Primary-secondary pumping and control circuits are simple in theory and practice. Operation is based on this simple rule: WHEN TWO PIPING CIRCUITS ARE INTER-CONNECTED, FLOW IN ONE WILL CAUSE FLOW IN THE OTHER, T0 A DEGREE DEPENDING UPON THE PRESSURE DROP IN THE PIPING COMMON T0 BOTH.
PRIMARY-SECONDARY BASICS
The Monoflo fitting provides a fixed orifice between the riser connections of the radiation circuit into the main and is installed in piping which is common to both the radiation circuit and the main circuit. Flow in the radiation circuit occurs because of the orifice pressure drop. A typical Monoflo detail is illustrated in Figure 1.
Were the Monoflo fitting removed and the riser tees moved closer togethers , there would be practically no pressure drop between the points of riser connection. Due to the elimination of pressure drop in the piping common to both circuits, there would be practically no radiation circuit flow. So:
WHEN TWO CIRCUITS ARE INTER-CONNECTED, FLOW IN ONE WILL NOT CAUSE FLOW IN THE OTHER IF THE PRESSURE DROP IN THE PIPING COMMON T0 BOTH IS ELIMINATED.
This simple and definitive statement provides the basic ground rule for design of Primary-secondary systems. The fundamental circuit is illustrated in Figure 2.
All primary-secondary control methods are finally referenced to use of a “common piping” inter-connection between the primary and secondary circuits. Common piping is defined as a length of piping common to both the primary and secondary circuit flow paths; purposely designed to extremely low pressure drop.
The common piping length is quite short and can vary as between a close nipple and to an approximate maximum length of two foot. This provides for a minimum of pressure drop in this piping length and insures hydraulic isolation of the secondary circuit from the primary circuit. Flow in the primary circuit will not cause flow in the secondary because of low pressure drop in the common piping.
A secondary circuit pump is used to establish secondary circuit flow. This pump is illustrated in Figure 3.
The secondary circuit pump is sized to provide design flow rate through the secondary circuit with reference to secondary circuit pressure drop only. In the sketch shown in Figure 3 this includes pressure drops; A-B, B-C, C-D, D-E, EG and H-I. Since the common piping pressure drop (A-I) is slight, it will have no effect on secondary circuit pumping requirements and the secondary circuit can be considered separately and in hydraulic isolation from the primary circuitry.
In primary-secondary application the primary and secondary circuits are treated separately. Secondary circuit pump heads have no effect on the primary circuit pumping head requirements and vice versa.
This singular fact permits design of the large system as though it were a number of small systems. The function of the primary circuit simply becomes one of heat conveyance to or from the secondary, while the secondary circuit serves the terminal heat transfer units.
Since the secondary circuits are energy head isolated from the large primary pumps, the control problem in the secondary circuits is minimized; pressure ratio increases across control valves, etc. can be set low because secondary pump heads are low. In effect, control isolation is achieved with a remarkable decrease in operating problems.
The simple design procedures that will follow “rules and definitions” will establish other design advantages:
- Design to “deep” primary circuit temperature drops with corresponding reductions in primary pump and pipe size.
- Simple effective control methods in the equipment room; boiler and chiller applications.
- Outside air handling coil design methods for freeze protection.
- Application to heat-cool zone switch-over.
PRIMARY-SECONDARY RULES AND DEFINITIONS
Location of the Secondary Circuit Pump
The secondary pump should always discharge into the secondary circuit. This provides for an increase in secondary circuit pressure over that established in the cross-over bridge by the primary pump.
The common piping can be considered as the compression tank “No pressure change point.” It is consequently generally wrong to pump into the common piping from the secondary circuit because of a decrease in secondary circuit static pressure.
The Cross-over Bridge
The cross-over bridge is the cross connection between the primary supply main and primary return. It provides primary design flow rate to the common piping.
The bridge contains balance valves and may contain a flow indicator. It is quite often underslung to simplify the initial air venting problem.
