HVAC Concept Design for Small Surface Combatant

Transcription

1 HVAC Concept Design for Small Surface Combatant Clint Domine Directorate Navy Platform Systems 1.0 OVERVIEW The objective of the concept study is to estimate the Heating, Ventilation and Air Conditioning (HVAC) cooling load for a Small Surface Combatant. The small surface combatant used for this study was a concept approximately 80 metres in length and could accommodate a crew of 45. The parameters were chosen to reflect a realistic situation of operating in the tropics. Also the ramifications of integrating a Chemical, Biological, Radiological and Nuclear (CBRN) Citadel system were considered. 2.0 SCOPE The scope of the study encompasses estimation of the total cooling load and required chiller water plant cooling capacity. To aid with the study, the following items were calculated using relevant heat transfer equations and HVAC design theory: Compartment heat gain and losses Total connected cooling load System air flow In addition, the study will address the general requirements for implementing a CBRN citadel to provide protection from CBRN threat. The following topics will also be discussed briefly in the report: Citadel system design requirements CBRN filters and airlocks Cleansing station arrangements The concept study aims to provide estimates of the Small Surface Combatant HVAC system and CBRN Citadel requirements to stimulate further discussion and research. It is not in the scope of this report to determine the specifications of individual compartment chillers, sizing of condensers, Air Filtration Units (AFU), HVAC ducting, citadel wash down facilities and total weight of the HVAC system. This report also does not discuss specific CBRN Defence arrangements and ventilation requirements for machinery spaces. 3.0 DESIGN ASSUMPTIONS 3.1 General The total cooling load is based on the total ship peak cooling. The chilled water plant (CWP) may not be required to produce the peak cooling capacity at all times but it must be able to 1

2 cope with extreme tropical environments where there is a possibility the ambient temperature will reach 40 degrees Celsius. 3.2 Design External Ambient Temperature The external ambient temperature was selected to be representative of the operational environment of the Small Surface Combatant. It is assumed that the Small Surface Combatant will operate in littoral waters close to the equator where the climate is extreme tropical. The extreme tropical regions such as the Persian Gulf and north Australia are examples of where the Small Surface Combatant may be operating and where ambient temperatures of 40+ degrees C dry bulb can occur. 3.3 Seawater Temperatures The seawater temperature does not affect the cooling load calculations; it does however have a profound effect on the performance of the CWP. Seawater is used as the cooling medium in the heat exchangers of the refrigerant circuit. Near surface seawater temperatures can reach as high as 35 degrees C in tropical waters. A high sea water temperature will decrease the heat transfer between the refrigerant and the seawater thus reducing refrigerant cooling capacity. This will have an adverse effect on the chiller which produces the chilled water. 3.4 Internal Design Temperature The internal design air temperature was chosen to be +27 deg C 50% relative humidity. 3.5 Boundary Heat Load Temperature differences between adjoining internal spaces The boundary heat load is the heat that is transferred through the bulkheads, deck and deckhead. The boundaries between adjacent compartments and external boundaries that would be susceptible to solar heat gain were established. The temperature differences between adjoining internal spaces such as a bulkhead, deckhead, or deck separating an air conditioned and non air-conditioned internal space was obtained from Table 1 of ISO-7547:2002(E). The heat transfer coefficient, U for each bulkhead, deck and deckhead was obtained using guidance values on commonly used materials as specified in ISO-7547:2002(E). It is also assumed that the temperature difference between adjacent air conditioned compartments is zero. 3.6 Equipment Heat load The lighting heat load is due to the heat produced from the lighting in a compartment. The occupancy heat load is the heat gain due to personnel in the compartment space. The equipment heat load is the heat gain from equipment within the space such as boilers, pipes etc. The lighting heat load is generally accepted as 16 watts/square metre of deck area. 3.7 Human heat load The human heat load due to perspiration is the following: * Sensible = 60 watts/person * Latent = 90 watts/person 2

