Chapter 6: Cooling

6.1. Theory and Applications

6.2. Commissioning the Cooling System

6.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparatins and Cautions

Time Required to Test

6.2.2. Design Issues Overview

6.3. Typical Problems

6.3.1. Condensate Drainage Issues

6.3.2. Carry-over Issues

6.3.3. Refrigeration Piping and Specialty Issues

6.3.4. Dehumidification Control

6.3.5. Evaporative Cooling Issues

6.4. Testing Guidance and Sample Test Forms

 

Figures

Figure 6.1: Typical Draw-Through Air-Handling-Unit Drain Trap. 12

Figure 6.2: Draw-Through Air-Handling-Unit Drain Trap Operation. 13

 

6.1. Theory and Applications

Cooling is one of the primary functions provided by air-handling systems. With mild ambient temperatures and a properly configured system, an air handling unit can often provide cooling for a significant portion of its operating cycle using outdoor air in an economizer cycle. This process is the subject of Chapter 3: Economizer and Mixed Air. When outdoor conditions are not suitable to meet the cooling requirements of the load, then other mechanized processes are required. Common approaches to mechanical cooling include:

·       Chilled water or glycol: Refrigeration equipment generates cold water or glycol, which is then pumped to coils located in the air handling units to cool and dehumidify the air stream. There are small packaged air-cooled chillers in addition to the larger chillers common in central chilled water plants serving large buildings, campuses, and industrial sites.

Capacity control is typically achieved by modulating the flow of water through the coil or by bypassing air around the coil. Generally, modulating the water flow is the most desirable approach since it has the potential to save chilled water pumping energy. Where dehumidification requirements lead to the need for reheat, mixed air or return air can bypass around the cooling coil for discharge temperature control while the chilled water coil valve is controlled to provide adequate dehumidification.

·       Direct expansion refrigeration: Refrigerant flows through the evaporator coil in the air handling system, often referred to as the DX coil (short for Direct eXpansion), to cool the air. Compressors and condensers, included as a part of the air handling package or located remotely, move the refrigerant through the piping and coil system and allow the heat absorbed in the air handling unit to be rejected to atmosphere.

Capacity control is achieved on the refrigeration side by an expansion device at the coil coupled with compressor unloading and hot gas bypass. Compressor unloading systems are generally step devices, which limit capacity modulation. At low load conditions, the compressors will cycle and unconditioned air will pass through the system during the off cycle, which can cause problems. Hot gas bypass can be used to maintain compressor operation continuously regardless of load, but this is an energy intensive solution since it maintains an active cooling coil at the expense of false loading the compressor. Face and bypass dampers are sometimes encountered for air-side capacity control, but they must be used with extreme caution since the low air flow rates that occur across the coil when the dampers are in bypass can cause severe problems in the refrigeration circuit.

·       Heat pumps: Heat pumps are a variation on the direct expansion refrigeration approach typically found in package and unitary equipment. When the air handling system requires cooling, the system works like a traditional direct expansion system. But, if the air handling unit requires heat, the system reverses and uses the coil in the air handling unit as a condenser and the coil used to reject heat to the atmosphere as the evaporator. The system cools the outdoors to heat the air stream in the air handling unit. Heat pumps allow electric heat generation for significantly less cost than electric resistance heating.

·       Direct and indirect evaporative cooling: Systems that use the cooling effect associated with evaporating water are often used in environments with relatively low web-bulb temperatures. The direct approach cools the air by spraying water onto a media and allowing it to evaporate into the air stream. The air leaves the process cooler and with a higher specific moisture content. The process lends itself to locations where the ambient wet bulb is below 65-70ºF and with high make-up air requirements since the air is not recirculated.

Indirect evaporative cooling uses a direct evaporative process to cool a secondary air stream, usually the exhaust stream from the area served or an outdoor air stream. This secondary air stream is then used to cool the primary air stream serving the space using a heat exchanger. Some systems combine heat recovery with the indirect evaporative cooling package. The wet economizer process is a special case of the indirect evaporative cooling approach.

