Document gbMOdYzoXj3MVv2rj4YkBwq7L

234 CHAPTER 19 1962 Guide And Data Book Table I .... Heat Gain from People (for Marine Air Conditioning Loads) Oogroo of Actively Dancing Persons frxtanp (Mess F''nm* and 1-hnrnij R/inrrwi) Watters Moderate Activity fLoin* ship'soffice, chart rooms, etc.) light Activity (Staterooms, crew's berthing, etc.) Hoot Koto (BW/HSJ Swwfbfc Latent ratal 245 605 850 220 330 550 300 700 1000 200 250 450 195 205 400 . effect of outdoor air reduces the operating load of the re frigeration plant. Outboard bulkhead -- exposed to conditioned area. Forward bulkhead = 221 sq ft, exposed to 120 F (2 in. insu lation). Aft bulkhead * 136 sq ft, exposed to 98 F (2 in. insu lation). Aft bulkhead = 51 sq ft, exposed to 98 F (no insulation). Inboard bulkhead * l45sqft,expoeedto85F(noinsu]ation). Glass -- 45 sq ft Solution: The heat transmission is tabulated for convenience. Surface Area, sq ft Deck under Forward bulkhead Aft bulkhead Aft bulkhead Inboard bulkhead 40 221 136 51 145 Temp. DifC F deg 3 38 16 16 3 U 0.50 0.23 0.23 0.40 0.30 Trans mission, Btu/hr 60 131 Load Determination Glass 45X95 Btu/(hr)(sq ft) 4275 The cooling load estimate for air conditioning consists of the usual factors, as dwcngwH in Chapter 26 of the 1861 Guide and Data Book, including: 1. Solar radiation. 2. Heat transmission through decks and bulkheads. 3. Heat dissipations of occupants. 4. Heat gain due to lights. 5. Ventilation air. 6. Motors or other electrical heat producing equipment. Hie heating load estimate for air conditioning should con sist of the following: 1. Heat losses through decks aad bulkheads. 2. Ventilation air.' No allowances are made for heatgain from wanner adjacent spaces. Heat Transmission Coefficients. The overall heat transmis sion coefficient, U, between the conditioned space and the outside of the boundary in question depends upon the con struction, material and insulation employed. The composite structures common to shipboard construc tion do not lend themselves to theoretical derivationof such coefficients. Therefore, they are most commonly obtained from full scale panel tests. Heat Dissipation from People. The rates at which heat and moisture are dissipated from people depend upon their state of activity. Table 1 gives values that can be used at 80 F room dry bulb. . Heat Load Calculations. Example l illustrates a detailed load calculation for a typical marine air-conditioning system. Example 1: Determine the total load for air conditioning the officers' mess and lounge of a new cargo ship with the following design conditions. Location Outdoor Indoor Design Temp., F Drybulb Wetbulb 95 82 80 67 Heat, Btu/lb 46.0 21.7 Heat, tive Gr/lb Temp. 78 74 Volume * 3900 eu ft People -- 28, phis 2 waiters * 30 lights * 1660 watte Transmission Sources, Areas, and Conditions Deck over *> 620 sq ft, exposed to conditioned area Deck under -- 40 sq ft, exposed to 85 F (no insulation). Remainder 1b exposed to conditioned area. Total Transmission Load The sensible and latent beat loads are calculated as follows: Sensible Space Load Stub TranfimissioD *= 7222 People * 28 X 220. Btuh per person -- 6160 2 X 300 Btuh per person * 600 Ughte - 1560 X 3.4I X 0.75 (use factor) - 4000 Sensible Outdoor Air Load 17,967 Minimum outdoor air * 15 cfm per person 30 people X 15 *450 cfm Air quantity required (based on a 17,967 temperature difference of 30 Fdeg)*-------------- --- 555 cfm 1.08 X 30 For 100 percent outdoor air, the sensible load is 555 X 1.08(95-80) - 9000 Btuh Total Sensible Heat Load * 17,967 + 9000 - 26,967 Btuh Latent Space Load Btuh People -- 28 X 330 Btuh per person -- 9250 -- 2 X 700 Btuh per person * 1400 Latent Outdoor Air Load* 655(0.67)(146-78) Total Latent Heat Load * 10,650 + 25,300 Total Heat Load * 26,967 + 35,930 10,650 =25,300 Btuh -35,950Btuh * 62,917 Btuh * 5.25 tons of refrigeration EQUIPMENT SELECTION The principal equipment required for an air-conditioning system can be divided into four broad categories: (1) the air handling portion consisting of fans, filters, central heating and cooling coils and sound treatment; (2) the distribution network including air ductwork, water and steam piping as required; (3) the terminal treatment consisting of heating and cooling coils, terminal miring units and Hiffi*aing outlets; and (4) the refrigeration equipment. Factors to be considered in the selection of equipment are: 1. Installed first cost. 2. Space available in fan rooms, passageways, machinery rooms and staterooms. 3. Operating costs including system 'maintenance. 4. Noise levels. 5. Weight. The wider use of unitary central equipment has reduced required fan room floor space, reduced operating noise levels and installation costs. Fans must be selected for stable per formance over their full range of system operation and should be provided with adequate isolation to prevent transmission of vibrations to the deck. Effective sound treatment is es- Marine Air Conditioning 235 genital since fan rooms are often located adjacent and near living quarters. In general, construction of all equipment is considerably heavier than required for land applications and must be cap&ble of withstanding the corrosive environment of the salt air. Materials such as stainless steel, bronze alloys, steel hot dip galvanized after fabrication, etc., are used extensively. Centrifugal refrigeration machinery using Refrigerants 