3 3.8 Fresh air required per man The overall fresh air per person required is kg/sec ( m 3 /second). The minimum fresh air per person requirement is kg/sec. 3.9 Fan heat load The fan used for distribution of the air also produces sensible heat and will vary depending on ducting arrangements and filtration back pressures. For the purpose of this design study it is assumed to be 14% Outdoor air load (fresh air) The supply air entering the cooling apparatus is a mixture of fresh air and recirculated air from the compartment. The fresh air or outdoor air load is the sensible and latent heat that is contained in the fresh air supplied to the cooling apparatus of the air handling unit. It is an additional heat load that must be taken into account along with the compartment heat load. It is not in the scope of this study to calculate the bypass factors and mixture ratios which govern the amount of recirculated and fresh air entering the system. For the purpose of this design study, the fresh air percentages ranging from 50% to 100% will be assessed to provide a range of cooling loads. Outside air load (OATH) =.19 ( Fresh air percentage) Q ( h h ) Where, 1 c outside design Fresh air percentage = fresh air proportion of the airflow entering cooling apparatus Q c = the compartment airflow (kg/second) h outside = The enthalpy of the outside air (kj/kg) h = The enthalpy of the design air condition (kj/kg) design Using a psychrometric chart, the enthalpy difference for an internal ambient design temperature of 27 deg C 50% relative humidity and an ambient outdoor air temperature of 40 deg C 70% relative humidity is 69 kj/kg Electronic Equipment Heat Load In determining the cooling load, the heat gain from electronic equipment such as communications, navigation, radar, and electronic cabinets must be accounted for. It is not in the scope of this study to accurately calculate the electronic equipment heat load. Typically, the electronic equipment will have a dedicated fan coil unit and heat exchanger attached through which a refrigerant or chilled water line would pass through. For a vessel of 80 metres in length and combining the capabilities of a Hydrographic, mine hunter, and patrol boat the electronic equipment cooling load was estimated to be 58 kw. It was also assumed that the electronic equipment cooling load was constant regardless of the air 3

4 distribution arrangements. For the breakdown of the electronic cooling loads refer to Table 1 below. Electronic Component Estimated load Hydrographic survey 9 system Radar 17.5 Communication 31.6 Equipment Estimated Total 58.1 Table 1: Electronic cooling load breakdown 4.0 HVAC COOLING LOAD CALCULATIONS 4.1 General To determine the cooling capacity of the CWP to supplying the required cooling to achieve the design ambient air temperature, the total cooling load of the ship is to be calculated. The total compartment cooling load is the work that the CWP must produce to maintain the desired air temperature for all air conditioned compartments. Heat transfer equations were used to determine the total compartment heat gain. 4.2 Compartment Volumes The volume for each compartment on the ship was calculated. Annex A contains the volumes for all compartments including air conditioned spaces, citadel compartments, and non air conditioned space such as machinery spaces. 4.3 Compartment Heat gain calculation For each air conditioned compartment, the following sensible and latent heat loads were calculated and added to produce the compartment total heat load, Qt: Boundary heat load Lighting heat load Occupancy heat load Equipment heat load Fan allowance For breakdown of compartment total heat loads please refer to Annex B. 4.4 Compartment Supply air quantities Once the compartment heat load data was collated for all air-conditioned compartments, the individual heat loads was summed to produce the total compartment heat load. The total air quantity is the total compartment heat load divided by the enthalpy difference. TAQ = Total Heat Gain/Enthalpy Difference 4