There are also processes that combine direct and indirect cooling approaches to yield a supply dry bulb temperature that is 10ºF or more below the wet bulb temperature of the secondary air. Generally, the process uses a direct evaporative cooling in the secondary air stream (usually exhaust from the area served) followed by a heat exchanger, which allows the evaporatively cooled secondary air to indirectly cool the primary air. Then primary air passes through a direct evaporative cooling process.

Evaporative cooling can improve the capacity and reduce the demand of a direct expansion chiller or condensing unit by precooling the condenser air. Evaporative cooling can also be used in series with other refrigeration processes to improve their effectiveness and reduce demand. The evaporative process will add energy from the spray pump motor, the secondary air fan motors, and added air flow resistance, but the reduction in energy requirement in the downstream refrigeration process can often yield a net energy savings.

·       Wet economizers: This process is a special case of the indirect evaporative cooling process. From the air handling unit perspective, the cooling source looks like a chilled water system. But the chilled water is generated via cooling towers or dry coolers. The chilled fluid is then pumped to the loads and serves a traditional chilled water coil. Some systems employ a heat exchanger between the circuit to the cooling towers or dry coolers and the circuit serving the loads. Other systems use the water directly. Advantages of this approach include:

1   The ability to generate chilled water without the operation of a chiller This can be a significant factor in dry environments where evaporative cooling processes are highly effective. It also can be significant for processes facilities like clean rooms where the nature of the load served makes the filtration requirements and pressure control problems associated with handling large variable volumes of outdoor air impractical, thus eliminating traditional economizer approaches.

2   Elimination of the outdoor air and relief duct systems associated with the economizer process Only ventilation make up air and its associated exhaust need to be provided to the air handling system. The same pipes that bring chilled water to the system when it is generated by the chiller plant provide economizer cooling.

3   Elimination of the economizer dampers and its related control systems The cost and complexity associated with a traditional economizer is no small matter as can be seen from Chapter 3: Economizer and Mixed Air.

Disadvantages of wet economizers include:

1   The need to operate cooling towers in sub-freezing weather: Operating cooling towers in subfreezing weather is not a casual undertaking. Using the towers to generate chilled water for the fall, winter and spring months will add a lot of wear to the towers.

2   Additional cooling tower and fan energy: Obviously, the need to operate cooling towers on a year round basis will add operating cost. However, this may be offset to some extent by reductions in fan energy associated with eliminating all but the minimum outdoor air duct and relief system. The average filter pressure drop may also be lower since the filters will last longer and load more slowly when they don’t have to deal with the large volumes of outdoor air associated with a traditional cycle.

3   Additional pumping energy: The other obvious energy burden associated with a wet economizer cycle that is absent in a traditional economizer approach is the need to run pumps to move water to the loads and the cooling towers. Dry coolers will also have spray tree pumps that must operate to achieve the evaporative cooling effect necessary for many of the operating hours.

In some situations, wet economizer pumping and tower fan energy costs may be viewed as an attractive alternative to the control systems, operating issues and the cost and real-estate represented by a traditional economizer system, especially when one central plant can serve multiple air handling systems on a large site.

·       Well water: While becoming less common, there are instances where running well water through the air handling unit cooling coil is used to provide cooling. The advantage is low energy cost, at the cost of water usage and disposal costs. From the air handling unit perspective, the process is nearly identical to a chilled water cooling process although the potential for scale, corrosion, and other problems related to using raw water continuously need to be addressed. Cooling coils constructed with cleanable tubes may be desirable in this application.

·       Recovered cooling energy: It is often possible to recover cooling energy from HVAC processes with high exhaust flow rates. In most cases, it will be necessary to supplement this recovered energy with additional cooling, but the recovery process can make a significant difference in the size of the equipment required and the electrical demand and energy consumption associated with it. Thus it can generate both first cost and operating cost savings.

The ASHRAE Systems and Equipment Handbook contains chapters with detailed information on most of these cooling technologies.

6.2. Commissioning the Cooling System

The following sections present benefits, practical tips, and design issues associated with commissioning the cooling section of an air handler.

6.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements lists practical considerations for functional testing. Key Preparations and Cautions address potential problems that may occur during functional testing and ways to prevent them.