11 aod 114 for air conditioning loads in excess of 150 tons usually proves most economical. For the majority of cases where the load is under 150 tons, one or more reciprocating machines are employed. The compressors, in general, must be capable of operating satisfactorily under shipboard list, trim, pitch and roll conditions and be constructed for marine service. The use of direct-connected reciprocating compressors has become standard practice due primarily to savings in space over belt-driven units. Hermetic centrifugal machines offer a similar advantage over open compressors and usually result in a substantially lower first cost. To date, these have only bfn used to a limited extent. Steam-jet refrigeration has been used on foreign passenger liners, but has never been applied to any American ships, except a few naval vessels. The absorption machine designed for chilled water service, utilizing either low pressure or high pressure steam, with lithium bromide as the absorbent and water as the refrigerant, offers many interesting possibilities for shipboard application, particularly where very quiet operation is necessary on military vessels. With regard to the refrigerantemployed on merchantships. Refrigerant 12 is most widely used, due to its availability in all ports of call. However, Refrigerant 22, used extensively ' on land applications, is now more readily available at ports around the world. An increased use of this refrigerant should be expected because of the following advantages: 1. Since Refrigerant 22 has a lower specific volume than Re frigerant 12, the same compressor has approximately twice the capacity of Refrigerant 22. This permits savings in compressor costs, space, and weight. 2. Toe size of refnger&nt lines and valves will be reduced. 3. liquid-suction heat exchangers commonly used with Refrigerant 12 systems to increase the thermodynamic efficiency of the cycle will not be required with Refrigerant 22. Sheet Metal Ducts Sheet metal ductwork on merchant ships has been con structed to various gages of steel and aluminum alloys. For low pressure ductwork, hot dipped galvanized, copper bear ing steel has been used most extensively. Where protected, the minimum thickness of the material for Don-watertigbt ducts is usually determined by the width or diameter of ducts according to the following: Size Less than 6 in. 6-12 ini 12*f-24 in. 24 in. and over Steel 22USSG (0.0313 in.) 20 USSG (0.0375 in.) 18 USSG (0.050 in.) 16 USSG (0.0625 in.) It is important that the fabrication of the ductwork and transitions sections be such that all seams are airtight. With the increased use of high velocity systems on mer chant ships, round, small diameter, prefabricated pipe and fittings are being employed. One of the effective ways of minimizing air leakage for these high pressure systems is the use of an economical pressure-sealed clamp for joining fittings to ab piping. The principal advantage of the round, small diameter duct- Rg. 1 .... Single Zone Central (Type A) System work is that it occupies a fraction of the space required for the huger sheet metal duct. Saving in weight also is an important consideration. TYPICAL SYSTEMS Comfort air-conditioning systems installed on shipboard are classified as: (1) those serving passenger staterooms, (2) those serving crew's quarters and similar small spaces, and (3) those serving public spaces. A brief description of the types of systems used is given in the following sections. Single-Zone Central System Public spaces are treated as one zone and can effectively be handled by this simple arrangement. Exceptionally large spaces may require two systems. Central station systems of either the built-up type or factory-assembled fan-coil type are generally employed for large public spaces. Fig. 1 is a schematic diagram of a typical system, which also is known as the Type A system. Under certain conditions it is found desirable to use all outdoor air, avoid the use of return ducts, and penalize the refrigeration plant accordingly. Outdoor air and return air are both filtered before being preheated, cooled and reheated as required at the central station unit. A room thermostat maintains the desired temperature by modulating the valve regulating the Sow of steam to the reheat coil. During mild weather the dampers frequently are arranged to automatically admit 100 percent outdoor air. Multi-Zone Central System This system (Fig. 2) is also known as the Type C system. Spaces are divided into zones in accordance with similarity of loads and exposures. Each zone has a reheat coil to supply air at temperature adequate for all spaces served. A thermo stat regulates the amount of steam or hot water circulated to the reheat coil. Manual control of air volume is the only means for occupant control of conditions. Filtering is same as for Type A system. The internal sensible heat load components of the spaces, particularly solar and lights, in any one zone are subject to large variance. Also, the methods used for thermostatic con trol of rehesters cannot compensate for these large variations. Therefore, it is impossible for this system to always satisfy individual space requirements. Also, volume control is con ducive to noise, drafts and odors. Terminal Reheat System The terminal reheat system is commonly known as the Type D system. A diagram of this system is shown in Fig. 3. In this