By Kooltronics

Forced Ventilation Air Cooling
In clean, non-hazardous environments with acceptable ambient temperatures, a simple forced-air cooling system utilizing ambient air is usually adequate. Combined with a low-cost air filter, such devices generally meet the heat removal needs of typical electronic and electrical equipment (Fig. 1).

FIGURE 1

Closed-Loop Cooling

In harsh environments involving high temperatures, heavy particulates, oil, or chemicals capable of damaging components, ambient air must be kept out of the enclosure. Sealed enclosures are generally used, with closed-loop cooling consisting of two separate circulation systems in a single unit. One system, sealed against the ambient air, cools and recirculates the clean cool air throughout the enclosure. The second system uses ambient air or water to remove and discharge the heat.
 

A comprehensive discussion of Closed-Loop Cooling appears later in this Design Guide.

Many applications using sophisticated electronic/electrical components require a closed-loop cooling system to dissipate heat buildup without introducing outside contaminated air. Closed-loop cooling is required when  equipment  is  operated in hostile environments containing dirt, oil, humidity or corrosives, which adversely affect the performance or ultimate survival of the components. The presence of airborne particulate matter compounds the difficulty  of  controlling the temperature of the equipment in the enclosure.

HEAT EXCHANGERS

For installations that can operate at above-ambient temperatures, Heat Exchangers provide moderate-cost closed-loop cooling. Available in both air-to-air and water-to-air versions, there are models covering a wide range of cabinet sizes and performance capacities. Depending upon the model selected and the heat load, near-ambient to moderately-above-ambient temperatures can be achieved.

For applications that can utilize Heat Exchangers, the advantages compared with Air Conditioners include:

• Lower initial cost
• Lower power consumption
• Simpler construction
• Fewer operating components
• Lighter weight

Advanced air-to-air Heat Exchanger designs for cooling enclosures include two types of heat transfer methods. One design consists of a finned-tube coil which contains liquid refrigerant. The warm air exhausted from the equipment cabinet to the Heat Exchanger is directed past the coil, causing the refrigerant to boil and absorb heat. The resultant refrigerant vapor rises to the upper portion of the tubes, where the heat is removed by the cooler ambient air and the refrigerant condenses back to liquid, completing the cooling cycle in a continuous process.

The most recent developments in enclosure Heat Exchanger design employ high-efficiency heat transfer elements fabricated of embossed convoluted metal foil or thin-film polymer material, constructed into two totally separate air paths. The air leaving the hot enclosure is directed through one side of the exchanger, where the heat passes through the element walls into the ambient-side airstream and is dissipated.

Figure 3 illustrates heat transfer in air-to-air Heat Exchanger applications.

WATER-TO-AIR HEAT EXCHANGERS

If ambient air cannot be utilized directly as a cooling medium, another cost-effective method of cooling is a water-to-air system (Fig.4). Water is used to remove heat from air circulated within the electronic or electrical enclosure.

Cooling water is circulated through a finned-tube coil, which is installed in a compartment isolated from the enclosure to protect the contents from possible leakage of water. As the heat-laden air circulates through the coil, the heat is absorbed by the water and carried away, in a continuous process.

Water-to-air systems are easy to install and usually require minimum maintenance. The water used must be reasonably clean and cold enough to ensure proper operation of the cooling system under the most severe anticipated conditions.

In cases where sufficiently cold water is available, below ambient-temperature cooling can be achieved.

Air Conditioners are required where the equipment operating temperature must be kept at or lower than the ambient room temperature, and/or the cabinet must be sealed from oil, dust, fumes and other contaminants.

Specially packaged vapor compression Air Conditioners (Fig. 5) protect the components and furnish the required cooling. Such Air Condition-ers employ hermetic refrigeration systems with additional controls. They provide enclosure and air-path geometries for direct installation to the equipment cabinet and accomplish the following:

1. Isolate the interior of the equipment enclosure from ambient conditions
2. Cool the air within the enclosure to the optimum temperature for the sensitive components
3. Circulate the air within the enclosure to equalize temperature and increase heat transfer from hot components
4. Automatically vary cooling rate to maintain close control of equipment temperature
5. Reduce humidity harmful to sensitive components

In addition, the field experience of many compressor manufacturers has indicated that the frequent start/stop cycling, typical of standard Air Conditioner operation, shortens compressor life.