Key Commissioning Test Requirements

The purpose of the cooling system test procedures will vary with the requirements of the system served. Regardless of the approach used, verifying the proper control sequence and integration into the overall air handling unit sequence and terminal equipment sequence is essential. In addition, capacity verification may be required. Tests targeted at verifying that the installed conditions will allow the coil to perform as intended may be more cost effective that tests targeted at documenting absolute capacity.

1 Verify cooling element (coil, heat exchanger, etc.) capacity if required by the specification. Capacity tests results should be evaluated with consideration of the accuracy of the instrumentation and the actual conditions at the time of the test.

2 Verify that the control element range matches the requirements of the control sequence and does not overlap the range of other elements served by the same signal to prevent unintentional simultaneous heating and cooling.

3 Verify proper sequencing of mechanical cooling with the economizer to minimize the potential for cooling and dehumidifying unnecessary volumes of outdoor air.

4 Verify proper sequencing of mechanical cooling with the other heat transfer elements in the air handling system to minimize the potential for simultaneous heating and cooling. Proper sequencing also ensures that there will be no ripple effects associated with inappropriate cooling element control. One example is unnecessary reheat energy triggered by over cooling and over-dehumidifying the supply air stream.

5 Verify the operation and performance of any freeze protection associated with evaporative cooling equipment.

For water/glycol based systems, testing will also:

1 Verify the stroke of the control valve to ensure that it closes completely. Control valve leakage testing should reveal no detectable leakage. Some of the larger globe valve designs, especially balanced double-seated designs, are not capable of complete and total shut-off. Most valves of this type have specifications for maximum leakage tolerances. Valves that are rated bubble tight should be capable of producing no detectable leakage when stroked fully closed.

2 In some instances, flushing and pressure testing the coil may be required.

For evaporative systems, testing will also:

1 Verify the performance of the spray pumps and spray system per design sequence of operations.

2 Verify the evaporative cooling effectiveness.

3 Verify the performance of the water quality control, level control and blow down system.

4 Verify that the performance of the evaporative media and heat exchangers will not be compromised by dirt accumulation, or that filters have been included to avoid such accumulation.

For DX systems, testing will also:

1 Verify the adjustment and performance of the refrigeration control devices such as the expansion valve, the hot gas bypass system, the compressor unloading system, the head pressure regulating system, etc.

a The expansion valve should be capable of maintaining the recommended level of superheat under all load conditions.

b The hot gas bypass control system should only function after the compressor system has unloaded to minimum capacity and there is no other way to maintain an active coil face without cycling the compressor off.

c For systems that must operate at low ambient temperatures, the head pressure control systems should demonstrate that it has been adjusted to ensure proper performance of the compressor and expansion valve.

2 Verify the installation and proper connection of the refrigeration piping to the Dx coil including appropriate accessories like sight glasses, refrigerant dryers, service valves, solenoid valves, and wells for expansion bulbs and testing superheat. Sight glasses should be free of bubbles and of indications of moisture under all load conditions.

3 Verify the evacuation and subsequent charging of the refrigerant circuit. The evacuation test should reveal no significant gain in pressure other than what can be attributed to ambient temperature change after the system has been evacuated.

4 Verify that the pump down cycle (typical on larger systems, but not necessarily provided on small tonnage and fractional tonnage equipment) allows the compressors to evacuate the evaporator coil prior to shut down.

Key Preparations and Cautions

Cautions

1 In most states, a license is required to perform any work that might release refrigerant to the atmosphere. Fines and other penalties can be substantial. Therefore, if you are not certified for this work, you should either become certified or retain a certified technician to work with you for any testing that requires connecting to the refrigeration circuit.

2 Refrigerant that is accidentally discharged from a system can be a health hazard and suitable precautions should be taken when working around refrigerant piping that is under test. Potential problems include allergic reactions, asphyxiation, and freeze burns.

3 When working around evaporative or other water-based cooling systems, suitable precautions should be taken to prevent inhalation and contact with equipment or materials that could contain spores or other disease carriers. Respirators, gloves, and eye protection may be desirable if there is evidence of mold, mildew, algae or other growth. This problem is more likely with existing systems that are being retro-commissioned than with new equipment.

