oil is needed to actuate the float mechanism, which ensures that only a small amount of oil is ever absent from the compressor crankcase at any given time. When the oil level in the sump of the oil separator drops to a certain level, the float forces the needle valve closed. When the ball and float mechanism on an oil separator goes bad, it may bypass hot discharge gas directly into the compressor’s crankcase. The needle valve may also get stuck partially open from grit in the oil. This will cause high pressure to go directly into the compressor’s crankcase causing high low-side pressures and low high-side pressures.
Megohmmeters for Preventive Maintenance
Megohmmeters (meggers) are electrical meters used to check the resistance and condition of the motor windings and the condition of the refrigeration and oil environment around the motor windings. A megger is nothing but a giant ohmmeter that creates a very large dc voltage (usually 500 volts dc) from its internal battery. The meter will read out in megohms (millions of ohms). Any motor winding or electrical coil can be checked with a megger. A megohmmeter’s main function is to detect weak motor winding insulation and to detect moisture accumulation and acid formations from the motor windings to ground before they can cause more damage to motor winding insulation.
When dealing with HVACR hermetic and semi-hermetic compressor motors, as contaminants in the refrigerant and oil mixture increase, the electrical resistance from the motor windings to ground will decrease. Because of
this, regular preventative maintenance checks can be made with a megohmmeter and can signal early motor winding breakdown from a contaminated system when accurate records are kept.
One probe of the megger is connected to one of the motor winding terminals, and the other probe to the shell of the compressor (ground). Note: Make sure metal is exposed at the shell of the compressor where the probe is attached so that the compressor’s shell paint is not acting as an insulator to ground. When a button is pushed and held on the megger, it will apply a high dc voltage between its probes and measure all electrical paths to ground. It is important to disconnect all wires from the compressor motor terminals when megging a compressor motor.
Also, read the instructions that come with the meter to determine what time interval to energize the megger when checking winding or coils. If possible, it is a good idea to run the motor for at least one hour, disconnect power, disconnect all electrical leads, and then quickly connect the megger to the motor. This will give a more meaningful comparison between readings for the same compressor on different days, because of the approximate same winding temperatures.
Good motor winding readings should have a resistance value of a minimum of 100 megohms relative to ground. In fact, good motor winding resistance should be between 100 megohms and infinity.
Figure 1 lists megohm readings with varying degrees of contamination and motor winding breakdown. Because of the very high resistance of the motor winding insulation, a regular ohmmeter cannot be used in place of the megger. A regular ohmmeter does not generate enough voltage from its internal battery to detect high resistance problems like deteriorated winding insulation, moisture, or other system contamination.
Listed below are some other important tips service technicians should know about the use of a megohmmeter:
• Never use a megger if the motor windings are under a vacuum.
• Meggers can be used for other electrical devices other than electric motors.
• Always consult with the meter manufacturer or user’s manual for detailed instructions on megging other electrical devices like coils.
• Meggers are often used in preventive maintenance programs, especially
before a contractor signs a preventive maintenance contract to determine
condition of the electrical devices.
• Any megger with a higher voltage output than 500 volts DC should be
used by an experienced technician. A high voltage for too long of a time may
further weaken or fail motor windings and the winding insulation could be
damaged by the testing procedure.
System Containing Air
Another similar scenario would be a refrigeration system containing air, as in Table 3. Air is a noncondensable and will get trapped in the top of the condenser. This will cause high head pressures and high condensing temperatures because of reduced condenser volume to desuperheat, condense, and subcool. Thus, the liquid at the condenser’s bottom will be hotter than normal and will lose heat faster to the ambient. This will result in an increase in condenser subcooling.
Table 3 shows 40? of condenser subcooling, but these amounts will vary depending on the amount of air in the system.
Again, in this example, high condenser subcooling is not caused from an “amount” of liquid being backed up in the condenser, but from the liquid in the condenser’s bottom simply losing heat faster.
System with Dirty Condenser
Table 2 shows a refrigeration system with a dirty condenser causing restricted airflow over the condenser. A similar condition would be a defective condenser fan motor starving the condenser of air. Both conditions caused the head pressure and thus condensing temperature to increase. Even the liquid at the condenser’s bottom will be hotter because of the elevated condensing temperatures. This creates a greater temperature difference between the liquid at the condenser’s bottom and the ambient (surrounding air) designed to cool the condenser and its liquid. This will cause the liquid at the condenser’s bottom to lose heat faster, causing more condenser subcooling. In this example, high condenser subcooling is not caused from an “amount” of liquid being backed up in the condenser, but from the liquid in the condenser’s bottom simply losing heat faster.
This phenomenon happens because the temperature difference between the liquid at the condenser’s bottom and the surrounding ambient is the driving potential for heat transfer to take place. As more and more air is restricted from flowing through the condenser, the amount of condenser subcooling will increase.
Notice that the system check sheet shows higher than normal condenser subcooling of 15?. This system check sheet looks very similar to an overcharge of refrigerant because of the increased subcooling amounts, but do not be fooled by it. When a high head pressure and high condenser subcooling is experienced in a refrigeration system, the service technician must not assume an overcharge of refrigerant. The technician must first check to see if the condenser is dirty or a condenser fan is inoperative because of similarities of symptoms in both scenarios of an overcharge of refrigerant and restricted airflow over the condenser.
