Evaporative Air-Conditioning phần 7 docx

Cooling Comfort Heating/cooling rooftop makeup air units provide summertime cooling to commercial kitchens through direct evaporative air-conditioning.. Where temperatures are exceedingl

Commercial Kitchen Evaporative Air-Conditioning

Makeup air, or exhaust air replacement, is typically needed in food service facilities-by building and health codes, if for no other reason-and EAC can be used to good effect in meeting these requirements Proper kitchen ventilation in particular has safety implications and requires a well-designed makeup air system In concert, EAC and makeup air systems can improve the comfort, safety, and efficiency of commercial kitch-ens while affording energy savings, improving worker morale, and providing customers better comfort

Cooling Comfort

Heating/cooling rooftop makeup air units provide summertime cooling to commercial kitchens through direct evaporative air-conditioning These systems are ideally suited to commercial kitchens because they provide 100 percent fresh air ventilation and cooling, requiring exhaust for proper application, rather than recirculation of air With its large summer cooling potential of 15°C or more, this system can increase employee comfort substantially In many areas of the world, direct evaporatively conditioned air can be introduced through a makeup air unit into the kitchen at between 5 and 10°C below outdoor dry-bulb temperatures or lower depending on ambient conditions

Cost Savings

Although VAC in kitchens is relatively common, it is certainly not the most efficient approach to comfort, because most of the air introduced through the VAC system is immediately exhausted by the stove hood before it can recirculate Thus, constantly running fresh air through a VAC substantially increases the load on the unit and requires purchasing refrigerated air-conditioning capacity well beyond what would

be required for the same internal load with a recirculating system In addition, a typical rooftop refriger-ated-heated makeup air unit would be oversized on the heat mode as well and would increase the initial capital cost By contrast, an EAC unit could supply kitchens with makeup air without water circulation through the pads when cooling was not needed

Sizing

Engineers generally size the makeup air unit to match a predetermined amount of air being exhausted by the hood ventilator selected Proper selection of a makeup air unit of the correct size (the quantity of air that can be delivered by a given unit at the necessary introduction temperature) is the key decision pa-rameter Temperature rise and air delivery performance vary with different units The amount of permis-sible temperature rise necessary in a given application is determined by the winter design conditions for the area in question and by the desired indoor temperature level

Laundry and Dry Cleaning

Evaporative air-conditioning in laundry and dry-cleaning operations cross a variety of ambient air-condi-tions Evaporative air-conditioning systems perform important cooling and ventilating funcair-condi-tions Evapora-tive air-conditioning may provide features that give the laundry operator greater cost-effecEvapora-tive versatility

in the interior environment as compared to vapor compression air-conditioning and ventilation alone

Extreme Heat Conditions

The heat levels of laundry and dry-cleaning plants are high ln addition to the solar gain and human heat load occurring in a typical (140 to 900 square meter) facility, the equipment used generates a large amount

of heat Heat levels can become unbearably high for finishing (ironing) personnel in particular Workers

are exposed to heated areas from steam above 80°C Every plant has a boiler to provide steam for finish-ing textiles These units in larger plants are rated as high as 5.3 to 10.5 million KJ/hour (5 to 10 million BTUh) Although boilers are walled off, at least part of their heat is added directly to the heat load of the plant Other equipment that contributes to the heat environment includes washers and tumblers, and in dry cleaning plants some of the solvent reclamation equipment

EACs can introduce air approximating the ambient wet-bulb temperature for cooling Some combi-nation of spot and area EACs can reduce heat stress conditions to acceptable levels Where temperatures are exceedingly high, such as at workstations near large boilers, shielding can be used in combination with spot cooling to reduce radiant heat Properly directed evaporatively cooled air washing over hot surfaces can reduce radiant heat as well The climate will determine whether the heat load or the ventila-tion load will require the larger capability In moderate climates, the ventilaventila-tion load may be greater than the airflow required for cooling; the opposite may be true in hot climates