4 For Dx systems, crank case heaters should be activated and verified as recommended by the manufacturer prior to starting any compressors. (Usually, this is required for at least 24 hours prior to start-up.)

5 Testing should proceed in a logical sequence that verifies primary interlocks and safety systems prior to verifying more complex control processes, integrated control functions, and tuning loops.

6 Some test procedures, either by design or by failure of the element under test to perform as intended, can cause air handling system discharge air temperatures to vary significantly above or below normal, which can pose the following problems:

a Occupant discomfort.

b Disruption of the process served by the system and potential damage to product.

c Where heating coils are used to simulate cooling load, they can cause inadvertent activation of fire dampers. See Section 11.4.2: Fire and Smoke Dampers, and Section 11.4.3: Air Hammer, under the discussion on fusible links. They can also cause inadvertent activation of heat detectors and rate of rise detectors and subsequent false fire alarm and building evacuation. Test plans should provide for these contingencies by taking steps such as disabling key fire detection elements for the test and ensuring that fusible links have been selected to tolerate any temperature that can be produced in the system.

7 Overly rapid stroking of valves and dampers during a test process can cause air and water hammer problems in the duct and piping systems serving the cooling element.

8 Functionally testing an electrically driven refrigeration processes element during the winter months for buildings and systems equipped with electric heat can cause several significant problems including:

a Distribution system load conditions that exceed design and switch gear ratings can trigger trips in the primary switchgear resulting in unscheduled and unanticipated outages.

b Demand peaks well in excess of those that would normally be encountered during the normal operation of the building due to the demand that the coil places on the system concurrently with the refrigeration equipment. See the additional discussion on this topic in Chapter 5: Preheat.

9 Overly vigorous adjustment of the superheat on an expansion valve can cause liquid refrigerant to pass through the evaporator without being vaporized and damage the compressor. All superheat adjustments should be made gradually with time allowed for the effect of each adjustment to stabilize and by someone qualified to perform the adjustment.

10 Dx cooling controllers should be carefully inspected to ensure that there is no logic or setting that will rapidly cycle the compressor when the program is activated. Built-in interlocks in the refrigeration equipment should not be relied upon to offer this protection, although many systems include them. At initial start-up, it may be desirable to be prepared to respond to this type of problem to prevent damage to the compressor or its starter.

11 Test sequences that subject the system to large volumes of untreated air should be avoided, especially if the system or area it serves has been subjected to conditions that could have lowered surface temperatures below the dew point of the outdoor air. For example, shutting down the cooling system on a 100% outdoor air unit after performing a capacity test that has cold soaked the duct and the area served by the system without shutting down the fan will most likely cause condensation in the duct and on the surfaces in the area if it is humid outside. Humid might be hot and humid as one might encounter in Florida, or it could be the conditions that exist on a rainy 60ºF day in the Pacific Northwest during the spring or fall. In both cases, the dew point of the outdoor air is probably below the surface temperature in the duct system down stream of the cooling coil and could even be below the surface temperatures in the zone served if the cooling capacity test has been running for a while. The resulting condensation could lead to IAQ problems and damage finishes, materials and supplies.

12 Evaporative cooling processes typically require that more attention be paid to the potential for freezing including taking note of the actual extremes of site temperatures that exceed design conditions.

Test Conditions

1 Some of the tests associated with the cooling source serving the cooling element(s) can be accomplished prior to the completion of the air handling unit and terminal system(s). For example, pressure tests, flushing, some control valve shut-off test processes, make-up and blow down control tests, and cooling source flow tests can all be accomplished without air handling unit and terminal equipment operation.

2 Other tests, like discharge control loop testing and tuning, cooling element capacity control, and capacity testing will require that the air handling system and its terminal equipment be operational and capable of moving the design volume of air, but not necessarily fully under control. Safety systems should be operational to protect the machinery and occupants in the event of a problem during the test sequence.

3 Testing the integrated performance of the cooling element with the rest of the system will require that the individual components of the system be fully tested and ready for integrated testing. In many cases, the building or at least the portion of the building served by the system must be substantially complete and under load.

