May 1996 Volume 2 Issue 9
USING REFRIGERATION SUPERHEAT
Large refrigeration systems generate large amounts of heat. For these systems to operate properly, this heat must be disposed of. In most cases it is simply discharged from the system’s condenser into the environment. Sometimes, however, recovering part of this wasted heat and using it to displace purchased fuel makes economic sense. In this issue of $mart Energy User we explore that possibility.
WHAT IS SUPERHEAT?
The mechanical compressor in a refrigeration system takes in cool, low-pressure refrigerant gas from the system’s cooling coils — its evaporator — and forces the gas to occupy less space. In doing this, the compressor raises not only the pressure of the gas, but also its temperature. What comes out of the compressor is high-pressure gas that is quite hot. Hot, gaseous refrigerant can’t cool anything, so the next step in the refrigeration process is to get rid of much of this heat and condense the refrigerant back into a liquid. The heat that must be gotten rid of has two components:
· the refrigerant’s superheat — the heat that is released from the refrigerant gas in cooling it down from compressor output temperature (often 100 to 200 degrees C) to the condensing temperature of the refrigerant (typically 30 to 45 degrees C), and
· the refrigerant’s latent heat or “heat of vaporization” — the heat that must be removed from the gaseous refrigerant at its condensing temperature in order to turn it into a liquid.
The advantage of recovering superheat rather than latent heat is that superheat is relatively high temperature heat, and high temperature heat is useful in more situations than low temperature heat.
IS THERE A GOOD USE FOR THE HEAT WITHIN THE PLANT?
For heat recovery to be cost effective, the recovered heat must take the place of energy (fuel or electricity) that would otherwise have to be purchased. Possible uses of recovered superheat in food processing plants include
· heating or preheating wash water,
· preheating boiler feed water,
· heating or preheating domestic hot water,
· preheating process liquids (water or oil).
IS THERE ENOUGH SUPERHEAT TO MAKE IT WORTH RECOVERING?
That will depend, of course, on the characteristics of your particular refrigeration system, but a couple of examples might give you some sense of what is possible.
Example 1: Superheat recovery in the dairy barn
Refrigeration systems are used in dairy operations to cool the milk immediately after milking. Dairy operations also use large amounts of hot water for wash-down after milking. The refrigeration compressors used in these systems have capacities in the order of 3 to 5 horsepower (hp). In one manufacturer’s system, the recovered superheat from a 3 hp compressor will raise the temperature of 68 imperial gallons of water from 50°F to 130°F within 2.5 hours after the start of milking.
Example 2: Industrial recovery of superheat
In one industrial situation, the mechanical power
input to the refrigeration compressor is 67 hp (50 kW) and the recoverable superheat is calculated to be 68 kW. To compare this with the previous example, 68 kW of superheat will heat 723 imperial gallons of water from 50°F to 130°F every 2.5 hours.
THE ECONOMICS OF SUPERHEAT RECOVERY
In the dairy example, heating 68 gallons of water from 50°F to 130°F using electricity at $0.10 per kWh costs $1.60. Assuming two milkings per day and a use for all the hot water, superheat recovery in the dairy will save $3.20 per day or $1168 per year. The heat recovery unit costs approximately $2300 installed, making the simple payback just under two years.
In the industrial example, the economic viability of the scheme depends on the amount of #2 oil that the recovered superheat will displace. Calculations put the cost of the displaced oil at $3343 per year. The estimated cost of the heat recovery equipment, installation, and necessary changes to the existing system is approximately $10,000. This results in a simple payback of 3.0 years.
The key piece of equipment that must be added to a refrigeration system to recover superheat is the heat recovery unit or heat exchanger. There are many different heat exchanger designs, but all are devices that allow heat to move from one fluid to another while keeping the two fluids separate. In some designs, the two fluids are kept apart by metal plates. In others, one fluid flows through tubing that is surrounded by the other fluid. This is the type of heat exchanger shown in the figure below. The hot refrigerant from the compressor passes through a coil of copper tubing in an enclosure through which the water to be heated flows. Some of the heat in the hot refrigerant is transferred to the water, thereby raising its temperature.
In certain situations the hardware arrangement can be this simple. In others, refrigerant bypass lines, electrically operated valves, and control sensors may also be needed. In any event, this sort of addition to a refrigeration system should be carefully planned by someone having the necessary knowledge and skills.
If you have questions about superheat recovery or other energy-related issues, call Mike Proud or Ron Estabrooks of the Energy and Minerals Section at 368-5010 (toll free).