Evaporation and Load on the Air Conditioner - Research Paper Example

Case Study: Analysis of evaporation and load on air conditioner Affiliation Introduction The determination of evaporation in swimming pools is usually necessary for the purpose of design and analysis. It is important for applications such as unoccupied indoor pools, water reservoirs etc. There have been many equations for calculating the water evaporation of undisturbed pools. This phenomenon of water evaporation on the free water surface of an indoor pool is a major contributor to the energy balance of thepools surrounding. Incase the total pressure of humid air which is on water surface and equal to sum of partial pressure of water vapour(saturated) at a given temperature,as well as partial pressure of air, it is greater than the saturation of water vapour, then the water will evaporate only from its surface (Benabed, 2012 ) Usually, there is more evaporation on occupied swimming pools as compared occupied swimming pools. This difference is related to the occupant number, the activities on the surrounding environment and also the water conditions as well as the ambient air in the swimming pool hall. ASHRAE recommends the use of a correlation factor which ranges from 0.5 for residential pools to a value of 1.0 for the public pools. They also provide carrier evaporation rate equation for predicting the rates of evaporation of occupied pools. The equation used in this case is Where: is the evaporation rate at the surface of the pool, in kg/s A is the surface area of the body of water is the saturation pressure at the temperature of the surface of the water is the saturation pressure at the dew point temperature of the room is the activity factor given by a table in the book, assumed to be residential activity level equating For improving efficiency, of the pools environment, these evaporation rates need to be kept at a low level at the same time maintaining a thermal comfort for the swimmers inside the water and also the pool hall space.This role is played by the air conditioning equipment. This study however aims at analysing the various parameters of thedesign of a swimming pool. The pool is an Olympic size and is located in residential location with activity Factor = 0.5. Considering these data, the other parameters such as the power load on the air conditioner, evaporation rate,etc. will be analysed (Sartori,2000) In order to clearly elaborate the energy movements in a swimming pool, we consider the figurebelow. The figure describes the flow of energy in the indoor swimming pool. It also shows the transfer modes that might occur in such a system. Fig: energy balance model for an indoor swimming pool Air conditioning systems in indoor swimming pools are usually designed for theprovision of suitable thermal comfort conditions for users in the space pool hall. The indoor temperatures are kept at approximately 30 degcelcious (Mancic, 2014) The main factors contributing to energy loses of an indoor space pool hallinclude: Convection at the pool surface Qconv, evaporation at the pool’s surface QEvap, radiation from surface of pool, heat loss as a result of fresh water into the pool and heat flow rate from heating Qaux (Whitman et.al,2000) In the calculations, we will neglect the losses at the pools walls. The convection from thesurface of water is a function of the temperature of the air in the space pool hall. It will be assumed that the pool water is perfectly mixed and that the fluid is incompressible. The water density and its conductivity are considered constant (Honig,199). The temperature change of the pool water over a time t may be calculated by the formula: CpwVpdT/dt=Q’-* Also, the heat loses will be proportional to the flow of water evaporated from the water surface. Qevap=Apqevap=ApEr Where E’ represent the mas flow rate of the water that has evaporated and ris the latent heat of evaporation. The heat transfer through convection may be written as; Qconv=Apqconv=Apα (Tw Tair) Where α represent convective heat transfer coefficient, Tairis the indoor air temperature in hall of the pool and Twis the pool water temperature. Analysis of results Test Site The test site was an Olympic-sized swimming pool. Below is a list of the parameters obtained: 1. Pure Water (no chlorine, diatomaceous earth, other chemicals, etc.)- 2. Standard Olympic Size Swimming Pool; length 50m by width 25m- area of water surface=50 x 25=1250m^2 3. Pool is in a residential location; Activity Factor = 0.5 4. Water surface temp is equal to room temp of 30 degrees Celsius 5. Outdoor temperature/relative humidity (state two conditions) is assumed to be average dry bulb temperature of the metro Detroit area in Michigan, during the month of July on a mid-summers day approx. 1:30 PM 6. Standard air conditioning process where the mass of air entering the duct is equal to themass of air leaving the duct 7. Velocity of air is assumed to be zero. The air circulation will only be due to the convectional currents resulting from evaporation. The air conditioner analysis The figure below is a schematic flow diagram describing the air conditioning process. The air conditioner simply monitors the incoming as well as outgoing air. The basis of analysis is the content of moisture in the leaving as well as the entering air. The conditioner analysis will enable us approach on the calculation of therate of evaporation as well as the load exerted on it by the water vapour (Reynolds, 1977) Fig: air conditioning flow diagram According to the 2011 ASHRAE Handbook of HVAC Applications, the formula for computing the evaporation rate is given as; (Eq. 