Modeling of a Cross Flow Heat Exchanger

Cross Flow Heat Exchanger Experiment

By

Andres Rodriguez

Abstract

The design project of the group was to create an experiment for Thermo/Fluids Laboratory. The experiment will help aide the students in relating pressure drop and heat transfer in a cross flow heat exchanger. The experiment consists of three cross flow heat exchangers (bare inline, bare staggered, and finned staggered) that will cool hot water with current fan in the Laboratory. Optimal flow rates, pressure and temperature drops were calculated for each case. Currently, a Lab View system is in process of being completed that will verify calculations made and give the students a better understanding of what is happening. So far there is one heat exchanger complete and two in processes. The heat exchanger will be operated in an ambient environment along with the attachment ducts.

Description of Work

This semester began with attempting to sort out the instruments purchased from Mamac Systems. Last semester's order was reviewed in greater detail and it was found that the instruments were incorrectly order due to many factors that involved an unclear website and an inexperienced student, which led to purchasing sensors without transducers that output a voltage. The majority of the equipment will have to be return and replace.

The drawings of the shell of the heat exchanger (shell of exchanger is the same for each configuration, and only vary in way the tubes are aligned) were sent out to Northridge's Plant Management and after a few meetings to clarify drawings the shell of the inline configuration was created. The shell was made without the through

holes needed for the tubes, which was done by the group in the laboratory. Twenty eight 14" copper pipes were cut and solder to 90 degree elbows, which were modified to fit our arrangement. A ј" of the face of the elbows were removed and connected with couplings made from the extra copper pipe left to another modified elbow.

The analysis of the effectiveness/NTU method described in chapter 13 (section 5) of the Yunus Ð--engal's Heat Transfer book was revisited to find another temperature drop for the inline configuration with the bare tubes.

Theory

Extended surface theory

One of the heat exchangers that are to be manufactured is a staggered configured, finned tube heat exchanger. Fins are extended surfaces used specifically to enhance the heat transfer rate between a solid and a fluid. The 2 ways that are commonly thought of when it comes to increasing heat transfer are increasing the heat transfer coefficient which could be done by increasing the fluid velocity, but this is directly related to the operating costs of the blower or pump required to increase the velocity.

Another option is and which can be achieved relatively easier, is by increasing the surface area across which the heat transfer occurs, and this is achieved by adding fins, which in this project, are circular fins on the tubes. This method is also relatively more cost effective then the other ways to increase heat transfer.

Before the finned tubing is purchased, it was thought to be prudent to calculate certain parameters and gauge the feasibility of purchasing the fins, and also to realize the effectiveness of the finned tubing, with the configuration that that heat exchanger has.

The finned tubing dimensions and cost are as follows:

 Copper Fin height 3/8" x 0.015''

 6 fins per inch

 15" over all length with 2'' bare ends

 $22.00 per tube- (lowest cost from research)

*There are thirty of these tubes required for the heat exchanger

Fin efficiency can be defined as:

Q dot fin = Actual heat transfer rate from fin

Q dot fin max Ideal heat transfer rate from fin

The equations used to calculate a fin efficiency were :

= ( L+t/2) (h/kt)^1/2 , (r2+t/2)/r1

Where :

L= length of fin, t= thickness of fin, k= thermal conductivity of fin material (copper), h= heat transfer coefficient, r2 = outer radius, r1= inner radius

Once these 2 values were obtained, they were used in conjunction on a fin efficiency chart found in the text book 'Heat Transfer, by Cengel and Yunus' to read of a fin efficiency.

A fin efficiency of 90% and above is considered a good rule of thumb and the fin efficiency we obtained was close to 95%.

An area of concern when it comes to using finned tubing is the possibility of a large pressure drop within the core of the heat exchanger. This could affect the performance of the fins and the heat exchanger itself. Also in addition, the power required to move the fluid across the bank of tubes is a considerable operating expense and is directly related to the pressure drop.

The book on compact heat exchangers by the authors Kays and London, was referenced to compute the pressure drop across the heat exchanger core.

The original equation used for the pressure drop obtained from the book of compact heat exchangers was:

 P == G2 v1 {(1+2)[ (v2/v1 ) -1} + f ( _A_ )

2gc Ac

Variable definitions:

G- mass velocity, v1= specific volume at entrance

f- friction factor (obtained from graph)

v1= specific volume of air at entrance of core

v2= specific volume of air at entrance of core

ratio of