Shallow Well Power Consumption - GWHP

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Valveman

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Sounds about right to me. I would only add that because of the problems of using the affinity law with pumps that must produce a constant head, the difference in power consumption between VFD and a throttling valve always look better for the VFD on paper than in real life. See this quote from another pump engineer.

“7. Finally, beware using the affinity laws for calculating a new pump diameter or speed for
systems that have a high static head, the affinity laws apply only between two points that
are at the same efficiency.â€


You also would not need to bypass any flow for motor cooling with a throttling valve. With a VFD you are creating a smaller motor from a larger motor, so you still need ample cooling for a smaller motor running at full load. This is why you need a minimum of 1.2 GPM to keep the motor cool.

“A variable frequency drive will introduce harmonic currents into the
motor windings which will cause a nominal increase of five per cent in
motor heating and therefore five percent higher energy losses.â€

Using a throttling valve, you are derating a large motor by making it draw a smaller load. A derated motor can pump hot water without problems, so it needs very little cool water for proper cooling. If the motor is still spinning at full speed and running on smooth sinusoidal power, restricting the flow with a valve will decrease the load enough that only 2/10 of a GPM are required for adequate cooling. This eliminates the need for a bypass as long as at least 2/10 of a GPM are being used, and also proves how much cooler a motor runs when throttled by a valve, compared to when being heated by a VFD. And heat is what destroys a motor, be it sooner or later.
 

Valveman

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I also meant to add that you can still get the 1/3 HP pumps and motors. Putting the pump on the motor if needed is a simple 4 bolt process. If you can get an actual 1/3 HP motor, it will have less rotor weight and help save energy. Sometimes they just stamp a ½ HP with a 1/3 HP stamp, and the length is the same. Also, I don’t think you can get a ½ HP three phase motor to use with a VFD. If you have to use a 3/4 or 1 HP motor just to get three phase, there is more iron to spin, and it will add to the energy used.

I also have a system with three flow rates required. The three flow rates require 1/6th, 1/3rd, and ½ HP load. I have a 7S03-6 (1/3rd HP pump and motor) that I throttle with a valve to get the lowest flow rate, and it uses just over 1/6th of a HP. Then when I need ½ HP worth of water, the 1/3 HP pump is helped by a 1/6 HP circulator type booster pump, and the two pumps together give me the ½ HP water when needed. Most of the time the 1/3 HP pump runs alone and not throttled, which makes all three ways the most efficient I could possibly make it.
 

Sixlashes

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You can still purchase the Franklin 1/3 hp motor for about $215. It has a little higher service factor (1.75) than the 1/2 HP (1.6) but is still physically smaller. A 1/2 hp 3 phase motor is also available for the same price.

With a VFD you are creating a smaller motor from a larger motor, so you still need ample cooling for a smaller motor running at full load. This is why you need a minimum of 1.2 GPM to keep the motor cool.

This is what Franklin states on the Application Installation Manual in the section titled "Variable Speed Submersible Pump Operation, Inverter Drives" concerning minimum flow:

"Motor Cooling Flow: For installations that are variable-flow, variable-pressure, minimum flow rates must be maintained at nameplate frequency. In variable-flow, constant pressure installations, minimum flow rates must be maintained at the lowest flow condition. Franklin’s minimum flow requirements for 4†motors : 0.25 ft/s (7.26 cm/sec)."

Is this the basis for your statement above? Is the .25 ft/s where you get the .2 gpm? Can you interpret? This is a huge issue for me planning the installation.

Your application of using a booster pump to get your gpm up intrigues me. Did you add the booster to your existing pump, or plan it that way?

After much thought on this subject, will a day (or more) without air conditioning (or heat) while working on the more complex VFD system be worth what I saved in electricity?
 

Valveman

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I checked my books this morning when I got to work, and yes you can get a ½ HP in 3 phase. Years ago I had to special order 1 HP motors in three phase for a specific customer.

1.2 GPM with a 4" motor and a 4" casing or shroud, which is the .25 feet per second flow, must be maintained when using a VFD, because of the excess heat produced by the VFD.

