Refrigeration and Air Conditioning 3 E Part 11 pptx

294 Refrigeration and Air-Conditioning Impurities may be classified by size: Pollens 9–80 µ m Mould spores 3–50 µ m Fine ash 0.7–60 µ m Bacteria 1–10 µ m Tobacco smoke 0.1–7 µ m Viruses up to 0.1 µ m Filtration apparatus is available to remove any size, but the very fine particles require a deep, bulky and expensive filter, which itself sets up a high resistance to air flow and therefore requires high fan power. A practical balance must be reached to satisfy the requirements: 1. To remove a high proportion of impurities in the air 2. To hold a large weight of dust before having to be cleaned or replaced, so as to reduce the frequency of maintenance to an acceptable level (i.e. if maintenance is required too frequently, it may be neglected) 3. The filter must be cleanable or reasonably cheap to replace A high proportion of the weight of dust and fluff in the air is in large particles and so is fairly easy to trap. Filters for general air- conditioning duty comprise a felt of glass or other fibres, used in a dry state and termed ‘impingement filters’. Air passage through the fibres is turbulent, and dust particles strike the fibres and adhere to them. The filter material may be flat, but is more usually corrugated, so as to present a large surface area within a given face area. A typical filter in a comfort air-conditioning system is 50 mm deep and may collect 95% or more of the impurities in the air, down to a size of 1 µ m. Increased dust-holding capacity can be obtained by making the filter material in a series of bags, which are normally about 400 mm deep, but also made up to 900 mm where maximum retaining capacity is required. Some bag filters are shown in Figure 27.15. Finer filtration is possible, down to 0.01 µ m. Such filter elements are only used when the process demands this high standard. These fine filters would clog quickly with normal-size impurities, so they usually have a coarser filter upstream, to take out the larger dusts. They are about 300 mm deep, and require special mounting frames so that dirty air cannot escape around the edges. Very fine particles such as smokes can be caught by electrostatic precipitation. A high voltage is applied to plates or wires within the filter bank, to impart a static charge to dirt particles. These will then be attracted to earthed plates, and adhere to them. Impurities are generally cleaned off the plates by removing the stack and washing. Air movement 295 Electrostatic filters will not arrest large particles, and need to be backed up by coarser impingement filters for this purpose. As a filter element collects dust, the air resistance through it will rise, to a point where the system air flow is impaired. Users need to have an objective indication of this limit, and all filters except those on small package units should be fitted with manometers (see Figure 27.2). On installation, marks should be set to indicate ‘clean’ and ‘dirty’ resistance pressure levels. Dry impingement filters cannot be effectively cleaned and will usually be replaced when dirty. Thin filters of this type are used on some package air-conditioners and much of the dirt can be dislodged by shaking, or with a vacuum cleaner. The problem of air filtration on small packaged units is the low fan power available and the possible neglect of maintenance. Since users will be reluctant to buy new filters when needed, some form of cleanable filter is employed. One such type is a plastic foam. Where replaceable filters are used, it is good practice to always have a complete spare set ready to insert, and to order another set when these are used. This avoids the inevitable delay which will occur if new filters are not ordered until the need is urgent. Air filters are not used on cold store coolers, since the air should be a lot cleaner and small amounts of dust will be washed off the fins by condensate or by melted frost. Air-cooled condensers are not fitted with filters, since experience shows that they would never be maintained properly. In dusty areas, condensers should be selected with wide fin spacing, so that they can be cleaned easily. Figure 27.15 Bag filters (Courtesy of Camfil Ltd) 296 Refrigeration and Air-Conditioning 27.12 Cleanliness and cleaning of ducting Filters in air-conditioning systems do not remove all the dirt from the air, and this will settle on duct walls. There is an increasing awareness that ducting systems can harbour a great deal of dirt, and that this dirt will hold bacteria, condensed oils such as cooking fats and nicotine, fungi and other contaminants. Where ducting cannot be stripped down for cleaning, it is strongly advisable to leave frequent access holes for inspection and cleaning. Some guidance on this subject will be available from HVCA [57] in 1989. 