Industrial Applications

Evaporative air-conditioners and packaged heat/cool ventilating makeup air units combine many fea-tures which make them applicable in diverse industrial applications EAC systems can be the most eco-nomical approach to comfort, increased equipment efficiency, and code compliance under a variety of conditions The uses of EACs in heat treating, forging, casting, welding, milling, rolling finishing, clean-ing and assemblies is also due to safety and comfort concerns for employees and government regulations pertaining to employee welfare

Direct EAC is often used to combat problems such as:

* Intense heat, as in many forging, foundry and casting areas

* Low humidity, as in computer and electronic control rooms

* Airborne contaminants, as in welding, plating and cleaning areas

The hot, stagnant conditions present in many casting and furnace rooms may reach indoor dry-bulb

temperatures that exceed outdoor temperatures from 100 to 15'C, and may reach nearly 70°C in some

extremely hot casting areas Molten steel at 1,300'C, must be cast by workers In many basic metal plants where casting, annealing, forging, baking, and drying occur, shutdowns and work slowdowns are typi-cal during summer months In some factories work reductions and shift stoppages are required if tem-peratures climb too high Some plants curtail operations or even close during the peak summer cooling periods The reasons may be union contracts, walk-outs under hot conditions, and so on Government regulations vary in different countries, but many countries have legally enforceable occupational heat/ stress standards Even without government regulations, many industries attempt to relieve the problem

of heat in their plants out of basic practicality Using evaporative air-conditioning in the plant environ-ment can help prevent mandatory and spontaneous work reductions

Spot or area cooling with EAC systems, combined with other heat-control methods such as radiant heat shielding, can effectively meet government heat/stress standards EAC can increase summer-time productivity by as much as 40 to 60 percent in plant areas too large to air-condition with vapor-compression systems

EAC is used in industries including aluminum production and fabrication; electronics assembly; power generation; shop work such as welding, plating and milling; metal fabrication and battery manufacturing The benefits of cooling hot workers remain essentially the same in most applications: increases work effi-ciency, lengthened work seasons, lowered costs, code compliance and increases equipment efficiency

Factory Air-Conditioning Design Considerations

Spot cooling is used in industrial areas where people are in close proximity to high-heat processes and cooling of the entire installation is either inefficient or extremely expensive Spot cooling with EAC

provides two principal benefits: evaporatively cooled air and air movement The effect of air motion, air temperature and humidity must be combined to derive an index to worker comfort (i.e., effective temperature) Spot cooling essentially isolates the worker from the immediate hot environment by displacing hot air with a stream of cooled air Thus, the effect on the worker is determined by the outlet air temperature and velocity Calculations for spot cooling are based on the amount of air being deliv-ered, the heat rise and static pressure in the duct, and the size of the outlet All of these variables will determine the amount of comfort or relief felt by the worker

Ventilation Control

Makeup air systems, in addition to providing space cooling, help in ventilating high-fume areas and in reducing airborne contaminants in accordance with government standards Makeup air is typically re-quired by regulation in the design of areas containing plating tanks and paint-spray booths, for example, and it is highly recommended in all areas with industrial exhaust Makeup air is highly recommended in all areas requiring industrial exhaust "Air starved" buildings may not be able to provide sufficient flow to operate the hoods, spray booths, and appliances properly Inadequate makeup air will cause drastic reduc-tions of efficiency that will affect propeller fans and natural drafts; in some instances the flow may be reversed In addition, some contaminated areas require a large supply-air flow to the dilution of airborne contaminants with or without associated exhaust-for example, areas using industrial solvents When year-round air supply is needed, heating-cooling-ventilation makeup air systems or EACs are often specified

