Heat Load Calculation for Air Conditioning System
Refrigeration and air conditioning systems cover a wide variety of cooling applications, using both standard and custom-made equipments. Central air conditioning is more reliable for easy operation with a lower maintenance cost. Refrigeration and Air-conditioning accounts for a significant portion of the energy consumption in many manufacturing industries (like chemicals, pharmaceuticals, dairy, food etc.), agricultural & horticultural sectors (mainly cold stores) and commercial buildings (like hotels, hospitals, offices, airports, theatres, auditoria, multiplexes, data processing centers, telecom switching exchanges etc). Educational and research institutions also need human comfortness, as the population of student community increase year by year. The effective design of central air conditioning can provide lower power consumption, capital cost and improve air purity of a building. This paper establishes the results of cooling load calculation of different climate conditions by using CLTD method for a multi-story JMI library which is a part of an institute. Cooling load items such as, people heat gain, lighting heat gain, infiltration and ventilation heat gain can easily be putted to the E 20 MS-Excel program. The E 20 MS-Excel program can also be used to calculate cooling load due to walls and roofs.
And results were compared with the standard data given by ASHRAE and CARRIER Fundamental Hand Books, and results are satisfactory. It is also seen that comfort air-conditioning generally implies cooling of room air to about 25°C and relative humidity in the range of 50% to 60%. Industrial process air conditioning and precision air conditioning may require temperatures ranging from 19°C to 23°C with relative humidity values ranging from 50% to 60%.
Keywords:- CLTD, Cooling Load, , Human comfortness,Air conditioning.
The main purpose of air conditioning system is to provide a comfortable environment for people or machines (computer centers, library, metrology laboratories….) inside a closed space. Therefore, cooling (reduce the temperature of the room air), heating and cooling load calculations are carried out to determine the required capacity of heating and cooling systems, which can maintain the required conditions in the conditioned space. To determine the required cooling or heating capacities, one has to have information related to the design of indoor and outdoor conditions, specifications of the building premises, specifications of the conditioned space (such as the occupancy, activity level, various appliances and equipment used etc.) and any requirements of the particular application. For comfort applications, the requisite indoor conditions are fixed by the criteria of thermal comfort, while for industrial or commercial applications the requisite indoor conditions are fixed by the particular processes being performed or the products being stored. As discussed above, the design outdoor conditions are chosen based on design of dry bulb and wet bulb temperatures for summer, monsoon or winter months for cooling and heating load calculations, respectively.
ABody surface area, ft2 (m2)
HVACHeating, ventilating, and air-conditioning
W Useful rate of working.
QS Heat stored in the body.
QE Heat loss by evaporation.
QR Heat loss and gain by radiation.
QC Heat loss and gain by conduction and convection
U Heat transfer coefficient on body surface.
Tb, Ts Temperature of the body and surrounding.
hi Indoor surface heat transfer coefficient, Btu/(h . ft2 . OF)W/(m2 . K)
hO Outdoor surface heat transfer coefficient, Btu/(h . Ft2. OF) W/(m2 . K)
1.2 Cooling load Calculation
The cooling load experienced by a building varies in magnitude from zero (no cooling required) to a maximum value. The design cooling load is a load near the maximum magnitude, but is not normally the maximum. Design cooling load takes into account all the loads experienced by a building under a specific set of assumed conditions.
The assumptions behind design cooling load are as follows:
1. Design outside conditions are selected from a long-term statistical database. The conditions will not necessarily represent any actual year, but are representative of the location of the building. Design data for outside conditions for various locations of the world have been collected and are available in tabular form in various handbooks.
2. The load on the building due to solar radiation is estimated for clear sky conditions.
3. The building occupancy is assumed to be at full design capacity.
4. All building equipment and appliances are considered to be operating at a reasonably representative capacity.
The total building cooling load consists of heat transferred through the building envelope (walls, roof, floor, windows, doors etc.) and heat generated by occupants, equipment, and lights. The load due to heat transfer through the envelope is called as external load, while all other loads are called as internal loads. The percentage of external versus internal load varies with building type, site climate, and building design. The total cooling load on any building consists of both sensible as well as latent load components. The sensible load affects dry bulb temperature, while the latent load affects the moisture content of the conditioned space.
Fig 1: Calculation of various load
1.3 THE SOFTWARE: E 20 EXCEL SHEET
E 20 EXCEL SHEET is a computer tool produced by Excel, a company providing solution for heating, refrigeration and air conditioning. The aim of this program is to assist engineers to design HVAC systems for commercial and industrial buildings. It presents two tools in one: estimation of the loads and design system, and calculation of the energy usage and calculation of energy costs,.
E 20 EXCEL SHEET is able to perform the following tasks:
To determine be the optimum rate at which heat needs to be removed from space to establish thermal equilibrium & maintain a pre-determined inside conditions
To calculate peak design loads (cooling/heating).
To estimate capacity or size of plant/equipment.
To provide information for HVAC designs e.g. load profiles.
To form the basis for building energy analysis.
2. LITERATURE REVIEW
Wong et al. 1 studied the development of a new example weather year and generated the mathematical model to design occupants load profiles using Monte Carlo simulation method for subtropical climate. For determining the HVAC energy consumption in buildings this method and results were very useful.
Francesco Causone 2 investigated and designed radiant cooling load systems for removal of solar heat gain. They used heat balance method and time series method to calculate the cooling load and proposed a simplified procedure to calculate the magnitude of the solar heat load.
Fernando et al 3 proposed a new approach based on a stochastic simulation method for uncertainty in peak cooling load calculations. The stochastic solution was compared with the conventional solutions, and a universal sensitivity analysis was assumed to identify the most significant uncertainties.