28 Air-conditioning methods 28.1 Requirement The cooling load of an air-conditioned space comprises estimates of the sensible and latent heat gains, and is Q S + Q L . This rate of heat flow is to be removed by a cooling medium which may be air, water, brine or refrigerant, or a combination of two of these. (See Figure 28.1.) Example 28.1: All air A space is to be held at 21°C dry bulb and 50% saturation, and has an internal load of 14 kW sensible and 1.5 kW latent heat gain. The inlet grille system is suitable for an inlet air temperature of 12°C. What are the inlet air conditions and the mass air flow? Inlet air temperature = 12.0°C Air temperature rise through room, 21 – 12 = 9.0 K Air flow for sensible heat, 14 9 1.02× = 1.525 kg/s Moisture content of room air, 21°C, 50% = 0.007 857 kg/kg Moisture to pick up, 1.5 2440 1.525× = 0.000 403 Moisture content of entering air = 0.007 454 From tables [4], this gives about 85% saturation. Note that the figure of 1.02 in the third line is a general figure for the specific heat capacity of moist air, commonly used in such calculations. (The true figure for this particular example is slightly higher.) The figure of 2440 for the latent heat is, again, a general quantity in common use, and is near enough for these calculations. 298 Refrigeration and Air-Conditioning Example 28.2: Chilled water For the same duty, a chilled water fan coil unit is fitted within the space. Water enters at 5°C and leaves at 10.5°C. The fan motor takes 0.9 kW. What water flow is required? Total cooling load, 14.0 + 1.5 + 0.9 = 16.4 kW Mass water flow, 16.4 4.19 (10.5 – 5)× = 0.71 kg/s Example 28.3: Refrigerant For the same duty, liquid R.22 enters the expansion valve at 33°C, evaporates at 5°C, and leaves the cooler at 9°C. Fan power is 0.9 kW. What mass flow of refrigerant is required? Cooling medium in Cooling medium out (a) 0.025 0.020 0.015 0.010 0.005 Moisture content (kg/kg) (dry air) Specific enthalpy (kJ/kg) 80 60 0 10 20 30 40 Dry bulb temperature (°C) 24 20 15 10 0 20 40 Wet bulb temperature (°C) (sling) Q L Q S Air-conditioned space Q S sensible cooling load Q L latent cooling load 0 Figure 28.1 Removal of sensible and latent heat from conditioned space. (a) Flow of cooling medium. (b) Process line Air-conditioning methods 299 Total load, as Example 27.2 = 16.4 kW Enthalpy of R.22, evaporated at 5°C, superheated to 9°C = 309.39 kJ/kg Enthalpy of liquid R.22 at 33°C = 139.84 kJ/kg Refrigerating effect = 169.55 kJ/kg Required refrigerant mass flow, 16.4 169.55 = 0.097 kg/s Example 28.4: Primary air and chilled water For the same application, primary air reaches induction units at the rate of 0.4 kg/s and at conditions of 13°C dry bulb and 72% saturation. Chilled water enters the coils at 12°C and leaves at 16°C. What will be the room condition and how much water will be used? The chilled water enters higher than the room dew point temperature, so any latent heat must be removed by the primary air, and this may result in a higher indoor condition to remove the design latent load: Moisture in primary air, 13°C DB, 72% sat. = 0.006 744 kg/kg Moisture removed, 1.5 2440 0.4× = 0.001 537 kg/kg Moisture in room air will rise to = 0.008 281 kg/kg which corresponds to a room condition of 21°C dry bulb, 53% saturation. Sensible heat removed by primary air, 0.4 × 1.02 × (21 – 13) = 3.26 kW Heat to be removed by water, 14.0 – 3.26 = 10.74 kW Mass water flow, 10.74 4.19 (16 – 12)× = 0.64 kg/s 28.2 Air-conditioning and comfort cooling The removal of heat within an enclosed space must be considered as a multi-step heat transfer process. Heat passes from the occupants or equipment to the air within the space, and from there to the refrigerant or chilled water. It follows that the temperature differences at each step are a reciprocal function of the air mass flow. Where there is a high latent heat load within the space, the relative humidity will also vary with the air flow – the variation being higher with low air flow. 300 Refrigeration and Air-Conditioning Further unintended variations will occur with the flow of the primary cooling medium. With two-step (on–off) control of the compressor within an air-conditioning unit, the temperature will slowly rise while the compressor is ‘off’ until the compressor re- starts. The design engineer must consider the effect of such variations on the load within the space. This governs the selection of the cooling apparatus and method of control. A wide variation of equipment is available and the engineer needs to be aware of the characteristics and correct application of each. Close control of conditions may require diversion of the main air flow, see Figure 28.10, or moving human operatives outside the sensitive area. Coolant flow control should be modulating or infinitely variable, where possible. Where conditions can be allowed to drift, within the general limits of human comfort, see Figure 23.8, or any similar zone which is acceptable to a majority of the occupants. Such standards of air- conditioning are generally termed comfort cooling. 