4 Simulating a real cooling load (both the sensible and latent components) in the field is a practical impossibility, which places a complication on performance tests. In most instances where capacity verification is required, it will be verified based on achieving a target dry bulb and wet bulb temperature depression below the current ambient conditions, or based on documenting coil performance under the given conditions and then modeling the coil under those same conditions. Preheat coils or reheat coils and humidifiers coupled with full recirculation may provide a method to approach simulating a design load condition, but these techniques are not without their difficulties and risks for the system and area served, especially tests that cause the indoor temperatures and relative humidity to be elevated significantly above normal. Limitations to testing include outdoor conditions at the time of test and conditions under which is it possible to operate the in the test mode. It may be possible to combine the cooling capacity test with capacity testing of other elements in the system like the preheat element.

5 As an alternative, it is possible to wait for near design conditions and perform the capacity test at that time. This approach is a more realistic test of the coil and also allows the control functions to be evaluated under more realistic conditions. However, it requires that the test process and instrumentation be prepared in advance and that the test team can respond quickly to reach the site and perform the test before the weather changes. This approach also requires that the load served by the system be able to deal with a potentially disruptive test process with little or no advanced warning. This test routine can be difficult if significant travel time is required to reach the test site.

6 Valve leakage tests and tests that are targeted at verifying valve stroke, spring range, and sequencing should be conducted with the pumping system operating at its peak differential pressure. The differential pressure across the valve plug can have a significant impact on the close-off rating and shift the operating spring range of the valve.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will include the following in addition to the standard tool kit listed in the Functional Testing Basics:

1 Inclined manometers, Magnehelics™, Shortridge Air Data Multimeters™, and other instruments capable of measuring and documenting low air static and velocity pressures. This equipment can also be used to verify flow rates.

2 A stethoscope or similar sound sensitive device can be useful for listening for valve leakage sounds when verifying that the valve is fully closed.

3 For capacity testing, flow measuring equipment capable of measuring the flow of the cooling energy source to the necessary degree of accuracy will be required.

4 Evacuation verification of Dx coils prior to charging may require an absolute mercury manometer.

5 Verification of refrigerant charge requires an accurate scale.

6 Test equipment suitable for verifying water quality may be required to verify the performance of the water quality control system for evaporative cooling equipment.

Time Required to Test

1 Some of the simpler tests like an interlock test or a valve shut-off test can be accomplished in an hour or less with one or two people.

2 More complex tests like a capacity test can require several hours and several team members to set up and monitor all of the necessary functions, especially if multiple operating points are to be evaluated. This test can be complicated by the need to quickly travel to the site with short notice to run a test while ideal test conditions prevail.

3 Tests that require referencing back to a model require some time to either develop or support the development of the model that will be used to evaluate the coil’s performance. If the modeling capability does not exist in-house, then it may be necessary to retain the coil manufacturer’s services if the modeling requirement has not been included in the pricing package.

4 Field-testing to lab or factory standards is expensive and a practical impossibility in many instances.

5 Tests targeted at verifying refrigeration system evacuation prior to charging usually require that the system hold vacuum for a period of time ranging from 8 to 24 hours. If the test fails, then additional time will be required to allow for a repair and then a repeat of the test cycle. This process can be a significant cause of lost productive time and additional travel expense.

 

6.2.2. Design Issues Overview

The Design Issues Overview presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand, even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing.

Do the cooling element design temperatures and configurations ensure adequate dehumidification under all operating modes (if applicable)?

Most cooling systems are intended to perform dehumidification in addition to sensible cooling. The dehumidification provided is highly dependent upon the leaving temperature condition from the cooling element. Misapplied discharge temperature reset sequences can raise supply temperatures to the point where adequate dehumidification is not provided.

This can also occur if face and bypass dampers are used on the coiling coil without providing some means to control the cooling coil discharge temperature and ensure flow through the cooling coil to provide the desired dehumidification.

Dx systems with limited turn down capacity and no hot gas bypass capability can also have problems with dehumidification during the portion of the operating cycle when the compressor is off while the fan remains in operation. It can be particularly troublesome if the compressor is significantly oversized for the load it served, either by design or by the current load condition.

Is the quality of the water supply for the evaporative cooling element suitable?