1) Where: is the evaporation rate at the surface of the pool, in kg/s A is the surface area of the body of water is the saturation pressure at the temperature of the surface of the water is the saturation pressure at the dew point temperature of the room is the activity factor given by a table in the book, assumed to be residential activity level equating to 0.5 At the entrance of the air conditioner (state 1), T1 = 30ºC and Tdp = 21ºC The values of the relative humidity, humidity ratio and the specificenthalpy can thus be found using the Psychrometric Chart. The values that were obtained are as below: Considering the temperature at exit, and from table A-2 at 30ºC, Similarly, from table A-2 at 21ºC, the conditions for the air exiting the air conditioning duct can be generalized are assumed to be constant under the conditions that the system is operating in Detroit, Michigan during the month of July on a mid-summers day at approximately 1:30 PM. These conditions are however assumed as: T2 = 35ºC Relative humidity, humidity ratio and the specific enthalpy for state 1 are found; Below is the psychometric chart showing the values of each state. Fig: showing the obtained values To determine the evaporation rate, we substitute all the found and given values to the equation. This rate of evaporation is usually proportional to the airpower rating of the air conditioner. The mass flow rate of the air entering will be equal to themass of the air at exit. This value can also help in determining of other parameters such as the radiation rates and total energy loss of the system. Therefore, it can be found by dividing the mass flow rate of water by the difference of the humidity ratios at the entrance and exit of the duct as shown below. Determining the load on air conditioner, we will have to substitute all the previous calculated and found values into the equation below: (Eq. 2) Finally, the heating load on the air conditioning unit is calculated as: From the obtained data, the evaporation rate is determined. This rate however relates to the amount of water vapour content in the surrounding environment. These data can be used for the monitoring of the swimming pools conditions as well as design. The heating load on the air conditioner is somehow high. This value however indicates the power in terms of heat it dissipates to the swimming pool in order to maintain the required conditions.The analysis does not consider the water chemistry since the water is pure. This means that there is no gas evolved from the water. There is aneed to reduce the workload on the air conditioner in order to save on the power consumptions. This will include applying some design modifications on the swimming pools construction. The base will need to be constructed using a material with low heatconduction. The upper roofing should also use a material that allows more heat that it can lose. Reflective material on the outside surfaces can also apply especially during thesummer. Conclusion From this study, it is clear that theevaporation rate for any water body can be determined. It is clear that this rate of evaporation increase with an increase in surface area and other parameters such as the surface area.The results also depict that most of heat from a swimming pool is lost through evaporation with a small percentage lost by radiation and other forms. To attain a conducive environment inside a swimming pool, designers will need consider all these parameters. The air conditioning facility works to balance the inside humidity and maintain temperature. From this study, it is also noted that the air conditioning mechanisms vary depending on weather the pool is open or closed or depending on weather the surroundings is on a residential location or not. Lowering the energy consumption of a swimming pool is necessary and this can be achieved by properly analysing the parameters in this study. For instance, the load on the air conditioner can be reduced by use ofa certain kind of materials for some of its structure designs. However, climatic considerations should be put in account. For instance, the most operative solution of removing excess moisture, dehumidification exhaust moisture is more effective during summer when the dehumidification performance of the supply exhaust system of ventilation is not using the swimming pool when the ventilation’s drying capacity is insufficient. These considerations will greatly reduce energy consumptions of the swimming pool. This paper has reviewed on the analysis procedure for the calculations of solar evaporation rates of free water surfaces and this phenomenon not only applies to swimming pools but may be used to analyse other water bodies. Conditioning of an house works in similar principles, but some situations may vary. References Benabed, Mustapha. A Numerical Investigation Of New Film Cooling Hole Configuration At The Leading Edge Of Asymmetrical Turbine Blade: Part A. Heat Mass Transfer 49.4 (2012): 497-508. Web. Sartori, Ernani. A Critical Review On Equations Employed For The Calculation Of The Evaporation Rate From Free Water Surfaces. Solar Energy 68.1 (2000): 77-89. Web. Honig, Jurgen M. Thermodynamics. San Diego: Academic Press, 1999. Print. Whitman, William C, William M. Johnson, and John Tomczyk.Refrigeration & Air Conditioning Technology. Albany, NY: Delmar Publishers, 2000. Print. Reynolds, William C, and Henry C. Perkins.Engineering Thermodynamics. New York: McGraw-Hill, 1977. Print. Mancic, Marko et al. Mathematical Modelling And Simulation Of The Thermal Performance Of A Solar Heated Indoor Swimming Pool. THERM SCI 18.3 (2014): 999-1010. Web. Read More