28.3 Central station system. All air The centralization of all plant away from the conditioned space, originating from considerations of safety, also ensures the best access for operation and maintenance and the least transmission of noise. Since all air passes through the plantroom, it is possible to arrange for any proportion of outside air up to 100%. This may be required for some applications, and the option of more outside air for other duties will reduce the refrigeration load in cold weather. For example, in the systems considered in Section 28.1, there may still be a cooling load required when the ambient is down to 12°C dry bulb, but this is the design supply air temperature, so all cooling can be done with ambient air and no mechanical refrigeration. The distribution of air over a zone presupposes that the sensible and latent heat loads are reasonably constant over the zone (see Figure 28.2). As soon as large variations exist, it is necessary to provide air cold enough to satisfy the greatest load, and re-heat the air for other areas. Where a central plant serves a number of separate rooms and floors, this resolves into a system with re-heat coils in each zone branch duct (see Figure 28.3). It will be recognized that this is wasteful of energy and can, in the extreme, require a re-heat load almost as high as the cooling load. To make the central air system more economical for multizone installations, the quantity of cooled air to the individual zones can be made variable, and reduced when the cooling load is less. This Air-conditioning methods 301 HC Supply fan Heating coil Cooling coil Air filter Extract fan Figure 28.2 All-air system will also reduce the amount of re-heat needed. This re-heat can be by means of a coil, as before, or by blending with a variable quantity of warmed air, supplied through a second duct system (see Figures 28.4 and 28.6). In the first of these methods, the reduction in air mass flow is limited by considerations of distribution velocities within the rooms, so at light load more air may need to be used, together with more re-heat, to keep air speeds up. Within this constraint, any proportion of sensible and latent heat can be satisfied, to attain correct room conditions. However, full humidity control would be very wasteful in energy and a simple thermostatic control is preferred. Figure 28.3 Re-heat for individual zones C H H H TT T 302 Refrigeration and Air-Conditioning C H T Figure 28.4 Variable air flow with re-heat to individual zones Figure 28.5 Zone differences with re-heat 0.025 0.020 0.015 0.010 0.005 0 10 13 18 20 21 30 40 Dry bulb temperature (°C) 20 15 10 5 0 Wet bulb temperature (°C) (sling) Re-heat b c a 80 60 40 20 0 Specific enthalpy (kJ/kg) Load ratio 0.7 A B Moisture content (kg/kg) (dry air) d Example 28.5 A room is to be maintained at 21°C, with a preferred 50% saturation, using air at 13°C dry bulb, 78% saturation and re- heat. The load is 0.7 sensible/total ratio. (See Figure 28.5.) Air at the supply condition can be re-heated to about 18°C and will rise from 18°C to 21°C in the room, picking up the quantity of HH TT Air-conditioning methods 303 heat ‘B’ as shown. The final condition will be 50% saturation, as required (line abc). Alternatively, supply air is used directly, without re-heat. It now picks up the quantity of heat ‘A’ (about three times as much) and only one-third the amount of air is needed. The final condition will be about 55% saturation. This is still well within comfort conditions, and should be acceptable (line ad). With this variable volume method, the cold-air supply system will be required to deliver less air into the building during colder weather and must be capable of this degree of ‘turn-down’. Below 30% of design flow it may be necessary to spill air back to the return duct, with loss of energy and, in some types, cold air in the ceiling void when trying to heat the room. If the final throttling is at the inlet grille, the reduction in grille area will give a higher outlet velocity, which will help to keep up the room circulation, even at lower mass flow. One type releases the room air in pulses, to stimulate room circulation. The dual-duct system, having the second method of heating by blending cold and warm air, has reached a considerable degree of sophistication, normally being accommodated within the false ceiling and having cold and warm air ducts supplying a mixing chamber and thence through ceiling grilles or slots into the zone (see Figure 28.6). The blending of cold and warm air will be thermostatically controlled, so that the humidity in each zone must be allowed to float, being lowest in the zones with the highest sensible heat ratio. Example 28.6 A dual-duct system supplies air at 14°C dry bulb, 75% saturation through one duct, and at 25°C dry bulb, 45% H C Mixing Mixing Mixing box box box TTT Figure 28.6 Dual duct supplying separate zones [...]