Definition: Plant: meaning a facilities / operation where manufacturing is occurring. Cooling: 1) free of heat 2) capable of lowering temperature. System: an assemblage or combination of things or parts forming a complex or unitary whole: a cooling system Piping: a hallow cylinder used for the conveyance of water, steam, gas etc.
Piping is a critical function of any plant, and is one of those areas that sometimes are overlooked; this is especially true in the layout and size. The pipe sizes and especially the pump sizes must be able to supply the fluid necessary, at the proper flow /pressure and be able to compensate for variations.
In General:
The pumping of water creates two things which are in fact related; one is flow as measured in GPM / LPM (gallons per minute / liters per minute) and also pressure. While there are numerous types of pumps on the market, one has to be aware that most are based on RPM, and thus if one were to change a motor to a higher RPM rate than one would also see an increase in GPM/LPM and possibly also pressure.
In the area of piping there is data available which states the maximum allowable pressure, and also flow rates one should use with a particular size pipe diameter. The maximum flow rate through a pipe is based on manufactures recommendation that a velocity of 5 Feet per second (FPS) not be exceeded. This is due to the fact that above this speed/velocity a pipe can wear out, and warranties for the pipe may be voided.
Examples of maximum flow and pressure ratings: Pressure (psi)/flow (gpm), based at 5 ft. per second velocity.
|
Type |
½" |
¾" |
1" |
2" |
3" 4" |
|
SCH 40 |
600/6 |
480/9 |
450/13 |
280/53 |
260/115 220/200 |
|
SCH 80 |
850/4 |
690/7 |
630/11 |
400/47 |
375/105 320/180 |
From looking at the above one may want to ask what is the flow needed for the mold or other processing equipment. A simple calculation again of needs may indicate that the 2" line from the tank does not calculate because of the 5 machines which each require 15 gpm for minimum processing.
In addition to maximum flow there should be a minimum flow in the pipes to prevent the buildup of sludge and or biological inside the pipe, thus minimum flow should be at turbulent rate or velocity. This turbulent rate can be calculated and expressed in a number referred to as Reynolds Number. A number for a pipe as minimum for turbulent is 4000, anything below 2300 is considered laminar and between the two number is a transitional area. Please note that temperature, composition of fluid all play an effect in the calculation and that those number given are based on pure water at room temperature.
(Temperature and composition play with density of fluid which is used in the formula)
Other things:
1- If using water cooled chillers the rule of thumb for cooling them is 3 gpm per ton of cooling capacity, thus the 5 ton chiller used on the floor requires 15 gpm to be efficient.
2- If using a hydraulic machine, what are manufactures recommendations for flow of coolant water to machine for its heat exchanger. (note this may be published and non-published data) On a 900 ton machine for example 21 G/min, (79 L/min) while on a 200 ton it is 13 G/min (49.2 L/min.)
3- It is important to calculate out the pipe size, gallon per minute and costs of pump operation. While it is safe to say that a larger diameter pipe is easier flow, the cost of running a larger pumping system well cost more.
4- All pipes at the machine / or drop down at end of machine should be marked as to tower, chiller, to machine or from machine, so as all are identified properly.
5- All pipes at machine / drop down at end of machine should be equipped with shut offs, ball type lever are most efficient and least for pressure reduction.
6- The smallest orifice in the system shall be the point of controlling the flow. Designing a system that is as large as convenient to the manifold on the machine for your molds or other cooling needs is ideal.
7- Flow meters are great ways to measure the flow of fluid, but pressure may also be used to measure flow. When using pressure one should always look for a delta between "to and return sides" or what is being cooled.
8- Since pressure reading is easy to use, gauges or at least some sort of hook up for gauges may be considered at various points in the piping layout, especially at the point closes to the cooling plant, and at the furthest point from the cooling plant. (i.e. first machine and last machine within the circuit)
The tower circuit:
If one were to use only a tower to supply all the plant's needs, than understanding the equipment that it is supplying is critical. What is meant is that in supplying fluid to such things as hydraulic machines, thermulator, chillers, or other equipment which are equipped with a solenoid which opens and closes by itself for its needs of its cooling water. The issue being is that the pumps are supplying a constant flow/pressure of fluid to the plant and now the demand is varying by what is turned on and off. Since mold cooling is continual / constant the concern here is there variation to its fluid flow. If the cooling system is set up correctly than not to worry, as this on and off shall not affect the supply. If in fact we are at the limits (perhaps minimum flow) than it may be noticed that things changed but nobody changed anything.
The primary way is to set this system up with a single piping plan, with the main pipe and then a down pipe at each machine location, and then some sort of distribution at this point for the machine, molds and auxiliaries. At the down point a valve to shut off should be placed, so as to be able to isolate all items downstream of this valve. The return lines should match up with the supply as far as the valves and pipe size. (Never go smaller in diameter back to the cooling plant)
Another way to set up only using a tower is to use two complete systems, which than allows one system to be used for the machines, auxiliaries. The additional piping system would than be used for the molds only, thus no cycling of fluid due to solenoids. The down side is that it would require extra pumps and additional fittings ports to the pump tank and system components. But with two systems one now has separated out the molds, from the machines, thus one may find that some consistency is seen.
It is more economical to use a single system, but it does need to be design correctly and of adequate size so as to provide proper flow at all times.
Chiller circuit
If one is going with a central chiller than the piping is dedicated to the mold only, and all piping should be marked as such. The main line should be of adequate size along with the down lines to the press/mold with a shut off valve at wall prior to connection to piping to manifold. Sizing must be correct as in many cases diameters reduction is greatest in this area from the authors' experience, which leads to longer cooling times.
*** It is important to inform operators and personnel that chiller lines are not to be hooked to auxiliary equipment and only to molds. This is from a cost standpoint, why does one want to cool down fluid only to than heat it up via the thermulator. Secondary, if one hooks up the thermulator it may dump heated water into the chiller circuit thus causing the chiller to work harder.
Piping and plumbing of the pipes is crucial to an efficient cooling delivery system. As an example in one case for a molder it was pointed out that they had placed a small valve on a chiller supplying the plant. The total cooling for this chiller load was calculated out to be only 3 tons, and this chiller was 5 tons. The plant informed the author that they were updating their cooling to a 10 ton chiller, and not to worry. Upon return the new 10 ton chiller was in place, and also new piping eliminating the small valve in the system. Though their demand had not increased (load was still at 3 tons) they were getting better runs due to new 10 ton chiller in their mind. In my mind it was the elimination of the small valve.
SL Silvey
24082012.01
www.silveysplasticconsulting.com
360-882-3183