By their nature, evaporative cooling processes will accumulate the solids and other non-volatile elements inside of the equipment as the water evaporates. Water supplies with mineral content or other contaminates can lead to scaling and other problems that can make maintenance difficult, elevate the risk of harboring microorganisms in the equipment, and shorten equipment life. Special water treatment processes may be required to prevent these problems.

Are filters required for the direct evaporative cooling processes to ensure that the quality of the evaporative media and heat exchangers is not compromised by accumulations of dust washed out of the air stream?

By their nature, direct evaporative coolers will function as air washers, even if they have not been designed for this function. If this has not been provided for in the design of the cooling element, its performance and air flow through the air handling system can be rapidly compromised by an accumulation of dirt in the filter media and on the heat exchanger surfaces. Thus prefilters may be required, including an allowance in the fan system pressure specification to handle the added pressure drop, both clean and dirty.

Has the cooling element been protected from freezing?

Cooling elements that employ water must be protected from freezing. In many environments, this will mean that a freezestat must be provided at a minimum and that the operation of evaporative systems must be locked out so they are not inadvertently operated during freezing or subfreezing weather.

Systems with large outdoor air fractions may require preheat and the decision as to whether or not to provide it should consider both the design conditions as well as the recorded seasonal extremes.

Evaporative cooling systems may also require basin heaters, heat trace on the make-up, blow down and distribution piping, and isolation dampers to close off their inlet from the outdoors when the unit is not operating.

The final step in protecting cooling elements from freezing is to include a well thought out commissioning process for the components that provide the protection.

If the cooling element provides dehumidification, has it been configured to prevent IAQ problems and water damage?

Draining the condensate from dehumidification associated with the cooling process frequently becomes a commissioning and operational problem. Key considerations to design for good drainage include:

1 Drain pans need to extend far enough past the cooling coil to ensure that they collect all of the condensate, including droplets carried off of the coil element under all operating conditions.

2 Tall cooling coils may require intermediate drain pans to prevent excessive carry-over from the lower portions of the cooling element under heavy dehumidification loads. The intermediate pan catches and removes condensate generated by the upper portions of the cooling element so that it does not have to flow over the lower portions of the cooling element to reach a drain pan. Minimizing the water flowing through the lower portions of the coil reduces the potential for carry-over.

3 Drain pans should be constructed to ensure that all accumulated condensate will flow to the drain line connection.

4 Corrosion resistant drain pan construction is highly desirable. Stainless steel has better corrosion resistance and is easier to clean than galvanized metal. Similar considerations also apply to coil frames and other metals use to fabricate cooling elements.

5 Drain pans should be insulated to prevent condensation problems associated with the temperature of the condensate itself.

6 For air handling equipment located above sensitive areas, consider providing a secondary drain pan and/or moisture alarms that will notify the operating staff of any overflow problems with the primary drain pan. A few extra dollars in first cost can have a quick pay back water damage from an overflowing drain pan is avoided.

7 While the drain pan’s primary function is to remove condensate from a dehumidification process, it also provides a measure of protection from water damage in the event of a frozen coil. It may be desirable to take this aspect into consideration during the design and sizing of the drain lines serving the drain pan.

8 If the trap on the condensate drain is intended to provide protection from infiltration of untreated air into the unit (an important consideration on some process applications) then trap primers may be required to keep the traps full when dehumidification is not occurring.

9 Traps on rooftop equipment may be subject to failure due to freezing if not properly located or protected.

 

6.3. Typical Problems

The following problems are frequently encountered with cooling elements.

6.3.1. Condensate Drainage Issues

Most cooling processes dehumidify the air, which can generate considerable mounts of condensate. Thus, cooling elements that dehumidify have drain pans that drain this condensate away from the cooling element to prevent indoor air quality problems and water damage to the unit and the area located below the cooling coil. The design issues that lead to poor draining of condensate, a typical problem found during testing, are described below.

Since there is generally a pressure difference between the interior and the exterior of the unit, draining the condensate is not as convenient as simply making a hole in the bottom of the drain pan. Generally, a trap is required to ensure consistent drainage of condensate from the drain pan. Without the water seal provided by the trap, air flowing through the drain line into the unit can interfere with the drainage process and cause the drain pan to overflow, especially when there is a significant pressure difference between the inside and outside of the unit casing at the cooling element location.