Cross-over Bridge; Overhead
While the underslung bridge is generally preferred; overhead cross-over bridges are also employed:
The overhead cross-over bridge cannot become “air bound” and will continuously “air purge” providing the piping pressure drop from the primary supply main to the primary return main (ΔP in Figure 7, expressed in feet of water) is greater than height “H” in Figure 7. This is the usual case. Should height “HH” become greater than the estimated Δ P; or when downfed secondary circuits are used from an overhead cross-over, a manual air vent should be employed as illustrated in Figure 8.
Overhead cross-over bridges should be designed to a minimum velocity on the order of 2’/sec. in order to drive any accumulated air down the cross-over return and into the primary return main.
Cross-over Bridle Length
The cross-over bridge can be as long as necessary for inter-connection between the primary and the secondary circuits.
Cross-over Bridge Pipe Sizing
The cross-over bridge is generally pipe sized to a piping friction loss rate ranging from 100 M”/ft. (approx. 1 ft. per 100 ft.) to 500 M”/ft. (approx. 4’ per 100’) and to the required primary flow rate.
When required primary flow rate is equal to secondary flow, the cross-over bridge, common piping and secondary pipe sizes are equal as illustrated in Figure 10.
Quite often, the primary flow rate will be considerably less than secondary flow. When the common piping is a part of the cross-over piping, special application procedure should be followed to prevent any possibility of “jet flow” through the common piping.
This is generally accomplished by sizing the common piping to secondary circuit pipe size and extending this pipe size in the cross-over bridge at least 8 pipe diameters upstream and approximately 4 pipe diameters downstream; as shown in Figure 11.
Common Piping Length and Flow Characteristics
Common piping is designed for minimum pressure drop and can vary in length as between a short nipple and approximately two feet. Common piping flow rate and direction characteristics will be established by the relationship of primary to secondary flow rates. There are three basic evaluations that should be made:
- Primary flow greater than secondary flow.
- Primary flow equal to secondary flow.
- Primary flow less than secondary flow.
An example illustrating consideration a; primary flow rate greater than secondary flow is shown in Figure 12.
Common piping flow can best be determined by the “tee law”; a simple statement that flow into a tee must equal flow away from the tee. Tee “A” from Figure 12 is shown in Figure 13.
A similar evaluation at Tee “B” allows complete evaluation of common piping flow (see Figure 14).
It will be noted that secondary supply temperature must be equal to primary supply temperature so long as primary flow is only slightly greater than secondary flow. Most chilled water systems are designed with a constant supply water temperature requirement; primary supply water flow rate is consequently set at a slightly higher value than the secondary. This insures a continuous slight common piping bypass and establishes that secondary supply temperature is set by primary supply temperature.
Should the secondary circuit pump be stopped the common piping flow rate would immediately increase to 150 GPM and the entire primary flow would bypass the secondary circuit.
Consideration b; Primary cross-over flow equals secondary. The second consideration is for the case where primary flow equals secondary flow. The same circuit is used as previously except that the primary cross-over flow rate is decreased to 100 GPM. An evaluation at tee “A” is shown in Figure 15.
The over-all circuit can be shown as illustrated in Figure 16.
When the primary flow rate is set equal to the secondary there will be no flow rate in the common piping. Secondary supply temperature will again be equal to primary supply and secondary return will equal primary return temperature.
Consideration c; primary cross-over flow less than secondary. The third evaluation is for the condition where the primary flow rate is less thin the secondary. The same circuit is used as previously except that primary cross-over flow is decreased to 50 GPM, while the secondary is maintained at 100 GPM.
The evaluation at tee “A” is shown in Figure 17.
The over-all circuit can be shown as illustrated in Figure 18.
The most important characteristic of system design where secondary circuit flow rate is greater than primary is the mix occurring at tee “A”. Common piping flow, at a temperature equal to secondary circuit return mixes with primary supply water to provide a mixed secondary supply temperature. This most important characteristic provides smooth reset controllability, establishes “deep” primary circuit temperature drop possibilities and can be used to great advantage in the numerous P-S control arrangements made possible.
A second important conclusion that can be drawn from Figure 1 is that primary cross-over return temperature must be equal to secondary return. In general P-S design establishes that the primary cross-over bridge flow rate will be equal to or less than secondary flow. This means that primary cross-over bridge return temperature for the full load design condition will always be equal to secondary return.
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