Most manufacturers ship the TXV sensing bulb attached to the suction line of the coil. This is to prevent the sensing bulb capillary tube from breaking in shipment.
To properly install the coil, set it in place on the top, bottom or side of the furnace. Completely cover the TXV with cold rags or brazing gel. Braze your copper lines together and let everything cool off. Then, remove the sensing bulb from the suction line inside of the plenum or coil box and gently move it to a horizontal suction line just outside of the plenum or coil box.
You can strap the bulb to the suction line at the 10 or 2 o’clock position (see picture) and wrap it with insulation tape. Make sure that the capillary tube is not kinked or damaged. You should not have the sensing bulb of a TXV in the air stream of a furnace or air handler.