Equipment Protection

To last and run efficiently over their design lifetime, many pieces of equipment require appropriate cooling Again the use of EAC is a valid option, certainly when the use of vapor-compression cooling is too expensive, humidity ratio is not the limiting factor, and ventilation alone is not enough For ex-ample, considerable waste heat is generated in power generation equipment Whether the power is generated by gas or steam turbines (run on fossil fuel, nuclear energy, or even solar power), the tem-peratures of the installations in which the turbines run must be kept under 41°C, which is the maximum operating condition for the windings in common alternators EAC may be used to keep room tempera-tures lower than 41°C allow generators to operate at overload outputs; the general rule is that approxi-mately 6 percent overload capacity is available for every 4.5°C below rated ambient temperature (usu-ally 41°C) achieved by cooling Other types of equipment also operate more efficiently under cooler conditions Electric motors, particularly high-horsepower units (200 HP and larger), can require direct cooling of the windings for proper operation

Agricultural Applications-Poultry

Indoor confinement of agricultural livestock is a growing trend worldwide because it yields higher qual-ity and improved productivqual-ity The comfort and well-being of indoor livestock is also becoming of para-mount importance from an ethical point of view Environmental control of livestock housing such as poultry has become an increasingly critical technology in which EAC plays an important part

One of the most commnon areas for applying EAC is in poultry houses Of the farm buildings commonly found in the poultry business, evaporative air-conditioning improves conditions in four major types: the broiler house, the hatchery, the laying house and the processing plant (see Figure 8.1 for an overview)

Better Growth Rates and Feed Conversion Proper evaporative air-conditioning of broiler houses

allows birds to achieve a weight gain of from 5 to 8 percent with a corresponding cut in the growth period

of 2 to 8 percent Closely related to poultry growth rates is the factor of feed conversion

Reduced Mortality Rates Improved Hatch Rates, Increased Egg-Laying Rates and Egg Size High

temperatures-37°C and above-will kill poultry EAC has been found to decrease poultry production

Figure 8.1 Typical Evaporative Air-Conditioning Applicationfor Poultry Houses

Source: ECI.

mortality rates by 35 percent or greater The use of environmental control with EAC in the hatcheries has been shown to improve hatch rates from 3 to 10 percent A commercial egg-laying house, or egg ranch, depends in part on lay rate-the number of eggs laid per hen per day-for its profitability Appropriate EAC systems have reportedly improved overall poultry egg-laying rates by as much as 15 percent in-creased overall quality, and boosted average egg size from 5 to 6 percent

Improved Conditions for Workers EAC in poultry houses improves the life and comfort of the

birds, as well as their overall productivity, but it also improves conditions for people working in these houses during the summertime heat In addition, EAC provides ventilation and other benefits of particu-lar value in the poultry environment

Greenhouses

Excessive summertime temperatures can reduce plant growth and, if high enough, can kill the plants Temperatures above 29°C constitute a danger to the health and growth of many greenhouse plants, and sustained temperatures above 35°C are a serious threat to most types of plant life EAC provides signifi-cantly lower indoor air temperatures that enhance plant viability, reduce mortality, improve plant size and increase weight

Fan and Pad versus Evaporative Air-Conditioners

Evaporative air-conditioning is used in horticulture, floriculture and other high-productivity greenhouse agricultural systems, where the environmental conditions are critical for production Basically two sys-tems, with different areas of application, are used Fan and pad systems have one air inlet into the green-house, where the wetted medium is installed and ventilating equipment on the far wall of the building (see Figure 8.2) This approach causes a significant temperature gradient from the inlet side to the venti-lation side of the greenhouse because of the heat the air picks up as it travels the length of the greenhouse The other system uses external packaged coolers that maintain a positive pressure in the greenhouse They are installed outside the greenhouse and blow humidified air through many polyethylene ducts into the greenhouse (see Figure 8.3) The system maintains a constant overpressure inside the green-house, with exhaust air leaving the greenhouse at an exit louver This approach supplies an even tem-perature gradient in the greenhouse, since the cooled supply air is delivered through the poly ducts throughout the greenhouse before it picks up additional heat from the greenhouse This approach creates

a more uniform growing environment inside the greenhouse than does the fan and pad system

Figure 8.2 Evaporative Cooling Pad Section of Rigid Cellulose Pads

I-=-Pads shown are along west wall of New Mexico State University grower greenhouse maintained by SWTDI in Las Cruces, New Mexico.