You need to look at the hot water applications to see what franklin says about derated motors. It says that if you derate a ½ HP motor by as little as 25%, you can safely pump water that is 140 degrees. 140 degree water, moving at 3 feet per second, does not cool a motor as well as water cooler than 86 degrees, and moving much slower. Franklin says with their "super stainless motor" (less that 2 HP), that a shroud is not needed. I have had to do the test myself to know that a derated motor, running on standard AC power, will have plenty of cooling at flow rates down to 2/10 of a GPM, when pumping cool water.

When used with a CSV, some pumps with a low differential pressure have a minimum flow of ½ of a GPM. Some of these motors have been running for nearly 15 years this way without a failure. Even though 2/10 of a GPM is sufficient, I always try to fudge it up a little, and that is why the standard minimum flow with a CSV is 1 GPM, which is really 5 times more than required.

Yes I planned on using the 1/6 HP booster with my 1/3 HP submersible, and it works great. I have a pressure relief valve that holds the 1/3 HP pump back to 56 PSI, where it delivers 3 GPM to fill a stock tank. If the house uses water the pressure will drop to 45 PSI, where the pump is delivering 6 GPM. Then if the house and the heat pump are both running, I need 9 GPM, and the 1/6 HP booster comes on. When the house and heat pump are no longer using water, it reverts back to pumping 3 GPM to the stock tank. The system uses 1.9 amps when the one pump is restricted to 3 GPM. It uses 2.8 amps at mid range flow, and 4.2 amps at max flow, with both pumps running.

Murphy is the one who controls when a pump system fails. It will always be at the worst time. Usually you have a house full of company, on Christmas Eve, and you end up replacing everything quickly, instead of repairing, just to keep everyone happy. Then all the money you thought you were saving, went to getting the thing back to working quickly.

I also want to say that I installed a 3 ton unit when a 4 ton was recommended. I was afraid the 4 ton unit would cycle on and off too much, which is bad for the heat pump and the well pump. The 3 ton unit keeps the house plenty cool or warm but, it will run almost continuously when the temp outside is 104 degrees, or extremely cold. It cycles slowly on days when the temperature is more normal but, I was afraid a 4 ton unit would have cycled a lot on really hot days.
 

Sixlashes

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valveman

It sounds like your 3 ton unit is right in the sweet spot for your application. If it is sized exactly by the book, during 2-5% of the hottest days, it will run continuously and barely not keep up, i.e. inside drybulb a couple degrees about normal setpoint. You made a good choice. HVAC contractors are notorious for oversizing, thus resulting in the unit short cycling. Where I live, that is a bad scenario since it leaves a lot of humidity in the house.

104 degrees !?!? I have been whining to the wife about working outside in the high 90's. :eek:

Since I have ICF walls here in Florida, I am very conscience of getting rid of the humidity. I am installing the 4 ton, 2 speed unit to accomplish what you did by not oversizing. According to the the Manual J, I have a sensible heat gain of 30,400 Btu at the outside design air temp of 95 degrees. The Model 049 is certified at 26,700 Btu on part load and 34,500 Btu on full load with my water temp. Even with the Manual J over rating the heat gain (common), the part load should never short cycle where is counts and give me problems.

I am still unsure on the issue of overheating the pump while the unit is in dehumidification mode. I know the unit will need close to 1.6 gpm of cooling water flow when the reheat coil water temperature is balanced. The possible problem is during the period of time it takes to modulate the reheat water up to the required temp. The geo unit uses an electronically modulated mixing valve and recirculating pump to get the water temp up. What I don't know is what the minimum cooling water flow rate is. Since this occurs when the unit first turns on, I think it could last for several minutes while the condenser coil gets up to running temp. I will contact Climatemaster to see if they will share this with me.

Assuming the cooling flow rate drops below 1 gpm, or even .5 gpm for 1 to 5 minutes, should I use a differential pressure bypass valve set to let a minimum of 1 gpm through? Or should I use a tank and pressure cutoff switch set to only cutout during this warm up scenario?

I will be using the dehumidification mode a lot during the spring and fall when the thermal mass in my walls keeps the cooling load minimal while the outside humidity is high.

