This final segment of our chiller barrel series contains guidelines for replacing these gaskets.
Cut a disc from suitable material.
To obtain the dimensions for this disc, you can use the old gasket, if it is in usable condition. If not, you will have to measure on the inside of the head, move 1/4 in. from the inside of one bolt hole, and measure 180 degrees across to the same point on another bolt hole. This is your diameter that must be transferred to a sheet.
Do this using a large compass. If you do not have a compass, you can make one using a ruler, yardstick, or straight edge. Take a ballpoint pen and a white paint marker, tape them to the yardstick, and use this as your compass. Center the ballpoint pen and mark your diameter with the paint marker. Two ballpoint pens can be used if necessary, but lines are much harder to see.
Lay the material on a flat surface.
A piece of cardboard on a bench or the floor works well. Using a utility knife with a new blade, make your first cut along the outside diameter mark you have laid out using light pressure. Follow along the same line. On the second cut the blade will pass through on 1/8-in. material; 3/16 in. will take three cuts and 1/4 in. will take four. Work your way around the disc cutting one-fourth of the diameter at a time.
Once you have a disc, you must now lay out the partition webs; if you have a gasket in good condition, you can lay it on top of your disc and trace the webs with the paint marker.
If you do not have a usable gasket, there are two alternatives to transferring the pattern to the disc. In the first method, put a good coat of oil on the head. Then, holding the gasket very tightly at one edge, roll it down over the head and press on the backside of the gasket. Peel the gasket back up without allowing it to slip, and you will have a pattern to cut along.
In the second method you again hold the gasket, but now you push firmly on the back of the gasket with your thumbs along the inside diameter of the head and on both sides of the partitions.
In both cases you must make sure that the disc does not slip while you are getting the pattern. Always make sure that you keep the disc centered when patterning. Both methods can be done with the head in a vertical position, but are much easier to do with the head lying flat, and should be done with two people.
Once you have your pattern, cut out the sections of the gasket using the cardboard and utility knife. When you come to the corners be very careful, one slip and you will be starting over. Always cut away from the corners.
Gasket replacement
1. Ensure that all the refrigerant has been recovered and that the vessel has been isolated. There is no need to drain the barrel.2. Carefully remove insulation to expose bolt heads. Cut carefully and mark the position of insulation so that it can be reused.
3. Cut or unsweat liquid and suction lines as necessary for removal of heads. Measure line sizes and get slip couplings.
4. Put witness marks at 12 o’clock on the tube sheet and on the head to ensure proper alignment during reassembly. Support the head and remove all but two bolts. Carefully remove the final two bolts. Be careful that the head does not slip down or swing out. Caution: Heads are very heavy; handle with extreme care.
5. Once the head is secured to where you can safely work on it, peel off the old gasket. Scrape the remaining gasket material and adhesive from the head, being careful not to gouge the head.
A wire brush on an electric grinder works well for removing the old adhesive. Clean gasket surfaces on the head and tube sheet with acetone or approved solvent and clean rags. Clean the gasket with solvent until the surface is shiny black. Allow to dry.
6. After you ensure that you have the correct gaskets, apply a thin (0.002-in.), uniform coat of approved adhesive to the head and one side only of the gasket.
7. Carefully place the gasket on the head. This is best done with two people, since the gasket should not be moved once it has been placed. Ensure that the gasket’s webs are centered on the partitions.
Allow the adhesive to cure for at least one full hour. Cut out unused dividers from the gasket after curing.
8. Guide the head into place, remembering to put couplings on liquid and suction lines. Be careful not to disturb the gasket. Tighten the bolts until the gasket is just touching the tube sheet all the way around.
9. Torque the bolts in a criss-cross pattern to value A, then value B, and value C. Torque once more to the final value. Do not tighten the bolts any further.
10. Rebraze all connections using 45% or higher silver-content solder and the proper flux for steel-to-copper joints, and 15% for copper to copper. Charge the evaporator to 150 psig with nitrogen and check for leaks using a leak reactant.
If a gasket leak is found, torque the bolts 10 ft/lb higher and recheck. Evacuate to 1,500 microns, break the vacuum with nitrogen, and evacuate to 500 microns. You can now recharge and start the chiller.
11. Reinsulate. You can use the leftover adhesive for this.
Replacement procedure for grooved heads/tube sheets
1. Ensure that all refrigerant has been recovered and that the vessel has been isolated. There is no need to drain the barrel.2. Carefully remove insulation to expose bolt heads. Cut carefully and mark the position of insulation so that it can be reused.
3. Cut or unsweat liquid and suction lines as necessary for the removal of heads. Measure line sizes and get slip couplings.
4. Put witness marks at 12 o’clock on the tube sheet and on the head to ensure proper alignment during reassembly. Support the head and remove all but two bolts. Carefully remove the final two bolts, being careful that the head does not slip down or swing out.
Caution: Heads are very heavy; handle with extreme care.
5. Once the head is secured to where you can safely work on it, peel off the old gasket. Scrape the remaining adhesive from the head (a 1/4-in. wood chisel works well for this), being careful not to gouge head.
Remaining adhesive can be removed using 3M or CRC automotive brake cleaner. On smaller units, use just the solvent. Clean the tube sheet with brake cleaner. Clean the gasket with solvent until the surface is shiny black. Allow it to dry.
6. Verify that you have the correct gaskets. Cut out any unused gasket dividers, cutting along the lines of the webs and the inside diameter of the gasket so there is no overhang on the groove. Apply a thin (0.002-in.), uniform coat of approved adhesive in the groove of the head and one side of the gasket.
Carefully place the gasket into the groove, starting from the outside at the top and bottom and working towards the center, then the partitions on the side. Once the gasket is in place, run a small, flat-blade screwdriver between the wall of the groove and the gasket to make sure that the gasket is seated in the groove. If the gasket is not fully seated it will probably leak. Allow the adhesive to cure one full hour.
7. Guide the head into place and align it with the tube sheet. This is critical, and the head must remain aligned and fully supported until all bolts are completely tightened. Be careful not to disturb the gasket.
Tighten the bolts until the gasket is just touching the tube sheet all the way around. Torque the bolts in a criss-cross pattern to value A, then value B, and value C. Torque once more to the final value. Do not tighten the bolts any further.
8. Rebraze all connections. Charge the evaporator to 150 psig with nitrogen and check for leaks using a leak reactant. If a gasket leak is found, torque the bolts 10 ft/lb higher and recheck. Evacuate to 1,500 microns, break the vacuum with nitrogen, and evacuate to 500 microns.
You can now recharge and start the chiller. Reinsulate using the leftover adhesive.