Figure 6.1 illustrates the features of a typical trap applied to a draw-thru unit where the pressure at the cooling element location generally will be lower than the pressure outside of the casing.

Figure 6.2 illustrates how this trap operates. Similar principles apply to traps on blow-through cooling elements except that the pressure at the cooling element will generally be higher than the pressure outside the unit and the location of the trap outlet relative to the connection to the drain pan needs to be adjusted accordingly.

Figure 6.1: Typical Draw-Through Air-Handling-Unit Drain Trap

 

Figure 6.2: Draw-Through Air-Handling-Unit Drain Trap Operation

Overflowing drain pans lead to a variety of short term and long term problems including:

·       Water damage to the unit and the areas surrounding it.

·       IAQ problems associated with standing water in undrained portions of the unit casing and moisture entrapment in the casing insulation materials.

·       IAQ and water damage problems associated with water carried through the fan and blown into the discharge duct system.

At the beginning of the dehumidification cycle, the trap will be dry and reverse flow through the trap could potentially interfere with drainage until the trap becomes primed with condensate. In most instances, this will not be a problem because the dehumidification loads tend to be non-instantaneous in nature (for example: a light load is initially encountered which allows the trap to prime despite reverse flow and without backing-up condensate in the drain pan.)

If the trap is dry, then unfiltered, untreated air can flow into the unit in draw-through applications. In most commercial situations, this relatively small volume of untreated air is not a concern. But, the contamination introduced thought this pathway can cause contamination on systems with large drain outlets serving sensitive processes like a clean room. If the trap must provide a seal in addition to ensuring consistent drainage, then the design needs to be arranged to keep the traps primed and protect them from freezing.

6.3.2. Carry-over Issues

When a cooling element is dehumidifying, it will tend to have a coating of water on the surfaces in contact with the source of cooling. Typically, these surfaces will also have air flowing past them. As a result, water droplets tend to be blown off of the element. The smaller droplets will evaporate and the larger droplets will fall from the air stream in a short distance. The condensate drain pan extends beyond the cooling element location to catch these drops as they fall out of the air stream. Where space constraints prevent an appropriate condensate drain pan extension, eliminators can be installed downstream of the coil to provide a surface to intercept and remove the droplets from the air stream. Using eliminators adds pressure drop and thus an energy penalty.

If the accumulation of water on the surface becomes too heavy or the airflow velocity becomes too high, water can be carried past the drain pan or even blown back off of the eliminators, resulting in water damage and IAQ problems. Conditions that can lead to carry-over include:

·       Excessive coil face velocities Generally, the rule of thumb is to keep coil face velocities below 450 to 500 fpm both to prevent carry-over as well as to minimize the energy burden associated with the coil pressure drop.

·       Excessive dehumidification load This condition can be created by coil entering conditions that are in excess of design or by operating the coil with a refrigerant temperature lower than design (usually reflected by chilled water that is colder than the design condition or a saturated evaporator pressure lower than design).

·       Poor condensate removal If condensate is allowed to accumulate on the coil surface due to an inadequately arranged drainage system, then carry-over can occur. This typically happens with tall coils that are not provided with intermediate drain pans, if the drain lines from the intermediate pans become clogged or if the drain pans are not draining quickly enough.

6.3.3. Refrigeration Piping and Specialty Issues

Unit erected refrigeration systems often encounter problems that are related to poorly configured and installed refrigeration-piping circuits. Often the design documents do not provide an adequate level of detail to describe the specialties required and the refrigerant related piping details necessary to ensure reliable performance. In addition to causing problems with cooling capacity, poorly executed piping systems can lead to compressor failures. Items to consider include:

·       Expansion valve, distributor and capillary tube selection.

·       Proper expansion valve equalizer line connection.

·       Hot gas bypass selection and control.

·       Liquid line solenoid valve selection and control.

·       Pressure and temperature port requirements for proper set-up and adjustment.

·       Service valve and isolation valve requirements to minimize refrigerant loss during procedures that open the piping circuit.

·       Subcooling requirements.