These factors have led to the develop-meant of techniques for close control of internal temperature over a wide range of ambient conditions, without turning the refrigeration compressor on and off and without employing electrically-controlled solenoid valves.

All temperature control for KOOLTRONIC Air Conditioners is accomplished by means of proportioning valves activated by pressure signals within the Air Conditioner hermetic system. Generally, load temperature can be controlled to ±5°F over a range of power dissipation, without the use of thermostats. Frequent start/stop cycling of the compressor is eliminated, thus assuring longer compressor life.

Recent developments in temperature requirements for enclosed components has led to the addition of adjustable Low Temperature Control thermostats in all KOOLTRONIC Air Conditioners to prevent over-cooling. EMI/RFI suppressors are included to control the line transients associated with compressor cycling and thermostat operation.

Hermetic Air Conditioners are available in air-cooled and water-cooled models.

AIR-COOLED AIR CONDITIONERS

Heat removed from the enclosure containing hot components is discharged by circulating the ambient air through the condenser coil and returning the heated air to the ambient. There are vertical and horizontal types for exterior mounting to side panels, doors, or the top of the enclosures, and for various internal positions.

WATER-COOLED AIR CONDITIONERS

Intended primarily for extreme operating conditions of high-ambient temperatures or severe contaminants, these units utilize water as the medium for heat dissipation. The heat is absorbed by cool water circulating through a coaxial condenser coil, following which the heat-laden water is discharged or recalculated after cooling

Heat is removed by enclosure Air Conditioners and discharged by means of the vapor compressor refrigeration cycle. This takes place in a hermetically sealed system, utilizing either an air-cooled or water- cooled condenser heat exchanger. A schematic ofa typical Air Conditioner is presented in Figure 6.

The compressor forces refrigerant, in vapor form, into the condenser heat exchanger, where it is cooled either by ambient air, driven between the fins by a specially engineered blower, or by cool circulating water. As it cools, the refrigerant condenses into liquid, which is passed through a filter to remove impurities and excess moisture. The liquid refrigerant flow is metered by a thermostatic expansion valve or capillary tube, to control its admission to the evaporator.

The thermostatic expansion valve is a feedback control device which regulates refrigerant flow on the basis of conditions sensed at the evaporator outlet. The capillary tube is a long, small-diameter tube which employs pressure differential and complex two-phase internal flow dynamics to meter refrigerant to the evaporator.

The refrigerant enters the evaporator as a liquid beginning to vaporize. As the blower-driven heated air from the enclosure passes through the evaporator heat exchanger, the refrigerant absorbs the heat and the conversion to vapor is completed. Any moisture present in the air is removed by condensation. This cool, dehumidified air is then returned to the cabinet.

After the heat absorption phase, the refrigerant passes into an accumulator, where any remaining vaporized liquid is separated. The gas then returns to the compressor, to repeat the cycle in a continuous process.

The total heat rejected at the condenser is all of the heat entering the system from the enclosure at the evaporator heat exchanger, heat of compression and heat rejected by the compressor motor, since the compressor is cooled by the refrigerant gas returning from the evaporator.

Typical air temperatures during the cooling process are: cooled air to enclosure, 75°F; heated air returning from enclosure, 100°F; ambient air cooling condenser,105°F; air rejected from condenser, 140°F.

TEMPERATURE CONTROL

Typical refrigeration and Air Conditioning systems control temperature by on/off compressor cyclingas air temperatures fluctuate between minimum and maximum thermostat settings. Compressor start-up often introduces substantial transients into the circuit powering the equipment to be cooled. Thermostat or relay operation results in electromagnetic inter- ference. Both of these factors can adversely affect the function of electronic equipment. On/off compressor control necessitates choosing between large temperature excursions or frequent compressor cycling.

Furthermore, frequent start/stop operation exposes internal compressor components to electrical and mechanical strains not encountered during continuous operation. The use of electrical controls to handle high compressor start-currents results in eventual erosion of the control contacts themselves.

In order to eliminate the possibility of these problems, KOOLTRONIC Air Conditioners feature a continuously operating compressor and non-electric proportional control system, which result in more stable equipment temperatures and prolonged life for the compressor and the control system. Both blowers and the compressor start simultaneously with the application of power to the unit, and continue to operate until power 
is removed at the time of equipment shutdown.

