MD 15116 - 2006 Computer Room Air-conditioning Systems
Appendix A - Supplementary Material
This appendix provides explanatory material that should be of assistance in using this guideline. It is intended to help explain the rationale behind some of the design requirements that are prescribed in this guideline.
This supplementary information may also describe alternative methods of meeting the design requirements, together with discussions of the advantages and disadvantages of each method.
A2.0 Air Distribution
A2.1 Supply Air
Raised floors may be used as distribution plenums and are particularly suited where computer equipment is to be supplied with cooling air directly.
Supply air that enters the computer equipment directly from below the raised floor has the advantage of reducing turbulence and air movement in the computer room and thus contributing to worker comfort. However, the temperature and relative humidity of this supply air is critical.
A2.1.1 Supply Air Outlets in Raised Floor
- Perforated floor panels - these have high induction rates and consequently may be located close to computer equipment without the danger of unmixed supply air entering the computer equipment air inlet, and with less discomfort to the operator.
There is a wide divergence of opinion among raised floor manufacturers regarding free area of perforated floor panels.
Some perforated floor panels may be equipped with volume control dampers.
- Floor diffusers and floor registers - these have better directional control and longer throw and are normally equipped with volume control dampers, but may be too drafty for operators working nearby, and are also not mounted completely flush with the raised floor. For these reasons they are usually located away from cart traffic or occupied areas.
A2.2 Return Air
Using the ceiling space as a return air plenum has several advantages:
- Return air grilles or small canopy hoods can be installed directly above heat-generating computer equipment and the heat generated can be captured almost directly into the return-air stream.
- A portion of the heat generated by the lights can also be captured directly.
A3.0 The Necessity for "Dry" Coil Operation
A3.1 "Dry" Coil Operation
Due to the extreme Canadian climate, computer rooms are generally located in interior building spaces and equipped with vapour barriers to maintain high humidity levels throughout the year. Due to low occupancy, the ventilation requirements are minimal. Hence the latent heat gain is very low.
Consequently the cooling load is almost 100% sensible heat and dehumidifica-tion is not normally required. It is, however, important that cooling coil capacity should provide "dry" cooling coil operation to prevent unnecessary dehumidification, and that simultaneous dehumidification and humidification should not occur.
The reasons for these requirements can be illustrated by reference to Example 1 below. See Figure A1.
Example 1: Dry and Wet Coil operation
Room temperature: 24°C
Room humidity: 50% RH
Supply air temperature: 16 deg.C
Bypass flow around cooling coil: 3.5%
Temperature rise between cooling coil discharge air and room supply air: 1.5deg.C
Sensible Heat Factor (SHF): 1.0 i.e. there are no latent heat gains or losses from the computer room to the surrounding rooms.
Total dbt. Difference = ((24 - (16 - 1.5))/0.965= 9.85 °C.
Dbt. of supply air leaving cooling coil = 24 - 9.85 =14.15 °C.
Dry Coil Operation
Refer to the psychometric chart in Figure A1 that depicts "dry" coil operation. Air enters the cooling coil at condition R and leaves the coil at condition C. It mixes with bypass air at condition R to form a mixture at condition M. There is a temperature rise of 1.5 deg.C between the coil and the entry into the room, as the air picks up energy from the fan heat, the cables, and heat transfer through the floor slab. Hence it enters the room at condition S.
All of these processes occur at a constant Sensible Heat Ratio of 1.0, therefore the psychometric process follows a horizontal line R-S-M-C.
Wet Coil Operation
Consider the processes that occur if there is wet coil operation - i.e. if condensation occurs at the coil, as shown in Figure A2. Since there is condensation, the psychometric process no longer follows a horizontal line, and the air leaves the coil at condition C1. There is a change in enthalpy between conditions C and C1, depicted as LHc in Figure A2.
Air is now supplied at condition S1, and it has to return to condition R during the cooling process. Hence, the psychometric process follows the horizontal line (at constant SHF), and then an enthalpy change is required to return to condition R from R1. This is shown as LHh in the Figure A2.
Thus, it is seen that two enthalpy changes are required, LHc and LHh, so there is, in effect, a double penalty on the cooling system.