Source; ECIL

Additional Crops

EAC can allow an extra growing season for greenhouse crops where summers normally would be too hot, thus increasing annual yields Where shading is normally required to lower indoor temperatures during summer, it may be reduced or eliminated, depending on the crop, when EAC is used This can

Figure 8.3 External Evaporative Air-Conditioners on a Research Greenhouse, New Mexico State University,

Las Cruces, New Mexico

f.

-

Source: ECI.

further increase plant yields Increasing the velocity of air movement permits shade requiring plants to

be grown at higher than recommended light levels without reducing plant quality

Uniformity among individual plants in a crop is enhanced by EAC Variation among individual plants

in a crop is reduced by lower summer temperatures This effect benefits commercial growers attempting to increase control over the produce they sell Both size and the date of harvesting of a crop are more uniform The former is a benefit in business planning, simplifying pricing, while the latter affords the grower more control over the seasonality of the crop, allowing better matching of target dates and deadlines

Within limits, high relative humidity (RH) is good for plants; however, RH decreases as temperature rises EAC provides two benefits to plants under these conditions The cooling reduces heat stress on a plant, thereby reducing the need for its own "evaporative air-conditioning"-that is, transpiration by its leaves The increased RH means greater saturation of the air surrounding the leaf, inhibiting the vapor-ization of water from the leaf itself

Because of the critical nature of temperature in the maintenance of healthy plants, cooling systems that fail to maintain conditions necessary to the health and development threaten the success of a greenhouse

Fine Tuning Greenhouse Environments with Evaporative Air-Conditioning

In warm climates, EAC is useful to ensure that heat-sensitive plants are maintained within safe limits Cooling may be necessary for plant survival In milder climates, however, EAC may be used for special greenhouse applications If a greenhouse that is oriented to low-temperature-preferring plants is desired, EAC can be an essential component, no matter the outdoor climate To maintain such planned environ-ments, EACs can be operated at night or on cool days For example, air that is 27°C and 30 percent RH can

be cooled to 19°C through EAC Even at night-when cool-house plants require 7° to 13°C tempera-tures-an EAC either with or without the pump operating can help maintain proper conditions

Introduction to Evaporative Cooling

Let us first look at the principle of evaporative air- In Figures A1.2 and A1.3, the effect of EAC is

conditioning, which can be explained by way of psy- explained In the cases shown the DB temperature

chrometric charts These charts present the moisture is 40°C, and the air is not saturated with water,

ratio versus the temperature, as it is registered by a RH is only some 15 percent One can add water,

normal thermometer (the dry-bulb temperature; and the temperature will drop until it reaches the

DB), in a certain situation The lines in Figure Al.1 saturation line What happens is that the heat in

connect the points with the same relative humidity the air is "absorbed" by the evaporating water The

RH At a given temperature air can contain an sensible heat is transformed into latent heat The

ef-amount of water vapor When this ef-amount of wa- fect is that the temperature is lowered to 20°C in

ter vapor reaches its maximum (100 percent RH), this case The difference between 400 and 20° is

the dew point is reached and the water starts to con- called the wet-bulb depression The wet-bulb

tem-dense The dew points are connected by the satura- perature, or WB, is registered by a normal

ther-tion line The higher the temperature in a certain mometer that is wrapped in a wet sock or other

volume, the more water it can contain in its gas- piece of textile The constantly evaporating water

eous stage In a desert this effect can explain why from the sock causes a drop of the temperature

in the morning, when the temperature is still low, The WB temperature is always lower than the DB

one sometirnes sees drops of water on the scarce temperature, except when the RH is 100

percent-vegetation In the afternoon, when the temperature that is, when the air contains the maximum amount

reaches its maximum, the air feels very dry and hot of water it can hold at a given temperature The

T1he same amount of water is in the air in terms of WB depression line connects the points with the

kilograms of water per kilogram of dry air, but the same enthalpy-the same amount of energy-the

feeling it gives is completely different sum of latent and sensible heat

Figure Al.l Psychrometric Chart and Saturation Figure A1.2 Complete Psychrometric Chart

Line

L < :3I' * gi{ 020 25aDW Point Temperature

1

Source: The Munters Corporation Source: The Munters Corporation.

