Warm water for washing the udders of dairy cows and hot water for washing and sanitizing milk handling equipment are required for the operation of a modern, commercial dairy farm. Heating the water for these purposes requires about one-third of the electrical energy that is used in a dairy operation. A significant portion of this expense can be economically eliminated through the use of waste heat recovery from the milk refrigeration system. At the same time, valuable energy can be saved.
Milk contains valuable energy: In this country, the most common unit of thermal energy (heat) is the the BTU (British Thermal Unit). A BTU is the amount of energy needed to heat one pound of water (or milk) by one degree Fahrenheit. This energy also becomes available as milk is cooled. A cow produces milk at about 102° F. When this milk is cooled to about 37°F for storage (a difference of 65°F), 65 BTUs of heat per pound of milk must be removed. Thus, if a cow produces 50 pounds of milk a day, 3250 BTUs of energy are available per day per animal.
A refrigerator transfers heat from a cold area to a warm one: All refrigeration units operate by pumping a special fluid around in a loop. This loop has a hot end and a cold end. At the cold end, the refrigeration fluid is at a low pressure and evaporates as it absorbs heat. It is then compressed to a hot, high pressure state and condenses as it gives off heat. In this way, heat taken from the milk is transferred to room air and often wasted.
While refrigeration heat must be removed from the condenser, there is no need to waste it. Commercial equipment is available that economically recovers part of this heat. This equipment falls into two basic categories: Desuperheating units and fully condensing units.
Desuperheating units can be inexpensively added to existing refrigeration equipment: In order to recover part of the heat in the refrigeration fluid, the hot vapor must be passed through a coil in a heat exchanger before it goes through the condenser. At the same time, cold water must be pumped through an adjoining coil in the heat exchanger so that heat is conducted from the hot refrigeration fluid to the cold water. This produces warm water, typically 95 to 105°F, that is stored until it is needed. Thus, the system is very simple. The heat exchanger pump operates only when the refrigeration unit operates, and all the refrigeration control functions remain connected to the condenser fan. However, this system can use only part of the available heat, typically 40 to 50 percent, to warm process water.
Fully Condensing units
Fully condensing units replace the condenser of an ordinary refrigeration system: A fully condensing unit is completely water cooled and eliminates the need for an ordinary, air cooled condenser. Nearly 100 percent of the available refrigeration heat is recovered from such a unit as hot water, typically at 120°F to 140°F. However, this heat recovery equipment must be properly adjusted to the requirements of the refrigeration system. It probably should be adapted only as an integral part of a new or replacement refrigeration unit.
A valve controls the flow of cold water through the heat exchanger so that this flow meets the requirements of the refrigeration system. This cold water, either from the bottom of the storage tank or from the cold water inlet, is needed to condense the hot vapor into cooler liquid refrigerant while absorbing and carrying away the heat of condensation. If this doesn't occur, the pressure and temperature of the refrigeration fluid will rise and the compressor will soon become damaged. As long as there is cool water in the bottom of the storage tank, the pump will circulate this water through the heat exchanger and the warmed water is stored for future use. However, once the storage tank is fully heated, another valve opens and excess warm water is discharged from the system while additional cold water is drawn into it to supply the needed cooling. This excess warm water can be used in the home, given to the cows as drinking water in the winter or put to other use on the farm. It does not have to be wasted.
The value of heat recovery depends on the size of the herd being milked and on the cost of the energy being replaced. It is convenient to express energy costs in terms of the amount that must be paid for a million BTUs (MBTU) of energy used to heat water.
Electricity is the most expensive way to heat water. However, it is frequently the only convenient method available to a dairy operator.
The amount of heat used for hot water on a dairy farm and the amount of heat that can be recovered from the refrigeration system both depend on the number of cows being milked. A large herd requires more energy input and produces more milk output than a small one. Consequently, a large dairy operation has more expense for energy to heat water and greater potential for saving on this expense.
Most dairy equipment companies now supply refrigeration heat recovery equipment. Typically, this equipment is of the add-on, desuperheating type that can be adopted at any time. As noted previously, such equipment is generally economical and its adoption is recommended. In addition, some dairy equipment companies supply fully condensing refrigeration systems. Such equipment is more efficient in energy collection than the desuperheating units and can save more money. However, such equipment is considerably more expensive and will only be economical if a new refrigeration system is required for other than energy conservation reasons. In either case, you should consult your dairy equipment supplier to find out what equipment will best fit your needs.
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This page was last updated on November 16, 2002