5 The enthalpy difference is the difference in enthalpies of the air entering and leaving the cooling apparatus. For this study the air entering the apparatus was assumed to be 29 degrees (50% Relative humidity) or a few degrees higher than the design ambient of 27 degrees. The air leaving the cooling apparatus is assumed to be the accepted value of 13 degrees C (dry bulb) 12 degrees (wet bulb). Using a psychrometric chart, the enthalpy difference between the two values is calculated as 23.5 kj/kg. This value however will change; however, as air entering the cooling apparatus is influenced by the proportion of fresh air and the number of air changes required. Once the total air quantity (TAQ) is calculated the apportioning of air to each air-conditioned compartment is based on the following relationship: H SR Compartment Air Quantity = TAQ H SRT Where; TAQ = Total Air Quantity H = Compartment Sensible load SR H SRT = Sum of Compartment Sensible Loads 4.5 Application of six minimum air changes per hour per compartment Once the air quantity for each compartment is calculated it had to be checked against the minimum requirement of six air changes per hour. All air conditioned spaces which had air quantities less than six air changes per hour were increased to the minimum of six air changes per hour. This effectively increased the total air quantity and subsequently produced a revised total air quantity. 4.6 Total Cooling Load The total cooling load is the sum of the compartment heat gain, outside air loads (OATH) and additional loads from electronic equipment. Total Cooling load = Grand total heat + Electronic Equipment Cooling loads Where; Grand total heat (GTH) = Qt + OATH Qt = Sum of compartments heat gain (sensible and latent loads) OATH = Outside air total heat 5.0 RESULTS - HVAC SYSTEM 5.1 General This section outlines the estimates of total compartment heat gain, supply air requirements and total cooling load capacity for the HVAC system without a citadel. The internal ambient and external temperatures from Section 3 of this document were used as the design temperatures. The calculation process described in Section 5 was used to calculate the total cooling load capacity. 5

6 5.2 Total Compartment heat gain The total compartment heat gain was calculated to be 94.1 kw. The sensible heat and latent heat components were 72.3 and 21.8 kw respectively. 5.3 Supply Air Requirements The total air quantity for the HVAC system alone was initially calculated to be 4 kg/second (12015 m 3 /hr) but after applying a minimum six air changes per hour per compartment it was recalculated to be 5.3 kg/second (15789 m 3 /hr). 5.4 Total Cooling Load Capacity The total cooling load capacities for fresh air proportions ranging from 50% to 100% were graphed in figure 1 below. It can be observed that with increasing fresh air proportion, the cooling load increases. This is due to the fact that with a higher fresh air percentage, a greater quantity of fresh air at a higher temperature and humidity is brought into the system which must be conditioned. Ideally, a balanced proportion of fresh and recirculated air will be chosen based on air change, fresh air per man, and power consumption requirements. If cost was not a factor, a 100% fresh air system will be desirable as air will not be recirculated reducing the amount of stale air or impurities being recirculated. A higher fresh air quantity, however, will mean the cooling coils will have to remove a greater quantity of heat and therefore power consumption will increase. Please refer to Annex C for breakdown of compartment outside air heat and grand total heat loads. Total cooling load vs fresh air percentage Grand total cooling load Fresh air percentage (%) Figure 1: Total Cooling load vs. fresh air percentage 6

7 6.0 CITADEL SYSTEM DESIGN & REQUIREMENTS 6.1 General A citadel is defined as the compartments within a vessel which form a group of interconnecting compartments enclosed by a vapour-tight boundary, within which filtered air can be circulated at a positive pressure to provide collective nuclear, biological and chemical protection. In the context of the small surface combatant the chosen citadel is a hardened COLPRO system because it provides the highest level of protection to the crew from a CBRN threat. The hardened COLPRO system is effectively a citadel that is integrated into the ship structure as opposed to an unhardened COLPRO system where by a toxic free area is achieved through the application of an inflatable CBRN liner. The primary role of the citadel is to prevent the ingress of CRBN agents into compartments by providing an overpressure, gas tight boundary, and CBRN filters. To prevent CBRN agents from entering citadel during personnel entry and exit, cleansing stations and air locks are used for decontamination. Cleansing stations, air locks, and CBRN filters impose an additional air requirement on the vessel s HVAC system. Also to maintain the overpressure air losses due to uncontrolled leakages and breathing air replacement requires additional airflow. This section provides an overview of the process in calculating the additional airflow required to maintain the CBRN citadel system. The following additional air flow requirement was calculated and added to the total air quantity calculated in Section 6: Losses due to uncontrolled leakages; and Breathing air replacement (Control of CO 2 ). In this section the following areas are also addressed: Cleansing station and airlock purging; Cleansing station arrangements; and; Citadel Subdivisions; and CBRN Filter requirements. 6.2 Citadel Compartments Areas generally considered part of the citadel are spaces vital to ship operations such as the bridge, communications, and crew living quarters. The number and type of citadel compartments will vary with each vessel and is constrained by factors such as compartment arrangement, available space, and budget. The table in Annex D is a list of compartments that form the Small surface combatant CBRN citadel. 6.2 Air supply/intake Requirements For a citadel equipped ship in normal open state, the supply air is sufficient to provide the required compartment air changes and fresh air flow requirements. In a closed down state such as during wartime, supply air is diverted via dampers through pre-particulate, particulate, and vapour filters before entering the air treatment/handling units. For efficient removal of CBRN agents, each filter is normally loaded with airflow ranging between 270m 3 /hr to 340 7

