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Calculations, design and sizing of cooling water/glycol finned coils.


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http://www.heatxchdesign.com/coolcoilshel3.php

This is the program for thermal design of cooling coils with helical fins. Cooling coils are basically fin and tube heat exchangers used in any air conditioning installations.  The cooling media is usually water from 42 F, also Ethylene or Propylene glycol water solutions of various concentrations can be used as the cooling media.  Cooling fluid runs in tubes and air runs over fins. Low coolant temperature could result in a coil fin surface temperature that is below dew point temperature of air going through finned surface of a coil, which leads to condensation of moisture in air. The condensation in cooling coils complicates design and selection of cooling coils comparing to heating coils, because we have to account latent heat which is necessary to condense moisture in air. In this case the coil is Wet. When no condensation happens during cooling down air passing through a coil, the coil is Dry. The program considers a coil -Wet, when at least one of two conditions is met: the relative humidity of air is more the 30% and/or Dew Point Temperature of incoming air is more than Entering Water/Glycol Temperature.

There are two methods of calculating heat transfer load in heat exchangers: LMTD and NTU.

The “Logarithmic Mean Temperature Difference “(LMTD) is a logarithmic average of the temperature difference between the hot and cold streams at each end of the heat exchanger. The larger the LMTD, the more heat is transferred.

LMTD method works very well for dry coil runs (one-phase steams), but it is very inaccurate for two-phase streams, in our case – air and moisture in the air, which needs to be condensed. To use LMTD method, we have to know all four temperatures for both steams. Usually we either know or assume outlet temperatures. Then we calculate LMTD with a correction factor (derived as per geometry of heat exchanger and directions of both streams), corrected LMTD or CLMTD. Mass flow of at least one stream is known. Knowing the temperature difference, mass flow and specific heat for the stream, we can calculate total heat transfer rate for the process, overall heat transfer coefficient and heat transfer area of the coil. Based on this data the program is trying to match the calculated coil heat transfer area to the given or assumed coil geometry (given heat transfer area), gradually changing outlet temperature of the stream in question in a loop until the heat transfer areas, given and calculated are matched.

The Effectiveness-NTU method takes a different approach to calculating heat exchange analysis by using three dimensionless parameters: Heat Capacity Rate Ratio (HCRR), Effectiveness (ε), and Number of Transfer Units (NTU). The relationship between these three parameters depends on the type of heat exchanger and the internal flow characteristics. 

The NTU method is useful when minimum data is provided for calculating heat transfer. In our case: the amount of moisture is to be condensed during the process and additional heat load which is necessary to do so and outlet dry and wet bulb temperatures are unknown.

First, the program calculates Heat Capacity Rate for each stream in the process. Then it selects the minimum and maximum Heat Capacity Rates for both steams. Usually the stream with a larger temperature difference has minimum capacity rate. HCRR is calculated as a ratio between minimum and maximum capacity rates. NTU is calculated as heat transfer area of a coil multiplied by overheat transfer coefficient and divided by minimum capacity rate.

Effectiveness (ε) can be derived from graphs or equations for certain types of heat exchangers using NTU and HCRR data. Heat transfer load for a cooling coil is calculated as maximum possible heat transfer load for Minimum Capacity Rate stream multiplied by Effectiveness (ε). Effectiveness (ε) value is in the range between 0 and 1.

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