Overcharged System
Table 1 shows an R-134a refrigeration system with an overcharge of refrigerant. Notice the 30 degrees of liquid subcooling backed up in the condenser. Because of the overcharge of refrigerant, the condenser will have too
much liquid backed up in its bottom, causing high condenser subcooling. By overcharging a system with too much refrigerant, increased liquid subcooling amounts will be realized in the condenser.
However, just because a system has increased subcooling amounts in the condenser doesn’t necessarily mean the system is overcharged. This will be explained in the next two system checks. Remember, the condenser is where refrigerant vapor is condensed and liquid refrigerant is formed. This backed-up subcooled liquid at the condenser’s bottom will take up valuable condenser volume, leaving less volume for desuperheating and condensation of refrigerant vapors.
Too much liquid subcooling at the condenser’s bottom will cause unwanted inefficiencies by raising the head pressure and the compression ratio. Higher compression ratios cause lower volumetric efficiencies and lower mass flow rates of refrigerant through the refrigeration system. Higher superheated compressor discharge temperatures will also be realized from the higher heat of compression caused from the high compression ratio.
Remember, most conventional condensers’ functions are to:
- Desuperheat compressor discharge vapors
- Condense these vapors to liquid, and
- Subcool refrigerant at its bottom.
SINGLE-DUCT – Variable Air Volume
A VAV system, as shown in Figure 10, controls temperature in a space by varying the quantity of supply air rather than varying the supply air temperature. A VAV terminal unit at the zone varies the quantity of supply air to the space. The supply air temperature is held relatively constant: while supply air temperature can be moderately reset depending on the season, it must always be low enough to meet the cooling load in the most demanding zone, and to maintain appropriate humidity. Variable air volume systems can be applied to interior or perimeter zones, with common or separate fans, with common or separate air temperature control, and with or without auxiliary heating devices. The greatest energy saving associated with VAV occurs at the perimeter zones, where variations in solar load and outside temperature allow the supply air quantity to be reduced. If the peak room load is not determined accurately, an oversized VAV system will partially throttle at full load and may become excessively noisy throttling at part loads.
Humidity control is a potential problem with VAV systems. If humidity is critical, as in certain laboratories, process work, etc., systems may have to be limited to constant volume airflow. Particular care should be taken in areas where the sensible heat ratio (ratio of sensible heat to sensible plus latent heat to be removed) is low, such as in conference rooms. In these situations, the minimum set point of the VAV terminal unit can be set at about 50% and reheat added as necessary to keep humidity low during reduced load conditions.
Other measures may also be used to maintain enough air circulation through the room to absorb sufficient moisture to achieve acceptable humidity levels. The human body is more sensitive to elevated air temperatures when there is little air movement. Minimum air circulation can be maintained during reduced load by (1) raising the supply air temperature of the entire system, which increases space humidity, or supplying reheat on a zone-by-zone basis; (2) providing auxiliary heat in each room independent of the air system; (3) using individual zone recirculation and blending varying amounts of supply and room air or supply and ceiling plenum air with fan-powered VAV terminal units, or, if the design permits, at the air-handling unit; (4) recirculating air with a VAV induction unit, or (5) providing a dedicated recirculation fan to increase airflow.
Variable Diffuser. The discharge aperture of this diffuser is educed to keep the discharge velocity relatively constant while reducing the conditioned supply airflow. Under these conditions, the induction effect of the diffuser is kept high, cold air mixes in the space, and the room air distribution pattern is more nearly maintained at reduced loads. These devices are of two basic types—one has a flexible bladder that expands to reduce the aperture, and the other has a diffuser plate that physically moves. Both devices are pressure-dependent, which must be considered in the design of the duct -distribution system. They are either powered by the system or pneumatically or electrically driven.
SINGLE-DUCT – Constant Volume
While maintaining constant airflow, single-duct constant volume systems change the supply air temperature in response to the space load (Figure 9). Single-Zone Systems. The simplest all-air system is a supply unit serving a single-temperature control zone. The unit can be installed either in or remote from the space it serves, and it may operate with or without distribution ductwork. Ideally, this system responds completely to the space needs, and well-designed control systems maintain temperature and humidity closely and efficiently. Single zone systems often involve short ductwork with low pressure drop and thus low fan energy, and single-zone systems can be shut down when not required without affecting the operation of adjacent areas, offering further energy savings. A return or relief fan may be needed, depending on the capacity of the system and whether 100% outdoor air is used for cooling as part of an economizer cycle. Relief fans can be eliminated if provisions are made to relieve over pressurization by other means, such as gravity dampers.
Multiple-Zone Reheat. The multiple-zone reheat system is a modification of the single-zone system. It provides (1) zone or space control for areas of unequal loading; (2) simultaneous heating or cooling of perimeter areas with different exposures; and (3) close tolerance of control for process or comfort applications. As the word reheat implies, heat is added as a secondary simultaneous process to either preconditioned (cooled, humidified, etc.) primary air or recirculated room air. Relatively small low-pressure systems place reheat coils in the ductwork at each zone. More complex designs include high-pressure primary distribution ducts to reduce their size and cost and pressure reduction devices to maintain a constant volume for each reheat zone.