8 m 3 /hr. Therefore the total supply air flow entering the citadel dictates the number of CBRN filters required to filter the air. The citadel air flow is usually air conditioned in a citadel system is the sum of the air flow required for the HVAC system including air changes, uncontrolled losses, and breathing air volume requirements. This leads to a total air quantity of m 3 per hour (5.9 kg/second). 6.3 Cleansing station and airlock purging requirements: General The purging air flow in the airlock shall be sufficient to provide at least five air changes per minute for removal of all contamination. This is equivalent to 300 air changes per hour (5 air changes/min x 60 mins/hr = 300 air changes/hr). The purging air flow for the airlocks and cleansing stations are only filtered air so it does not need to be added to the air-conditioned supply. The purging air requirements will only have an impact on the number of fans and filtration units required Airlock purging calculation To maintain a citadel system the Small Surface Combatant requires airlocks in the following locations: 1. 4 airlocks with 2 each in the fwd and aft cleansing stations; 2. 4 airlocks for the four machinery spaces (assuming that each machinery space is independent); 3. 2 airlocks for the contaminable spaces (hangar and paint storeroom); and 4. 1 airlock for access between the fwd and aft sub-citadels. This leads to a total of 11 airlocks for the Small Surface Combatant citadel. Each airlock occupies a volume of approximately 1 m 3. The total swept air lock volume will be 11 x 1 m 3 = 11 m 3. The required airlock purging airflow is 11 m 3 /air change x 300 air changes/hr = 3300 m 3 /hr Cleansing station air change calculation The largest volume compartment in the cleansing station generally receives a minimum of five air changes in the processing interval between men. The processing interval between personnel entering the cleansing station is specified to be no more than five minutes. The cleansing station air change requirement is five air changes per five minutes or 60 air changes per hour. The volume of a typical cleansing station with stretcher capabilities is 39 m 3. The largest compartment in the cleansing station is typically the first stage compartment at 24 m 3. Because there are two cleansing stations on the Small Surface Combatant (Aft and Fwd cleansing stations) the total volume is 2 x 24 m 3 = 48 m 3. For 60 air changes per hour the cleansing station air change equates to 48 m 3 /air change x 60 air changes/hr = 2880 m 3 /hr. 8

9 6.3.4 Total purging airflow requirement The total purging airflow requirement for cleansing station and airlocks is = 6180 m 3 /hr 6.4 Breathing Air Volume Requirements: Breathing Air Volume Formula The breathing air volume equation determines the quantity of filtered air that is required to offset the CO 2 generated from personnel breathing within a compartment. The breathing air volume varies depending on whether the person is resting, performing light work, or performing normal work. For this design exercise, the breathing air volume was assumed for personnel performing normal work loads at 1.25 m 3 /hr/person. Filtered air = Where: A BAV N B 2 B 1 BAV = breathing air volume; BAV resting = 0.5 m 3 /hr/person; BAV light work = 0.75 m 3 /hr/person; BAV Working = 1.25 m 3 /hr/person; N = number of persons A = CO 2 generated during breathing (4%) B 2 = Permitted CO 2 concentration in fully manned compartment; B 1 = CO 2 content in fresh air (0.03%) CO 2 permitted concentration in various types of compartments: Operational spaces 0.15% Berthing, resting spaces 0.25% Dining, lounge spaces 0.25% Workshops, offices, stores 0.45% Citadel Breathing Air volume calculation Breathing air volume was assumed worst case scenario so BAV working of 1.25 m 3 /hour/person was selected. The number of persons was taken as the total complement of the platform which was 45. An average figure of 0.25% was selected for the permitted CO 2 concentration in fully manned compartment. Given these parameters the filtered air required was calculated to be: = m 3 /hr