The control bypass valve, shown in Figure 6, permits refrigerant, in gas phase, to be injected at the inlet to the evaporator heat exchanger. This high-temperature, high-energy gas presents an artificial heat load and permits the effective cooling rate to be varied as necessary to maintain essentially constant temperature of the air returned to the enclosure. (It is normally factory set to deliver air at approximately 70°F.) This control also prevents evaporator freeze-upsdur-ing periods of low heat load or low ambient temperature.

Although the above control system works effectively at most times, there are instances of over-cooling due to low heat load or low ambient temperature. In order to prevent that condition, Low Temperature Control thermostats and EMI/RFI suppressors have been added to all KOOLTRONIC Air Conditioners.

When activated, the Low Temperature Control shuts off the compressor and condenser (ambient side) blowers. The evaporator (enclosure side) blowers continue to circulate the air through the enclosure and Air Conditioner. When the air temperature again reaches the level at which cooling is needed, the compressor and condenser blowers résumé operation.

AMBIENT TEMPERATURE RANGE

Most KOOLTRONIC Air Conditioners are designed to operate at ambient temperatures ranging from 50°F to131°F. Optional Low Ambient Kits allow operation in ambient temperatures as low as 0°F. 

Maximum operating ambient temperature decreases linearly with altitude at the rate of 3°F per1,000 feet between 2,500 and 7,500 feet, where maximum operating ambient temperature is 110°F. The ability to operate at high ambient temperatures permits KOOLTRONIC Air Conditioners to be installed in close proximity to furnaces and other heat-producing equipment.

For applications in ambienttem-peratures higher than the rated 
maximum, consultation with the KOOLTRONIC Engineering Depart-ment often provides the solution.

CONDENSATION

High ambient relative humidity does not affect the rated capacities of KOOLTRONIC Air Conditioners. They're designed for installation on reasonably tight enclosures of relatively limited internal volume.

Normally, only sensible heat loads are imposed on the Air Conditioner. Even at an ambient temperature of 95°F and a relative humidity of 100%, the air within a typical electronic equipment enclosure 21/2 feet square and 6 feet high will contain only small amount of water in vapor form. As the temperature of the air being circulated within the enclosure is reduced from 95°F to 70°F, theater will be condensed quickly in the evaporator heat exchanger and be disposed of through the drain in the condensate tray at the bottom of their Conditioner.

Unless the enclosure is totally sealed, some slow invasion of ambient air will take place through cracks and seams in the cabinetry and the front panels. However, even at ambient relative humidifies of 100%, the infiltration rate is normally so small that the effect on cooling capacity of latent heat of water vapor condensation in the infiltrating air
is negligible.

Cooling performance of the Air Conditioner is reduced if its capacity is used for the condensation of excessive moisture. This occurs if the enclosure is poorly sealed oris open for long periods, under high humidity conditions. A continuous flow of condensate denotes that these adverse conditions are present and should be remedied immediately.

In forced convection cooling of enclosures, cooler ambient air is drawn or forced through the components in an enclosure and discharged.  When electronic/electrical enclosures are sealed to keep out moisture, dust, dirt and other contaminants, the heat generated by the components is trapped and closed-loop cooling (air conditioner or heat exchanger) is needed to maintain the optimum environment for the components.

The heat generated by the components is the INTERNAL HEAT LOAD.  Because of the internal heat being generated, there is typically a temperature differential between the inside and the ambient air outside the enclosure.  Heat is conducted through the exposed area of the enclosure from the warmer to the cooler space, and is known as the HEAT LOAD TRANSFER.  Since Air Conditioners cool to below ambient, the Heat Load Transfer is a heat gain into the enclosure, whereas it is a heat loss out of the enclosure when Air-to-Air Heat Exchangers are used because they do not cool to below the ambient.  

When sizing an Air Conditioner or Heat Exchanger using the methods on the following pages, if the Internal Heat Load (in Watts or BTU/Hr.) is known, it is the value used in Step One.  When it is not known, there are several methods of estimating, including (1) Component Rated Heat Production;  (2) Air Flow Heat Rise;  (3) Sealed Enclosure Temperature;  (4) Incoming-Outgoing Power; and (5) Component Efficiency.  Of these, the most accurate and least time-consuming is Method (3), which is the method we recommend for uninsulated enclosures.  

