Lab # 1 Exchange principles con- and counter-current systems Michael Axelsson University of Gothenburg Department of Zoophysiology 1
INTRODUCTION Life depends on the exchange of mass and energy so a constant and permanent transfer of molecules such as gases (oxygen and carbon dioxide), ions (protons, cations like sodium, potassium and calcium or anions like chloride and bicarbonate) and energy (heat) is taking place between organisms and their environment. Mass transfer occurs by molecular diffusion while heat transfer occurs by conduction, convection and radiation. The flow of either mass or heat depends on the properties of the media where exchange is taking place, the difference in temperature or concentration and the distance. Because of this, mass and thermal exchange between an animal and its environment will always occur. Such ubiquity constitutes an important selection pressure towards the appearance of specialized exchange structures designed to avoid or improve the exchange of heat or molecules with the environment. The following are examples of exchange structures designed to avoid loss of heat or molecules: Vascular design of blood flow in the appendages of aquatic animals or animals living in cold climates to avoid heat loss Vasa recta in kidneys to supply nutrients to the inner medulla without removing the osmotic gradient Vascular heat traps that allow the maintenance of specific organ systems at temperatures above the temperature of the rest of the body of the animal Airway structure in desert animals to avoid excessive evaporative water loss The following are examples of exchange structures designed to facilitate heat or mass transfer: Salt glands in birds and reptiles that aid in the excretion of excess salts (mostly NaCl) Rete mirabile in fish aided to filling the swim bladder with oxygen at high pressures Fish gills that uptake oxygen from water and download carbon dioxide 2
Brain cooling mechanisms associated with respiratory passages in some birds and mammals Extensively vascularized and expanded respiratory epithelia in tetrapods to allow the exchange of respiratory gases With the exception of the last example, all others are based on a common scheme that has appeared recurrently in the evolution of many animal groups: the countercurrent exchanger. The purpose of this laboratory exercise is to study the basic function of the countercurrent exchanger and to compare it with its alternative arrangement: the concurrent exchanger. The system used for this lab is an obvious simplification of the anatomical arrangements encountered in nature and it is based on two copper tubes soldered together along its entire length (1 m). When the water inside the tubes flows in opposite directions, we talk about countercurrent exchange. When water flows in the same direction we talk about concurrent exchange. DESCRIPTION OF THE SYSTEM The actual system is located in the Department of Zoology at the University of Göteborg but it can be controlled remotely from any location in the world. A camera placed over the system allows direct visualization of the actual lab setting. Below is a schematic drawing of the setup with numbers that will be referred to in the text. Thermostated baths (1) are used to simulate two different 3
temperatures; in this case one bath is set to 5 C and the other to 37 C. Two peristaltic pumps (2) are used to pump water from the two baths through the heatexchanger model (3, see below for more details). The temperature in the exchanger is measured at 4 locations along each of the two tubes in the heat exchanger model (at 4 cm, 36 cm, 65 cm and 96 cm respectively) plus at the very entrance at three locations. The temperature is measured by an interface box (4, PICO TC08 thermocouple interface) and the information is entered into a computer (6). The computer controls the two pumps via a control box (5) and also sends an alarm signal to an external alarm box (7) every time a person logs on remotely. The computer records the temperature along the exchanger and controls the two peristaltic pumps (see below for more information about the program). OPERATION OF THE SYSTEM If you are going to run the lab from a remote location (any location outside the lab were the equipment is setup) you need a computer with access to the Internet and a minimum connection speed of 126 kbits/second (ISDN or higher). Log on to the following address http://vivaldi.zool.gu.se/labview_www/index.htm. A window looking like the picture to the right will show. On this page there are links to different things that you will need during the lab. Heat exchanger is the link to the program that runs the lab. If you 4
click on this link another window will open. If this is the first time you log on it will take some time before the program-window is shown (up to several minutes). The first time you log on the program automatically downloads two files from National Instruments, first the run-time module that let you run the program through the web browser, the second is a plug-in that let your web browser show the program. During this process a window will be shown asking if you would like to install the run-time module, just click the Yes button. When the run-time module and plug-in are installed the program will be shown (as in the figure below) and you can see the temperature in the heat exchanger. In the window that opened (and also in the initial window), there is a link to a web-camera that will allow you to see the lab. You can control the camera via you web browser. The are several preset positions that you can use to see different areas of the lab, and you can also use the PAN, TILT and ZOOM functions to navigate the lab yourself. There is also a link to a chat that will help you to communicate with other students that will be 5