53

Figure A.3 Wet-Bulb Depression of Ambient Air Figure A1.4 Saturation Effectivenessfor an 80

Percent Effective Evaporative Cooling Pad

5 0J s 15 lc 25 30X 20d 40 40 5 s la 2 t; 1 23 3 2 25~~~~~ 30 30/ 351 3 fi,C 43 A 5 / 5

Dwy-5BdbThTh, C Dl-mmd Tcn~ab sC

Source: The Munters Corporation Source: The Munters Corporation.

Figure A1.4 explains the cooling effect of an The outgoing air thus has a DB temperature of 40°C EAC When water and dry air are mixed in an EAC, - 16°C = 24°C

the air will cool down following the WB depres- Figure A1.5 explains what happens in different sion line The efficiency of a certain EAC defines situations when an EAC is used The arrows point thie degree of cooling In this case, the dry air of 40°C at certain combinations of DB temperature and RH

is led through a pad of corrugated paper, which is It is clear that not all these situations are "comfort-constantly wetted with water The appliance has an able." Only certain combinations of DB and RH are efficiency of 80 percent, which means it cools the actually sensed as comfortable by human beings, airwith O.80OXWB depressionof40°C-20°C = 16°C and this limits the use of the EAC technology for

F'igure A1.5 Saturation Effectiveness of 80 Percent Figure A1.6 Effect of Indirect Evaporative Cooling for Evaporative Cooling Pads at Different Ambient on Ambient Airstream

Conditions

tiuiai;~~~~~~~~~~~~~~~~~~~~~~~.U Lhgig,= EbcdtRs

Note: All are 80 percent effective; in drier conditions the Source: The Munters Corporation.

cooling effect is more pronounced than in hu ecid regions.

Source: The Munters Corporation.

Figure A1.7 Effect of Combined Indirect Evapora- Figure A1.8 Energy-Saving Effect of Using a tive Cooling Coupled with Direct Section Smaller Coil Coupled with Indirect and Direct

Evaporative Cooling Sections

Note: This allows evaporative conmfort cooling to be applied in Source: The Munters Corporation.

more huumid regions, as compared wit- direct EAC alone.

Source: The Munters Corporation.

certain applications In some cases, hovwever, such the effect that the direct cooling follows another

as greenhouses or cattle sheds, the RH can be in- (lower) WB depression line In this way the end of creased without any problem the arrow comes into the "comfortable zone" again EAC technicians have succeeded in reducing Figure A1.8 shows the effect of adding a cooling the amounts of water that are in the air, so as to coil after an indirect evaporative cooling section (us-increase the possibilities for applying EAC in hu- ing the indirect as a precooler) This allows for a mid areas also Figures A1.6 ard Al 7 explain the smnaller-sized coil to be used, thus saving energy over effect in an indirect-direct air-conditioner Before conventional systems A direct section can be added entering the wetting air stream, the air is first after this as well if needed Other systems reduce the cooled by a normal heat exchanger, in which wa- amounts of water by using desiccants, chemicals that ter and air are also mixed In this case, the DB tem- can remove the water from the air stream before it perature of the entering air is reduced from 37.7'C enters the direct cooler With these types of combined

to 26.1'C by the heat exchanger After this, the processes, EAC can be made more efficient in more

26.1 'C air is cooled in the direct cooler to a DB of humid areas The price of these coolers increases with 19.6 C The indirect cooler in the beginning has the more complicated technology