The function of a cooling system is to remove heat from processes or tools. Heat removed from one medium is transferred to a different medium, or course of fluid. Most frequently, the cooling medium is water. Nevertheless, the heat switch ideas and calculations discussed on this chapter can be applied to other fluids.
Environment friendly elimination of heat is an economic requirement within the design and operation of a cooling system. The driving pressure for the switch of heat is the distinction in temperature between the two media. In most cooling systems, that is in the range of 10-200 levels F. The heat flux is mostly low and in the vary of 5,000 to 15,000 Btu/ft2/hr. For exceptional circumstances such as the indirect cooling of molten metallic, the heat flux can be as high as 3,000,000 Btu/ft2/hr.
The transfer of heat from course of fluids or tools leads to a rise in temperature, or perhaps a change of state, in the cooling water. Most of the properties of water, together with the conduct of the contaminants it incorporates, are affected by temperature. The tendency of a system to corrode, scale, or support microbiological progress is also affected by water temperature. These effects, and the control of circumstances that foster them, are addressed in subsequent chapters.
Kinds of Programs
Water heated within the heat change course of will be dealt with in one among two methods. The water may be discharged at the increased temperature right into a receiving physique (once-by way of cooling system), or it may be cooled and reused (recirculating cooling system).
There are two distinct kinds of systems for water cooling and reuse: open and closed recirculating techniques. In an open recirculating system, cooling is achieved by means of evaporation of a fraction of the water. Evaporation leads to a lack of pure water from the system and a focus of the remaining dissolved solids. Water have to be eliminated, or blown down, in order to manage this focus, and contemporary water should then be added to replenish the system.
A closed recirculating system is definitely a cooling system inside a cooling system. The water containing the heat transferred from the process is cooled for reuse by way of an exchange with one other fluid. Water losses from such a system are normally small.
Each of the three types of cooling techniques-as soon as-via, open recirculating, and closed recirculating-is described in detail in later chapters. The precise approach to designing an acceptable treatment program for each system can also be contained in those chapters.
HEAT Transfer ECONOMICS
In the design of a heat transfer system, the capital price of constructing the system should be weighed towards the continued value of operation and upkeep. Incessantly, greater capital prices (extra change floor, exotic metallurgy, more environment friendly tower fill, and so on.) end in decrease working and maintenance costs, while decrease capital prices might result in larger working costs (pump and fan horsepower, required maintenance, etc.). One essential operating cost that must be thought of is the chemical treatment required to prevent course of or waterside corrosion, deposits and scale, and microbiological fouling. These problems can adversely affect heat transfer and might result in equipment failure (see Determine 23-1).
The following is an overview of the complex concerns involved in the design of a heat exchanger. Many texts are available to offer extra detail.
In a heat switch system, heat is exchanged as two fluids of unequal temperature strategy equilibrium. A higher temperature differential ends in a extra speedy heat switch.
However, temperature is barely certainly one of many factors concerned in exchanger design for a dynamic system. Different considerations embrace the world over which heat switch occurs, the characteristics of the fluids involved, fluid velocities, and the traits of the exchanger metallurgy.
Process heat obligation, process temperatures, and obtainable cooling water supply temperature are often specified in the initial levels of design. The dimensions of the exchanger(s) is calculated in response to necessary parameters such as process and water circulate velocity, sort of shell, structure of tubes, baffles, metallurgy, and fouling tendency of the fluids.
Elements within the design of a heat exchanger are related by the heat transfer equation:
Q = UA DTm
Q = fee of heat transfer (Btu/hr)
U = heat switch coefficient (Btu/hr/ft2F)
A = heat transfer surface space (ft2)
DTm = log imply temperature difference
between fluids (degrees F)
The rate of heat transfer, Q, is decided from the equation:
Q = WC DT + WDH
whereW = stream price of fluid (lb/hr)
C = particular heat of fluid (Btu/lb/degrees F)
D T = temperature change of the fluid (degrees F)
D H = latent heat of vaporization (Btu/lb)
If the fluid does not change state, the equation turns into Q = WC DT.
The heat transfer coefficient, U, represents the thermal conductance of the heat exchanger. The higher the worth of U, the more simply heat is transferred from one fluid to the other. Thermal conductance is the reciprocal of resistance, R, to heat circulate.
The total resistance to heat circulation is the sum of a number of particular person resistances. That is proven in Figure 23-2 and mathematically expressed below.
Rt = r1 + r2 + r3 + r4 + r5
Rt = complete heat move resistance
r1 = heat stream resistance of the method-side movie
r2 = heat flow resistance of the process-side fouling (if any)
r3= heat flow resistance of the exchanger tube wall
r4 = heat movement resistance of the water-facet fouling (if any)
r5 = heat circulation resistance of the water-facet movie
The heat circulate resistance of the method-facet film and the cooling water movie relies on tools geometry, movement velocity, viscosity, particular heat, and thermal conductivity. The effect of velocity on heat transfer for water in a tube is proven in Determine 23-3.
Heat circulate resistance because of fouling varies tremendously relying on the traits of the fouling layer, the fluid, and the contaminants within the fluid that created the fouling layer. A minor amount of fouling is generally accommodated in the exchanger design. However, if fouling is not saved to a minimal, the resistance to heat switch will increase, and the U coefficient will lower to the point at which the exchanger cannot adequately control the process temperatures. Even if this level just isn’t reached, the switch process is less environment friendly and probably wasteful.
The resistance of the tube to heat transfer is dependent upon the fabric of building only and doesn’t change with time. Tube walls thinned by erosion or corrosion could have less resistance, however this isn’t a significant change.
The log mean temperature distinction (DTm) is a mathematical expression addressing the temperature differential between the two fluids at each point along the heat exchanger. For true countercurrent or cocurrent stream:
When there isn’t any change in state of the fluids, a countercurrent circulate exchanger is extra environment friendly for heat transfer than a cocurrent stream exchanger. Due to this fact, most coolers operate with a countercurrent or a variation of countercurrent flow. Calculated DTm formulas may be corrected for exchanger configurations that are not really countercurrent.
Heat transfer equations are helpful in monitoring the situation of heat switch tools or the efficacy of the remedy packages. The resistance of the tube is fixed; system geometry does not change. If movement velocities are held constant on both the method aspect and the cooling water aspect, film resistance will even be held fixed. Variations in measured values of the U coefficient can be utilized to estimate the amount of fouling happening. If the U coefficient doesn’t change, there is no fouling happening on the limiting aspect. As the exchanger fouls, the U coefficient decreases. Subsequently, a comparability of U values throughout operation can provide useful information about the necessity for cleansing and can be utilized to observe the effectiveness of therapy programs.