An INTRODUCTION TO HVAC - Basics of Heating Ventilation and Air conditioning

An INTRODUCTION TO HVAC

In this post we are going to discuss about the Basic Modes of Heat Transfer, Sensible Heat and Latent Heat.

Heat transfer is described as, energy on the move because of temperature distinction. Heat transfer takes place at whatever point there is a temperature difference inside a system or at whatever point two systems at various temperatures are brought into warm contact. Heat, which is energy on the move, can't be estimated or noticed straightforwardly, yet the impacts created by it tends to be noticed and estimated. Since heat transfer includes transfer as well as change of energy, all hotness transfer processes should submit to the first and second laws of thermodynamics. Anyway, dissimilar to thermodynamics, heat transfer handles systems not in warm balance and utilizing the heat transfer regulations it is feasible to observe the rate at which energy is moved because of hotness transfer. According to the specialist's perspective, assessing the pace of hotness transfer is a key prerequisite.

Heating ventilation and Air Conditioning includes heat transfer;

            Subsequently a decent comprehension of the basics of hotness transfer is an absolute necessity for an understudy of Heating ventilation and Air Conditioning. This article refers to a short audit of hotness transfer applicable to Heating ventilation and Air Conditioning. Commonly, hotness transfer happens in three distinct modes: conduction, convection and radiation. In the majority of the designing issues heat transfer happens by more than one mode at the same time, i.e., these hotness transfer issues are of multi-mode type. We will now see all these three types of Heat transfer modes in detail.

Heat transfer mode by Conduction:

Conduction is a method of transfer of heat within a body or from one body to the other, due to the transfer of heat by molecules, vibrating at their mean positions. The bodies through which the heat transfers must be in contact with each other. There is no actual movement of matter while tranferring the heat from one location to the other.

            Conduction heat transfer takes place whenever a temperature gradient exists in a stationary medium. Conduction is one of the basic modes of heat transfer. On a microscopic level, conduction heat transfer is due to the elastic impact of molecules in fluids, due to molecular vibration and rotation about their lattice positions and due to free electron migration in solids. The fundamental law that governs conduction heat transfer is called Fourier’s law of heat conduction, it is an empirical statement based on experimental observations and is given by:

In the above condition, Qx is the pace of hotness transfer by conduction in x-heading, (dT/dx) is the temperature inclination in x-heading, A is the cross-sectional region ordinary to the x-bearing and k is proportionality constant and is a property of the conduction medium, called thermal conductivity. The '- ' sign in the above condition is an outcome of second law of thermodynamics, which expresses that, in an unconstrained interaction heat should continuously move from a high temperature to a low temperature (i.e., dT/dx should be negative). The thermal conductivity is a significant property of the medium as it is equivalent to the conduction heat transfer per unit cross-sectional region per unit temperature value. Thermal conductivity of materials differs fundamentally. For the most part, it is exceptionally high for unadulterated metals and low for non-metals. Thermal conductivity of solids is for the most part more prominent than that of liquids. Below Table shows run of the typical thermal conductivity values at 300 K. Thermal conductivity of solids and fluids shift mostly with temperature, while Thermal conductivity of gases rely upon both temperature and tension. For isotropic materials the worth of thermal conductivity is same in all dircetions, while for anisotropic materials like wood and graphite the worth of thermal conductivity is different this way and that. In Heating ventilation and Air Conditioning conductivity materials are utilized in the development of heat exchangers, while low thermal conductivity materials are expected for protecting refrigerant pipelines,refrigerated cupboards, building dividers and so forth..

Heat Conduction General Equation:

            Fourier's law of hotness conduction shows that to appraise the hotness transfer through guaranteed mechanism of known thermal conductivity and cross-sectional region, one requires the spatial variety of temperature. Furthermore the temperature anytime in the medium might differ

with time too. The spatial and fleeting varieties are gotten by settling the hotness conduction

condition. The hotness conduction condition is acquired by applying first law of thermodynamics

also, Fourier's regulation to an essential control volume of the leading medium. In rectangular

arranges, the overall hotness conduction condition for a leading media with steady

thermo-actual properties is given by:

In the above condition,  is a property of the media and is known as Thermal diffusivity, qg is the pace of hotness generation per unit volume inside the control volume and τ is the time.

Heat transfer mode by Convection:

Convection is the mode of heat transfer which occurs mostly in liquids and gases. In this method, transfer takes place with actual motion of matter, from one place within the body to the other. Often when we boil water we have seen bubbles, and currents develop in the water on careful observation. 

        Convection heat transfer happens between a surface and a moving liquid, when they are at various temperatures. From a severe perspective, convection is certainly not an essential method of hotness  transfer as the heat transfers from the surface to the liquid comprises of two systems working all the while. The first is energy transfer because of sub-atomic movement (conduction) through a liquid layer adjoining the surface, which stays fixed regarding the strong surface because of no-slip condition. Superimposed upon this conductive mode is energy move by the naturally visible movement of liquid particles by goodness of an outside force, which could be created by a siphon or fan (constrained convection) or produced because of lightness, brought about by thickness slopes. At the point when liquid streams over a surface, its speed and temperature contiguous the surface are same as that of the surface because of the no-slip condition. The speed and temperature far away from the surface might stay unaffected. The locale wherein the speed and temperature shift from that of the surface to that of the free stream are called as hydrodynamic and thermal limit layers, individually. liquid with free stream speed U∞ streams over a level plate. In the area of the surface , the speed will in general fluctuate from nothing (when the surface is fixed) to its free stream esteem U∞. This occurs in a thin district whose thickness is of the request for ReL-0.5 (ReL = U∞L/ν) where there is a sharp speed angle. This thin area is called hydrodynamic limit layer. In the hydrodynamic limit layer area the inertial terms are of same request extent as the thick terms. Comparatively to the speed slope, there is a sharp temperature slope in this area of the surface if the temperature of the outer layer of the plate is not the same as that of the stream. This district is called warm limit layer, δt whose thickness is of the request for (ReLPr) , where Pr is the Prandtl number, given by:

Heat transfer mode by Radiation:

            Radiation is another form of heat transfer. It does not require any medium and can be used for transfer of heat in the vacuum as well. This method uses Electro-Magnetic waves, which transfers heat from one place to the other. The heat and light from the sun in our solar system reach our planet (Earth) using radiation mode of transfer. Infact, radiation is the most potent method of heat transfer . For instance we can, consider our real life example. During winters, when we sit near a fire we feel warm without actually touching the burning wood. This is possible by radiation only.

Sensible Heat:

            Sensible heat is when energy is moved as hotness to an article, changing the temperature

in any case,but not its state. On the off chance that you can gauge the temperature of the hotness, it is reasonable. A body (strong, fluid or gas) of mass m and explicit hotness c is warmed to change its temperature from T1 to T2 without impacting its state. Without a doubt, the volume or the strain of the body is unaltered. The energy got by the body liable for its climbed temperature is given

by the connection:

Q = m*c*(T2-T1) in joules

Q = m*c*(T2-T1) 1055,06 in BTU

Latent Heat:

        Rather than sensible heat, Latent heat is defined as the energy delivered or ingested that changes the condition of a body during a steady temperature process. This cycle leaves temperature

unaffected - it will not get sequential. The most widely recognized types of Latent heat are Fusion and vaporization.