logged on at the same time (if you are doing the lab alone or if all people in the lab-group is present in front of the same computer you will not need this). When you activate the chat you will be asked for a login name and then the window with the chat will open. OPERATION OF THE PROGRAM To run the lab you need control of the setup, one person at the time can be in control and to gain control you right click on the program-window and select Request control of VI (VI stands for Virtual Instrument) in the window that pops up. You will then be granted control unless somebody else has control of the program at the moment. If so, you will be placed in a queue. When you have control you can use the mouse to click on the various control buttons (see description of the program below). In the top part of the program, information about the pumps are shown, in the case below both pumps are on and pumping and the direction of warm and cold water through the exchanger will be indicated by arrows (blue for the cold and red for the warm water). In the middle the temperature data are shown. The thick lines/dots indicate the temperature at each measuring point along the heat exchanger while the thin dotted lines indicate the temperature of the water entering the heat exchanger. There are four different tabs that you can select, Time diagram, Temperature diagram Pump settings and Logg, this will be explained later. In the lower part there is one button Save data, it is used to 6
start or stop saving data to the hard disk. At the right information about the time to the next measurement is shown; this is set to 5 seconds between each measuring point. In the next tab Temperature diagram the temperature data is shown inside 4 circles that represent the 4 measuring points along each tube and the flow direction is indicated by the large arrows or in the case when one or both pumps are off a white area and the text Pump X is off The temperature of the water going in to the heat exchanger is also shown outside of the corresponding arrow. When you click on the button Pump settings a window will show with the controls for the pumps as shown in the figure to the right. You can turn on and off the pumps individually, change the speed of each pump and change the flow set-up between counter current and con current. The speed of the pumps can be set to 1, 2 or 3 7
corresponding to low medium and high flow. Once you finished setting the pumps you close the window by clicking Close window. Below is s schematic drawing showing how the water (cold in blue and warm in red) is pumped through the exchanger during counter-current and con-current setup The fourth tab Log shows a list of the systems that are logged on to the computer. Every time someone logs in an alarm in the lab is set of and a green small windows saying New user logged on shows for 5 seconds in the lower white field next to the Save data button. In the Log tab there is also a button called Get attention that when pressed sounds an audio/visual alarm in the lab to get attention from the person that is locally responsible for the lab. This alarm is to inform the person responsible for the lab that somebody has logged in so that we can make sure that everything is working in the lab and we can also join the chat to help you with any problem. You can then follow the development of the temperature either by looking at the Time diagram or Temperature diagram. 8
Saving data When the temperature readings have stabilized i.e. there is not further changes you should save a short period of data that will be used for the calculations afterwards. To start saving data to the hard disk click on the Save data button now every 5 seconds one set of temperature data will be saved. After 1-2 minutes you can stop saving data by clicking on the same button that now reads Stop saving. You have to download the data to you local hard disk (or floppy) for later processing and to do this you right click on the diskette symbol to the left of the program and select Save Target As in the pop-up window, this will open up a window and you can now select were on your local machine you want to save the file and what you would like to call it. By default this file is called DATAFILE.TXT and you should rename it (give it a name that indicate what type of experiment you were running and speed of pumps and so on) The data files that you have saved are plain ASCII files and can be open in Excel or any other program that can open ASCII (text) files. To open the files in Excel go to Files > Open and then to the sub directory were you have saved the file and then in the tab File of types select All files *.*, this is important otherwise Excel only looks for its own files with the extensions.exl. 9
DESCRIPTION OF THE LAB EXERCISE When reading about countercurrent exchangers in textbooks the first concept that usually appears is that countercurrent exchangers are more efficient than concurrent exchangers. Think about this and come up with a way to quantify the exchanger efficiency. Efficiency is usually expressed as a percentage. Try to calculate the efficiency using the temperature data you obtain after running the heat exchanger in the countercurrent and the concurrent modes. When running the heat exchanger, wait until the temperature values stabilize. You can keep track of this using the Time diagram. Once the temperatures stabilize, write the numbers down in your notebook, plot them in an Excel graph and compare both exchange modes. Or use the Save data function and download the data file to your computer Now that you have done this exercise, try to answer the following questions: 1. Did you achieve a 100% efficient exchange in any mode? 2. Can countercurrent exchange have an efficiency of 100%? How? 3. Can concurrent exchange have a 100% efficiency? How? Flow through the exchanger tubes can be changed as specified in the previous section. Do you think that flow can affect efficiency? Try to make a prediction before running the exchanger at lower or higher flows and verify if you prediction is right. Describe how efficiency varies with alterations of flow. Plot your data in graphs to demonstrate your results and discuss the possibility that efficiency in counter current exchangers is similar to the efficiency in concurrent exchangers. Let s consider a real scenario, the fish gill 10
The two panels below shows one single gill lamella inserted between to Y-axis with oxygen partial pressure. The task is to draw the lines for the partial pressure of oxygen in the water and in the blood in the two different situations (countercurrent to the left and con-current to the right). The water and venous blood oxygen tension is given in the top of the figure. 11
References Illustration for this manual is from the following sources, some have been redrawn while some have just been scanned. Animal Physiology, Mechanisms and adaptions, R. Eckert & D. Randall, W.H. Freeman and Company, San Fransisco, ISBN 0 7167 1423 X Biology of fishes. 2 nd edition. Q Bone, NB Marshall and JHS Blaxter. ISBN 0 412 741140 7 Comparative Animal physiology, P. C. Withers, Suanders College Publishing, Forth Worth, Philadelphia, ISBN 0-03-012847-1 12