10 6.5 Uncontrolled Leakage Distribution: The uncontrolled leakages are based on a requirement of 900 m 3 /hr of filtered fresh air for every 3400 m 3 of citadel volume that is above the deep water line. It is assumed that the uncontrolled leakages were distributed evenly across the citadel areas of the ship. The breakdown of the citadel volume calculation is described in Annex D. The citadel volume, including 300 m 3 allowance for passageways, was calculated to be 3342 m = 884 m 3 /hr 3400 Therefore the air flow requirement due to uncontrolled leakages is calculated to be 884 m 3 /hr. 6.6 Citadel Sub-division Overpressure According to the overpressure requirements of Det Noske Veritas Part 6 Chapter 10 Nuclear, biological and chemical protection there shall be a positive overpressure of at least 500 Pa between the citadel and the external environment. 7.0 DISCUSSION From this HVAC design study, it can be observed that a chilled water plant with a cooling capacity of at least 300 kw is required to provide a design temperature of 27 degrees Celsius with 50% relative humidity. Increasing the amount of fresh air supplied to the HVAC system will increase the cooling load. At 100% fresh air system will require a cooling capacity of 454 kw. Another critical factor that determines the cooling capacity of the chilled water plants is the temperature of the seawater and the performance of the seawater condensers. It is vital that an appropriate condenser is chosen for the chilled water plant. An undersized or inefficient condenser will result in the refrigerant not being cooled sufficiently and consequently the cooling capacity of the plant will be reduced. High temperature seawater passing through the condensers will also reduce its heat transfer potential. 8.0 REFERENCES 1. International Standards Organisation 7547:2002(E) 2. DA9 Air conditioning load estimation, The Australian institute of Refrigeration heating and air-conditioning and heating (Inc.), Det Noske Veritas Part 6 Chapter 10 Nuclear, biological and chemical protection, Rules for high speed, light craft and Naval surface craft,

11 Annex A: Compartment Volumes Name Floor Area (m2) Height Width Length Fwd Bulk width Aft Bulk width Port Stbd Fwd Bulk (m2) Aft Bulk (m2) Port (m2) Stbd Head (m2) Com plim ent Volume (m^3) 03 Deck Bridge HVAC and Systems Public Head Deck HVAC and Systems Ships Op Office CO's Day Cabin Pantry CO's Sleeping Cabin Comcen Single Berth Officer HVAC and Systems Single Berth Officer Repair Base Deck Ships Magazine Public Head Battery Comp Electrical Equipment Compartment Unassigned Engineering

12 Admin Electronics Workshop Sickbay Public Head Hangar Small Arms Stowage Unassigned Paint Store Ships Office/ Pay Office Aviation Admin Office Avionics and Helo Workshop Emergency Generator Deck Naval Stores Naval Stores Berth (2) Berth (2) Berth PO Berth CPO Pantry Wardroom Public Head UUV Support and Battery Charging Demolition Magazine Boat Gear Store Diving Gear Store Repair Base

13 Damage Control Store Berth (2) Berth PO Berth PO Berth CPO Berth Officer Berth (2) Garbage Compactor/Store Dry Garbage Deck General Store Accommodation Store Laundry SS Mess SS Pantry and JS Servery Galley Stores Cool Cold and Dry Engineering Store Eng Workshop JS (2) JS (4) JS (4) JS (8) Unassigned Steering Gear General Store Canteen and Canteen Store JS Dining Scullery