... be heated in winter, the fluid must be a non-freeze solution 3 Heat pipes between the two ducts These comprise a coil made with closed pipes, filled with a volatile liquid This liquid will condense in one coil and evaporate in the other, at the same pressure and therefore at the same temperature All these methods will transfer heat in either direction, so providing heat recovery in summer and winter... specialized refrigerant Similar high-temperature dehumidification has been used in the drying of other fibrous materials and ceramics 30 Heat pumps Heat recovery 30 .1 The heat pump If the flow of refrigerant in a cooling system is reversed, the heat exchanger which was the evaporator becomes the condenser and vice versa (see Figures 9.4b and 28 .11) and the flow of energy is also reversed Reversing valves may... ‘white’, i .e with no discrete frequencies, and they are comparatively easy to attenuate Where machinery of any type is mounted within or close to the conditioned area, discrete frequencies will be set up and some knowledge of their pattern will be required before acoustic treatment can be specified Manufacturers are now well aware of problems to the user and should be able to supply this basic data and. .. energy which has been generated within a system Methods of recovery may be passive or active Mechanical heat recovery systems will generally be found under the description of Heat Pumps Heat pumps Heat recovery 32 3 The first step is energy conservation, which is the subject of Chapter 34 Recovery of rejected or wasted heat requires a careful analysis of the heat flow within the systems under survey... All devices using air should be protected by filters, or they will choke with dirt and become ineffective Heat exchangers for liquids will be double pipe, shell -and- tube and plates (see Figures 6 .3, 6.4 and 17.1) Waste fluids may be contaminated by the process, and heat exchangers for such fluids must be cleanable, and kept clean All heat exchange equipment should be fitted with indicating thermometers... rooms 3 Hot discharge gas from a refrigeration circuit can be used to heat water (see Figure 6.5) 4 Condenser heat can be diverted into a building, for heating in winter (see Figure 20 .3) 30 .3 Apparatus and methods Passive heat exchanger equipment for air heat exchange is described in Section 26.4 and shown in Figure 26.5 This can only be used where the ducts are adjacent Other methods are: 1 The rotating... higher temperature There is no reversal of the refrigerant flow, so the selection and design of the system can be optimized for the duty Some heat pumps, such as the dehumidifier, serve the double purpose of a useful load both on the evaporator and condenser side Again, there is no reversal of the refrigerant flow in operation so the components may be selected without compromise Applications can be classified... are made in wall-mounted form for perimeters or ceilingmounted form to cover open areas (See Figure 28.8.) Larger units may be free-standing Two methods are used to circulate the room air over the chilled water coil In the first, an electric fan draws in the air, through a filter, and then passes it over the coil before returning it to the space The fan may be before or after the coil The fresh air. .. the order of 0.7 with entering air at 50% saturation, and will give indoor conditions nearer 45% saturation if used in temperature climates with less latent load (see Chapter 35 ) Winter heating items fitted within room air- conditioners may be electric resistance elements, hot water or steam coils, or reverse cycle (heat pump) One model of water-cooled unit operates with a condenser water temperature... use the condenser heat to the best advantage 29.4 High-temperature dehumidifiers The kilning, or accelerated drying, of newly cut timber requires higher temperatures than will usually be found in refrigeration and air- conditioning systems Typically, the air will be above 50°C and may be up to 80°C Condensing temperatures of 85–90°C require R 134 a in open compressors, or the use of a more specialized . reduce the frequency of maintenance to an acceptable level (i .e. if maintenance is required too frequently, it may be neglected) 3. The filter must be cleanable or reasonably cheap to replace A. available to remove any size, but the very fine particles require a deep, bulky and expensive filter, which itself sets up a high resistance to air flow and therefore requires high fan power. A. a dry state and termed ‘impingement filters’. Air passage through the fibres is turbulent, and dust particles strike the fibres and adhere to them. The filter material may be flat, but is more usually