Christian Ghiaus et al 4 reported that the calculation of optimal thermal loads of intermittently heated buildings. An unconstrained optimal control algorithm was proposed which used feed-forward to compensate the weather conditions and model predictive programming (MPP) for set-point tracking.
Francesco Causone 5 investigated and designed radiant cooling load systems for removal of solar heat gain. They used heat balance method and time series method to calculate the cooling load and proposed a simplified procedure to calculate the magnitude of the solar heat load.
3. THEORETICAL ANALYSIS
The configuration of the building as shown in figure and for finding the overall heat transfer coefficient (U) we use below equation
QM – W = QE± QS ± QR ± QC ………………. ….. (1)
QR = ? (Tb4-Ts4) ………………………. (2)
QE = CdA (Ps-Pv) hfg.Cc …………………….. (3)
QC = UA (Tb-Ts) …………. (4)
CLTDcorr = (CLTD+LM) K+ (25.5–Ti) + (To–29.4 ) … (5)
Q =UA(CLTD)corr ………… …..(6)
Q =AxSHGF ax xSCxCLF ………………….(7)
Qs,person =qs,person x N xCLF …………… ………(8)
RTH = RLH + RSH …………… .(9)
Qlight = Total wattage of light X Use factor X Ballast factor
Qequipments = Total wattage of equipment X Use factor X CLF
Amount of infiltrated air (Vinf) = Volume of Space X Ac m³/hour
Latent heat gain due to the infiltration, Qs,inf = 50000X Vinf X (To – Ti) watts
Sensible heat gain due to the infiltration, Qs,inf = 20.44 X Vinf X (To – Ti) watts
4. E 20 EXCEL SHEET
This program is released as two separate parts, but they have same products. The ” E 20 EXCEL SHEET for design load” program provides system design and load calculating features. The full “E 20 EXCEL SHEET” program provides the similar system design capabilities plus energy analysis features.
4.1 E 20 EXCEL SHEET FEATURES
E 20 EXCEL SHEET estimates design heating and cooling loads for commercial and industrial buildings in order to estimate required size for HVAC system components. Ultimately, the program provides information regarding to selection and specification of equipment. The program performs the various tasks including heating and cooling loads.
5. RESULTS AND DISCUSSION
The results show the cooling load calculation of different climate conditions by using CLTD E 20 EXCEL SHEET method for a JMI Library which is a part of an JMI institute. Cooling load items such as infiltration, lighting, people and ventilation can easily be entered to the E 20 EXCEL SHEET MS-Excel program.
5.1 BUILDING PARAMETERS
City : JMI LIBRARY , NEW DELHI
Length of the room = 13.70m
Breadth of the room = 6.00m
Height of the room =4.26m
No of windows =4
No of door =2
No of fans = 8
No of tube lights = 24
Energy Units: W
Calculation Method: E 20 EXCEL SHEET
In this study, Refrigeration and Air conditioning of JMI Library in Engineering Department building located in New Delhi was considered for calculation of cooling loads. The hand calculation accuracy and features make it sufficient for actual design of HVAC systems.
The main conclusions can be drawn from the results of calculation of the present work are:-
1. The total cooling load for the Air conditioning of the JMI Library by hand calculation requirement is 6.30TR. and total cooling load for E 20 EXCEL SHEET programs requirement is 6.25TR.
2. By hand calculation method it is found that each TR can cover 1000 m² floor area.
TABLE 1: INPUT PARAMETERS
-6353704900TABLE 2:Cooling load sheet of 40 seated Engineering library
TABLE 3: RESULT
Graph 1: DBT VS SPECIFIC HUMIDITY
Graph 2: LOAD VS DBT
ASHRAE, Handbook of Fundamentals, Ch. 28. American Society of Heating, Refrigerating and Air-Conditioning Engineers, U.S.A. (1997).
Christian A., Gueymard and Thevenard D., “Monthly average clear-sky broadband irradiance database for worldwide solar heat gain and building cooling load calculations” Solar Energy, 2009; 83; 1998–2018.
Abdullatif E., Ben-Nakhi, Mohamed A., and Mahmoud, “Cooling load prediction for buildings using general regression neural networks” Energy Conversion and Management, 2004; 45; 2127–2141.
Westphal,F.S. and Roberto L., “The use of simplified weather data to estimate thermal loads of non-residential buildings” Energy and Buildings, 2004; 36; 847-854.
Aktacir M.A., Buyukalaca O, and Tuncay Y., “A case study for influence of building thermal insulation on cooling load and air-conditioning system in the hot and humid regions” Applied Energy, 2010; 87; 599–607.
Mui K.W. and Wong L.T. “Cooling load calculations in subtropical climate” Building and Environment, 2007; 42; 2498–2504.
Francesco C., Stefano P.C., Marco F. and Bjarne W.O., “Solar radiation and cooling load calculation for radiant systems: Definition and evaluation of the Direct Solar Load” Energy and Buildings 2010; 42; 305–314.
Fernando D.M., Jose M. C.L., Antonio C.A., “Uncertainty in peak cooling load calculations” Energy and Buildings, 2010; 42; 1010–1018.
C.P. Arora, a text book of “Refrigeration and Air-conditioning” publishedby Mc Graw-Hill 2009.
Domkundwar and Arora a text book of “Refrigeration and Air-conditioning” published by Dhanpat Rai and Co. (P) Ltd.
Tingyao Chen and Zhun Yu, “A statistical method for selection of sequences of coincident weather parameters for design cooling load calculations” Energy Conversion and Management, 2009; 50; 813–821.
Kulkarni K., P.K. Sahoo and Mishra M., “Optimization of cooling load for a lecture theatre in a composite climate in India” Energy and Buildings, 2011; 43; 1573-1579.