Operation of Cooling Coil with Moisture Loss from Computer Room
Because moisture is lost from a computer room when there is a breakdown in the vapour barrier, or when there is air infiltration into the space, the sensible heat factor becomes greater than 1.0, and the humidifier in the self-contained air-conditioning unit operates to replace this loss.
In order to maintain the computer room at design conditions under these circumstances, the moisture picked up by the return air must be removed by dehumidification as it passes through the cooling coil. This may necessitate some reheat.
Note that the cooling coil, although designed to operate as a "dry" coil will, under these circumstances, operate as a "wet" coil.
Consequently, operation of the reheat coil may be interpreted as a sign of excessive humidification. The visual alarm should alert the operating personnel of this condition.
A4.0 Types of Air-conditioning systems
A4.1 Self-contained Refrigerant A/C units
Multiple self-contained refrigeration units may be installed within the computer room. This type of system has the following advantages:
- Excellent performance.
- Inherent flexibility in matching cooling capacity to cooling load requirements.
- If one unit breaks down, the other units can take over the cooling load, ensuring reliable, fail-safe operation
- The A/C units may be located close to heat sources, resulting in higher cooling efficiency
- Good security, as the equipment is located in a secure area.
A4.2 Split Refrigerant Cooled A/C unit (DX System)
In this type of system, the condensers are remotely located with refrigerant piping to the main air conditioning unit and the cooling coils. This type of system is useful when the distances between the elements of the air- conditioning system are not too large.
One advantage of this type of system is that a heat recovery device can be used to recover heat from the condenser.
A4.3 Split A/C System Using Glycol or Chilled Water
This system is used when the distances between condenser and compressor are too great for the use of refrigerant cooled unit. This system is relatively maintenance free, when compared to a DX system.
Chilled water may be used as the circulating fluid when freeze protection is not required. However, when there is exposure to freezing conditions, a glycol loop should be used. Heat recovery devices can readily be incorporated into these systems.
Since cooling loads are primarily sensible heat, the chilled fluid temperature should be maintained as high as possible without compromising the performance of the system.
Fig. A3 shows one possible arrangement, with a heat recovery device. The dry cooler heat exchanger is provided for free cooling and energy conservation. The cooling coil controls should include 3-way valves to ensure adequate flow through the chiller at all times.
A4.4 Chilled Water-cooled Computer Room A/C Unit
Water-cooled A/C units may also be used, using chilled water from a central chiller plant. System should be closed system, use distilled water (or other water treatment - usually as specified by the computer equipment manufacturer), water/water heat exchanger and be designed to operate all year round, 24 hours per day. The use of City water for cooling is not recommended.
A5.0 Heat Recovery Methods
Several methods of heat recovery are possible, depending upon the type of building and the potential for using the recovered heat. Some of the possibilities are illustrated below.
A5.2 Arrangement #1: Heat Recovery for Domestic Water
With this arrangement, the recovered heat is utilized for heating domestic water. See Fig.A4.
A5.3 Arrangement #2: Economizer/Free Cooling
Another possibility for energy conservation for use with glycol or chilled water chillers is shown in Fig.A3, which illustrates the use of an additional dry cooler heat exchanger for periods when free cooling by outside air is possible.
A5.4 Arrangement #3: Using Additional Cooling Coil
Another possibility is to divert the condenser fluid from the refrigerant condenser to an additional cooling coil in the A/C unit. This is illustrated in Fig. A5.
Fig. A1 Psychrometric Process for Computer Room A/C Units with Dry Coil Operation, Appendix A, Section A3.1
Long description of figure A1 is available on a separate page.
Fig. A2 Psychrometric Process for Computer Room A/C Unit with Wet Coil Operation, Appendix A, Section A3.1
Long description of figure A2 is available on a separate page.
Fig. A3 Chilled Water or Glycol-cooled Computer Room A/C Units, Appendix A, Section A.4.3
Long description of figure A3 is available on a separate page.
Fig. A4 Heat Recovery System for use with Glycol-cooled Condensers, Appendix A Section A5.0
Long description of figure A4 is available on a separate page.
Fig. A5 Energy Conservation – Additional Cooling Coil to Utilize Condenser Fluid, Appendix A, Section A 5.4
Long description of figure A5 is available on a separate page.
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