·       Head pressure control during low ambient operation.

·       Suction line traps on systems with significant elevation changes between the evaporator and condenser.

·       Double suction riser design for systems that can vary capacity via unloading systems.

·       Refrigerant dryer selection and provisions for service.

Whether factory or field installed, there are specialty devices in most refrigeration systems that require some attention during start-up.

·       Proper superheat adjustment.

·       Proper hot gas bypass adjustment.

·       Proper head pressure regulator adjustment.

·       Proper charge.

·       Refrigerant system cleanliness and freedom from moisture.

·       Adequate provisions for monitoring and checking all of these parameters.

The ASHRAE Refrigeration Handbook as well as the Trane Company’s Refrigeration Handbook are excellent resources for further detail.

6.3.4. Dehumidification Control

The energy and comfort implications of inadequate humidity control are significant for commercial office buildings. Producing very cold conditions off the cooling element will dehumidify the air, whether it is required or not. Examples include low temperature air applications and efforts to provide additional sensible cooling due to the inability to meet the current load. The extra dehumidification causes the process to use more energy than is necessary. There is a direct energy impact due to the extra moisture that is condensed as well as an indirect energy impact, since the higher condensate burden will also increase the coil pressure drop. Supplying low temperature air will often result in a building with a relative humidity 10 to 30% lower than required, which works against the low temperature air fan energy savings. Carry-over of this extra condensate can also become a problem.

Processes that result in elevated leaving air temperatures can provide less dehumidification than is necessary. For example, reset schedules may elevate the discharge air temperature in an effort to save energy without limits for humidity control. Additionally, processes that control the cooling element directly from space temperature can lose control of humidity, especially when coupled with high outdoor air loads. These examples can become particularly troublesome in humid environments where the sensible load in the area served is relatively low while the outdoor air requirements are relatively high. At a minimum, the lack of dehumidification can cause comfort and product quality problems, but can frequently lead to IAQ problems in the system and in the building envelope.

6.3.5. Evaporative Cooling Issues

Evaporative cooling processes are inherently air washing processes. If inadequate filtration is provided, the media on an evaporative cooling unit that has not been designed to function as an air washer can quickly become fouled, leading to cooling and airflow capacity problems as well the potential for IAQ concerns. Most direct evaporative coolers rely on the sump water level to provide a seal at the bottom of the evaporative cooling section. If the water level is not properly controlled, air can bypass the cooling process by flowing under the media through the sump, resulting in loss of performance. If the water level is allowed to remain in continuous contact with the media, problems with slime and algae can occur.

Regardless of the exact approach used, maintenance is an important consideration for any evaporative cooling process. Issues to consider include:

·       Inspection requirements Usually evaporative equipment will require more frequent inspection at the air handling unit location as compared to a Dx or chilled water cooling coil. Most of this effort will be targeted at the water distribution equipment and ensuring it is thoroughly wetting the media, that the system is generally free of scale and debris, and that the water level control system is functioning properly. Water levels that are too high can cause problems with the wetted media and water levels that are too low can cause air to short-circuit around the evaporative cooling section.

·       Scaling The high evaporation rate associated with this process can easily lead to scale problems. Scale needs to be detected and removed from the system when it shows up to ensure good performance, equipment longevity, and minimize the potential for micro-biological growth, particularly Legionnaire’s Disease. Spray nozzle performance and thus system performance can be significantly degraded by scale or erosion at the nozzle.

·       Bleed rate, make-up, and general water quality Proper adjustment of these parameters is essential to prevent problems with scale and performance. Water treatment cannot be as freely applied in direct evaporative cooling systems since the water and air stream are in intimate contact. Thus algae and scale control are far more dependent on the quality of the water supply, indirect controls, and inspection. This can be particularly important in controlling the potential for Legionnaire’s Disease. Any water treatment chemicals that are use must be registered for use with evaporative coolers with an appropriate agency such as the US EPA.

6.4. Testing Guidance and Sample Test Forms

Click the button below to access all publicly-available prefunctional checklists, functional test procedures, and test guidance documents referenced in the Testing Guidance and Sample Test Forms table of the Air Handler system module.

AHU Testing Guidance and Sample Test Forms