The system uses conditioned air from a central unit generally at a fixed cold air temperature that is low enough to meet the maximum cooling load. Thus, all supply air is always cooled the maximum amount, regardless of the current load. Heat is added to the airstream in each zone to match the cooling capacity to the current load in that zone. The result is very high energy use. However, the supply air temperature from the unit can be varied, with proper control, to reduce the amount of reheat required and the associated energy consumption. Care must be taken to avoid high internal humidity when the temperature of air leaving the cooling coil is permitted to rise during the spring and fall.
When a reheat system heats a space with an exterior exposure in cold weather, the reheat coil must not only replace the heat lost from the space, but also must offset the cooling of the supply air (enough cooling to meet the peak load for the space), further increasing energy consumption compared to other systems. If a constant volume system is oversized, the reheat cost becomes excessive.
Bypass. A variation of the constant volume reheat system is the use of a bypass terminal unit instead of reheat. This system is essentially a constant volume primary system with a VAV secondary system. The quantity of room supply air is varied to match the space load by dumping excess supply air into the return ceiling plenum or return air duct, i.e., by bypassing the room. When bypass terminal units dump to a return air plenum, the return air plenum temperature is reduced and the plenum must be kept at a lower pressure than the room so that cooler air does not spill into the room through the return air grilles. A return fan is often used to create that negative pressure. While this reduces the air volume supplied to the space, the system air volume and fan energy remains constant. Refrigeration or heating at the air-handling unit is reduced due to the lower return air temperature. A bypass system is generally restricted to small installations where a simple method of temperature control is desired, a modest initial cost is desired, and energy conservation is less important.
AHU – Equipment Isolation
Vibration and sound isolation equipment is required for most central fan installations. Standard mountings of fiberglass, ribbed rubber, neoprene mounts, and springs are available for most fans and prefabricated units. The designer must account for seismic restraint requirements for the seismic zone in which the particular project is located. In some applications, the fans may require concrete inertia blocks in addition to non-enclosed spring mountings. Steel springs require sound-absorbing material inserted between the springs and the foundation. Horizontal discharge fans operating at a high static pressure frequently require thrust arrestors. Ductwork connections to fans should be made with fireproof fiber cloth sleeves having considerable slack, but without offset between the fan outlet and rigid duct. Misalignment between the duct and fan outlet can cause turbulence, generate noise, and reduce system efficiency. Electrical and piping connections to vibration-isolated equipment should be made with flexible conduit and flexible connections. Special considerations are required in seismic zones.
Equipment noise transmitted through the ductwork can be reduced by sound-absorbing units, acoustical lining, and other means of attenuation. Sound transmitted through the return and relief ducts should not be overlooked. Acoustical lining sufficient to adequately attenuate any objectionable system noise or locally generated noise should be considered. Chapter 46 of the 1999 ASHRAE Handbook—Applications, Chapter 7 of the 1997 ASHRAE Handbook—Fundamentals, and ASHRAE Standard 68 have further information on sound and vibration control. Noise control, both in the occupied spaces and outside near intake or relief louvers, must be considered. Some local ordinances may limit external noise produced by these devices.
AHU – Outside Air Requirements
common complaint regarding buildings is the lack of outside air. This problem is especially a concern in VAV systems where outside air quantities are established for peak loads and are then reduced in proportion to the air supplied during periods of reduced load. A simple control added to the outside air damper can eliminate this problem and keep the amount of outside air constant, regardless of the operation of the VAV system. However, the need to preheat the outside air must be considered if this control is added.
Another problem is that some codes require as little as 2.4 L/s per person [about 0.25 L/(s·m2)] of outside air. This amount is far too low for a building in which modern construction materials are used. Higher outside air quantities may be required to reduce odors, VOC’s, and potentially dangerous pollutants. ASHRAE Standard 62 provides information on ventilation for acceptable indoor air quality. Air quality (i.e., the control or reduction of contaminants such as volatile organic compounds, formaldehyde from furnishings, and dust) must be reviewed by the engineer.
Heat recovery devices are becoming more popular as the requirements for outside air increase. They are used extensively in research and development facilities and in hospitals and laboratories where HVAC systems supply 100% outside air. Many types are available, and the type of facility usually determines which is most suitable. Many countries with extreme climates provide heat exchangers on outside/relief air, even for private homes. This trend is now appearing in larger commercial buildings worldwide. Heat recovery devices such as air-to-air plate heat exchangers can save energy and reduce the required capacity of primary cooling and heating plants by 20% and more under certain circumstances.
The location of intake and exhaust louvers should be carefully considered; in some jurisdictions, location is governed by codes. Louvers must be separated enough to avoid short-circuiting of air. Furthermore, intake louvers should not be near a potential source of contaminated air such as a boiler stack or hood exhaust. Relief air should also not interfere with other systems. If heat recovery devices are used, intake and exhaust airstreams may need to run together, such as through air-to-air plate heat exchangers.