14 JS Rec Area Dry Store MCR and DCC Central JS (2) JS (4) JS (2) Baggage Store JS (6) Gym Steering Gear Echo Sounder/ Equipment Room FWD Auxiliary Machinery Generator Room Engine Room AFT Auxiliary Machinery Room

15 Annex B: Compartment Heat loads Index Compartment Comp. Sensible Heat Gain Qs Comp.Total Heat Gain Qt Comp. Sensible Heat Ratio #Men Revised Comp air quantity Comp. Air Quantity Mc Compt. volume 6 Air changes per hour (minimum) Units kw kw SHRc N m^3/s m^3/s m^3 m^3/s 1 Bridge Ships Operation Office CO's Day Cabin CO's Sleeping Cabin Comcen Single Berth Officer Single Berth Officer Unassigned 01 Deck Fwd Port Engineering Admin Electronics Workshop Sickbay Unassigned 01 Deck Fwd Stbd Ships Office Aviation Admin Office Avionics and Helo Workshop Berth (2) 1 Port Berth (2) 2 Port Berth PO 1 Port Berth CPO Port Wardroom Berth (2) Stbd Berth PO 1 Stbd Berth PO 2 Stbd Berth CPO Stbd Berth Officer

16 26 Berth (2) Stbd SS Mess Eng Workshop JS (2) Aft Port JS (4) Aft Port JS (4) Aft Port JS (8) Unassigned 2 Deck Aft Port Canteen and Canteen Store JS Dining JS Rec Area MCR and DCC Central JS (2) Aft Stbd JS (4) Aft Stbd JS (2) Aft Stbd JS (6) Gym TOTAL

17 Annex C: Outside air total heat and Grand total heat with varying fresh air proportions 50% Fresh Air 60% Fresh Air 70% Fresh Air 80% Fresh Air 90% Fresh Air 100% Fresh Air Compartment OATH GTH OATH GTH OATH GTH OATH GTH OATH GTH OATH Bridge Ships Operation Office CO's Day Cabin CO's Sleeping Cabin Comcen Single Berth Officer Single Berth Officer Unassigned 01 Deck Fwd Port Engineering Admin Electronics Workshop Sickbay Unassigned 01 Deck Fwd Stbd Ships Office Aviation Admin Office Avionics and Helo Workshop Berth (2) 1 Port Berth (2) 2 Port Berth PO 1 Port Berth CPO Port Wardroom GTH 17

18 Berth (2) Stbd Berth PO 1 Stbd Berth PO 2 Stbd Berth CPO Stbd Berth Officer Berth (2) Stbd SS Mess Eng Workshop JS (2) Aft Port JS (4) Aft Port JS (4) Aft Port JS (8) Unassigned 2 Deck Aft Port Canteen and Canteen Store JS Dining JS Rec Area MCR and DCC Central JS (2) Aft Stbd JS (4) Aft Stbd JS (2) Aft Stbd JS (6) Gym TOTAL

19 Annex D CBRN Citadel Compartments Item Compartment Volume Item Compartment Volume m 3 m 3 03 Deck 1 Deck (cont.) 1 Bridge Damage Control Store HVAC and systems Berth (2) Public head Berth PO Deck 36 2 Berth PO Ships operation office Berth CPO CO s day cabin Berth Officer Pantry Berth (2) CO s Sleeping Cabin Deck 8 Comcen Accommodation Store Single Berth officer Laundry Repair Base SS Mess Deck 43 SS Pantry and JS Servery Ship s magazine Galley Public head Stores Cool Cold and Dry Battery compartment Engineering Store Electrical equipment 47 compartment 10.5 Eng Workshop Unassigned JS (2) Engineering Admin JS (4) Electronics workshop JS (4) Sickbay JS (8) Public head Unassigned Small arms stowage General Store Ships office/pay office Canteen and Canteen Store Aviation admin office JS Dining Deck 56 Scullery Naval Stores JS Rec Area Berth (2) Dry Store Berth (2) MCR and DCC Central Berth PO JS (2) Berth CPO JS (4) Pantry JS (2) Wardroom Baggage Store Public Head JS (6) UUV Support and Battery Charging Gym Repair Base