Cold storage and freezer applications
By Paul Rohrs
Radiant Expert
There is a very real need for primary secondary piping and hydronic applications in the refrigeration and freezer industry. These applications are not common, but when they employ a waste-heat recovery principle, they can reduce our carbon footprint. This is an application where we do not need to put a fire in the belly of an appliance that burns fossil fuel, because we are going to extract heat from the freezer's refrigeration equipment.
These types of projects fall into the "Green Build" category that seems to be all the rage lately. Let's review one such recently completed project and discuss some of the finer points.
Universal Cold Storage is a 43,000 sq/ft (floor space) roll-in cold storage freezer facility. The objective of this project was to condition the soil below the freezer floor so that it wouldn't freeze. We didn't want an active zone of radiant floor heat fighting against the refrigeration process, but floor heat and hydronics are key for the long-term protection of the slab. If the soil froze and heaved it could fracture the structural slab where all of the frozen product is stored and transported via forklift. We are maintaining the soil temperature with a designed 20 delta t with 60 F supply and 40 F return and constant flow.
While the refrigeration process produces the necessary cooling for the freezer, heat is a natural byproduct of that process. We recover this heat with a brazed-plate heat exchanger that sends the accumulated heat from the ammonia lines on the upstream side of the exchanger to the downstream side, which has a 50% propylene glycol solution pumping thru the PEX tubing below the slab. No carbon-based fuels are being burned, which means "green," and plenty of it. Not only is the system efficient but it is also environmentally conscientious and reduces the cost of doing business.
The process began as the site was cleared; the demolition crew cut out the existing 6" slab and removed a total of 20" of grade. The reconstruction process started with 3" of sand. We attached our tubing directly onto the grade, which did vary on occasion. After 23,000 linear feet of tubing was installed and pressure tested, the general contractor poured a 3" non-structural concrete cap over it so that the tubing was protected, and so that they could begin some light duty traffic with scissor lifts and forklifts. As the project progressed, 6" of high-density insulation was installed and then completed with an 8" structural slab.
The basic components of the system were pretty straightforward. We used 3/4" tubing installed on 24" centers and piped back to two pairs of 23 loop manifolds that were located in the freezer bay itself. We then penetrated the insulated wall panel into a dry-storage bay that would be home to all of the mechanical equipment. We used a B&G quantek circulator that will be pumping 86 gpm at 63 ft/head. The heat that is extracted from the brazed plate HX is being fed by a PL55 in a primary secondary piping arrangement. The manifolds are 1 1/2" but are piped as a reverse return (first in - last out) arrangement that transitions to a 2 1/2" primary loop. The PL55 is piped with closely spaced tee's in 1 1/2" and is piped 25' vertically, 140' horizontally where it penetrates the roof and ties in to the HX. The upstream side of the heat exchanger utilizes the ammonia lines used for refrigeration and is a double-walled unit.
Although this project features a DDC (direct digital control) system that had the ability to control the equipment, I wanted constant flow in the primary pump and secondary pumps as the freezer called for 0 F temperatures. A 50% propylene glycol solution provided freeze protection to -20 F, but we also wanted some visual indicators in the mechanical area so that, if a pump eventually went down, we would have a red light come on so that a passerby could be the last line of defense.
Instead of a paddle-type flow switch, we used a Johnson Control P74 Series differential pressure switch. If we lost flow, the differential pressure switch contacts would then become NC and signal the red light. The control company took those same contacts back to their control room to indicate a loss of flow.
Flow switches have been around for quite a while and are available with many different types of flow rates and characteristics in mind. The differential pressure control serves this application well, as I can dial in the correct differential pressure via the pressure gauges, and it can be fine-tuned a little more readily. Flow switches have paddles that are immersed in the flow stream; it is typical to adjust the switch with a sensitivity adjusting screw.
This application utilizes equipment that is environmentally sound and will reduce energy costs for the life of the system. Universal Cold Storage began filling the nearly 8,000 pallet positions on May 1. This new facility, with a little more than 1.5 million cubic feet of space will be at capacity between August 1 and September 1. There is an obvious need for these types of facilities, so be prepared for projects like this in your area and note that there are many of these applications currently up and running. Even though it is not our normal everyday radiant project, primary secondary piping and hydronics can play a significant role in freezer and refrigeration applications.