How much head are you pushing with your pump(s)? I assume you are running 230V single phase. Are you using a control with a running capacitor.? I've read that helps in the efficiency of the motor.
 

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Hi sixlashes
107 here on Sunday, and the 3 ton unit is still keeping the 2,000 sq ft house nice and cool but, it doesn't stop running until the sun goes down. I spent some extra on insulation because of just using the 3 ton unit, and I am also happy with that.

I was going to suggest that you use a pressure tank to handle your really low flow rates. A tank that holds 30 gallons of draw down would deliver 1 GPM efficiently, and would only make the pump cycle twice each hour. However, now you need to use a pressure switch to control the pump, instead of a relay from the heat pump.

This is where a CSV comes into play. It will allow you to use all the draw down from the tank, and still keep the pump from cycling when flow rate 2 and 3 are needed.

I would say that if the flow rate drops below 1 GPM, you would need to use a pressure tank, or a bypass. A bypass will be wasting energy, the pressure tank will not. However, this size pressure tank will cost you about $500. It would take a long time for the slight energy savings to pay off the pressure tank.

My pump is lifting from 80', and the pressure relief valve is set at 57 PSI. This makes my 7S03-8 pump produce 3 GPM to the stock tank. This pump also supplies water to the house, so when a tap in the house is opened, the pressure drops below 57 PSI, the pressure relief valve closes down, and the well pump can supply as much as 6 GPM at 40 PSI to the house. This pump runs like this 24/7/365, and the 1/3 HP pump uses about $75 every month. When my heat pump comes on, the 1/6th HP booster comes on with a relay. This increases the flow to the heat pump from 3 to 4 GPM, which is what my system needs. This is a weak well that will only make 4 GPM over the long run. It has some storage in the well so I can use 6 GPM for about 30 minutes, or 10 GPM for about 10 minutes.

All the water goes through the heat pump before it goes to the house, so when the house is using water, this is also the amount of flow going through the heat pump. The 1/6 th HP booster runs anytime the heat pump is on but, is really only needed when no water is being used in the house. Running water through the heat pump before the house, means that my house water is warm in the summer and cool in the winter. The warm water in the summer is great. It is almost exactly the correct temperature for a shower, and the water heater rarely comes on. The line going to the sinks has cool water in them for a minute, so you can still get a cool glass of water. You just have to be careful with the washing machine because, the cold water is really about 96 degrees.

During the winter, the cold water in the house is about 45 degrees. You can really get a cold glass of water right out of the sink. However, the cold water in the shower is also 45 degrees, and the water feeding the water heater is also 45 degrees. This means even with a 50 gallon water heater, and the de-super heater working, you run out of hot water quickly when taking a shower. I have learned to turn off the heater for a few minutes when taking a shower, to save hot water and energy. This is a small sacrifice to make for being able to double use the water. I had no choice in this matter as the well only makes 4 GPM but, it does save considerable energy over having to supply 4 GPM to the heat pump on top of the 6 GPM the house needs. The only other thing I have noticed is that the warm water makes a ring at the water level in the toilets. A little chlorox quickly takes care of it.

I also have a spare well that only makes 1 GPM but, has 100 gallons of storage. So I have a 10 GPM, ½ HP with a CSV on that well. Only when we have company and there are lots of shower happening does the pressure drop below 40 and the spare well come on. The CSV makes it produce only the extra water needed over the 6 GPM being used. So if the house requires 8 GPM, the CSV on the spare well adds 2 GPM to the 6 GPM already coming, and the 100 gallons in the spare well will last 50 minutes. I actually have two houses and a barn on this water system, and even when the in-laws show up, I have never ran out of water even when 15 people are using water and taking showers.

Even during the hottest or coldest months, the heat pump has never used more than $75. This is great compared to the trailer house, which can easily use $250 on a hot month, and is only half the size of the main house.

I did have a 3 wire motor with a control box that lasted 14 years. I replaced it with a 2 wire motor and can't tell any difference in the electric bill.

Here is a picture of my system. The blue circulator is for the heat pump. The other circulator sends water to a wood burning stove so I can make free hot water when I want to.
 

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