The following steps should be followed:

(A) The Enclosure must be operating at its highest work load.
(B) All openings are sealed and air-moving equipment turned off.
(C) Measure the inside air temperature near the top of the Enclosure.
(D) Measure the ambient air temperature and determine the differential.
(E) Calculate the exposed surface area of the Enclosure (sides and top) in square feet.
(F) Refer to the Temperature Differential Heat Gain/Loss Chart, at the temperature differential determined in (D) and multiply the Watts/Square Foot by the exposed area (E).  This is the approximate Internal Heat Load in Watts.  For BTU/Hr, multiply by 3.413.

Air Conditioners for cooling electronic/electrical enclosures should be selected to provide adequate cooling for the anticipated worst-case conditions.  For best results, Panel-Mounted Air Conditioners should have the cool air inlet at or below the lowest significant heat-producing component and the return air port should be above the highest such component.

The capacity (BTU/Hr) should be approximately equal to the Internal Heat Load plus the Heat Load Transfer (see Sizing Closed-Loop Cooling Equipment), but should not be over-sized, as this will result in excessive cooling which may affect component performance or cause “sweating” of the Enclosure during high humidity conditions. Normally, no additional provision is needed for the humidity in the Enclosure at start-up because it will be dissipated very quickly from a properly-sealed Enclosure.

A determination should be made of the appropriate mounting position—side panel, door, internal or top.  Although most KOOLTRONIC Air Conditioners are equipped with Condensate Evaporators (or are available as an option) and all have condensate drains, it is preferable to not place an Air Conditioner directly above electronic/ electrical components if the application is likely to be subject to high humidity, which will infiltrate an imperfectly sealed cabinet.

The maximum allowable internal cabinet temperature (Ti) should not exceed the heat tolerance specification of the most sensitive component in your system.

NOTE:  This selection process applies only to indoor gasketed enclosures which are uninsulated. Before proceeding, read Sizing Closed-Loop Cooling Equipment.  For applications involving outdoor locations, solar load, non-metallic or insulated enclosures, consult KOOLTRONIC for assistance. 

The model numbers of KOOLTRONIC Air Conditioners reflect the refrigerant, as of the publication date. A 4 between the A and C signifies R134a; a 3 signifies R22. The Technical Data shown is as determined by testing for UL certification. As more R134a-type compressors become available, R22 units will be redesigned for R134a which may affect performance.
 

The proper selection of a Heat Exchanger is determined by how many WATTS can be dissipated at a given temperature differential. Since a Heat Exchanger uses ambient air or available water for cooling, the enclosure cannot be cooled below a temperature slightly above that of the air or water entering the Heat Exchanger. Therefore, the greater the temperature differential, the higher the capacity of the Heat Exchanger. Consequently, the smaller the temperature differential required in an application, the higher the capacity of the Heat Exchanger needed to achieve it.

The amount of heat to be dissipated is the Internal Heat Load less the Heat Load Transfer (heat conducted through the uninsulated walls of the enclosure).

The performance of Heat Exchangers is expressed in WATTS/°F.  In the preferred “Air In” method, the Watts/°F depicts the amount of heat the Heat Exchanger is capable of dissipating per degree Fahrenheit of temperature differential between the entering air or water temperature and the maximum allowable internal enclosure temperature.  The calculation process requires (a) the amount of heat to be dissipated, (b) entering air or water temperature, (c) maximum allowable internal enclosure temperature and (d) the dimensions of the enclosure.

The maximum allowable internal cabinet temperature (Ti) should not exceed the heat tolerance specification of the most sensitive component in your system. 

NOTE: This selection process applies only to indoor gasketed enclosures which are uninsulated.  Before proceeding, read Sizing Closed-Loop Cooling Equipment. For applications involving outdoor locations, solar load, non-metallic or insulated enclosures, consult KOOLTRONIC for assistance.
 

This is the rating (Watts/°F Air In) based on the temperature of the air into the heat exchanger, after exiting the  hottest area of the enclosure.  Refer to the Heat Exchanger Index and Selection Guide for the unit that most  nearly matches the required performance and dimensions.

Another rating, (Watts/°F Air Out) is based on the lower temperature of the cool air as it exits the heat exchanger into the enclosure.  This rating may be used if the coolest air can be directed for spot cooling of components in the portion of the enclosure where the air enters from the heat exchanger.