Evaporator performance based on cooling water at 2.4 GPM per ton from 54°F to 44°F in R-22 service at 35°F SST and 6°F superheat.

Condenser performance based on R-22 service at 105°F condensing temperature and using 3 GPM per ton of cooling water at 85°F

Condenser performance based on R-22 service at 105°F condensing temperature and using 3 GPM per ton of cooling water at 85°F

planning to replace your home heating appliance to save fuel? Are you choosing the most efficient warm air furnace or hot water boiler to

maximize your fuel savings? Are you aware that just replacing the appliance will not maximize your fuel savings? There is more to it than just

sizing off the old unit or measuring the radiation or duct size.

Below is the top three ways of improperly sizing the appliance. When the heating or cooling unit is over sized it cost's more to install, short

cycles which lowers the efficiency and increases the maintenance costs. They will all size the heating or cooling appliance incorrectly. The

proper way is a heat loss or heat gain calculation.

1. Measure the existing radiation

2. Measure the square footage of floor space and multiply or divide by a magic number

3. Put in the same size as the one being removed

The above does not work!!!! The rules of thumb listed above always oversizes the equipment. The need for a heat loss today is critical with the

fuel costs as high as they are.

Most consumers believe by replacing the heating appliance they will automatically save the maximum amount of fuel. This could be no further

from the truth. In most cases, just replacing the heating appliance without a heat loss, a fuel savings will be encountered, but you will never

maximize your fuel savings unless the unit is properly sized and properly installed. Choosing the right unit is more than an educated guess!

All areas of North America have a given outside design temperature. The reason for this is the greater the temperature differences between

the inside and outside of your home the greater the heat loss. As the outside temperature warms up the heat loss is reduced. When the

equipment is sized with a heat loss, it will heat the home with the lowest fuel input. As the outside temperature warms up the unit will start to

short cycle. To sum it up, the unit will short cycle when outside temperatures are warmer than design temperatures. The shorter the cycles the

less efficient the operation becomes. These shorter cycles also will shorten the life of the mechanicals such as the motors and controls.

With this known, imagine a heating appliance that is oversized. Let's assume we need 70,000 Btu's to heat your home at a design outdoor

temperature of 10f, and the actual unit was improperly sized and is 120,000 Btu's. This unit is almost twice the required heat loss and this is not

uncommon. When it is running the fuel input will be 120,000 instead of the 70,000, which is the actual heat loss requirement. This is the start of

your problems. At the outside design temperature of 10f, the unit will short cycle. As it continues to warm up outside the short cycling gets

considerably worse then if the unit were properly sized. Every hour of running time the unit will burn 50,000 btu's more than required. There

goes your efficiency and wear on the motors and controls increases driving maintenance costs up. The fuel bill is higher than needed. Fuel is

wasted every time the unit is running. Happy heating!

The proper sizing of the heating or air conditioning appliance will maximize the efficiency of the appliance. Let's assume we properly sized the

hot water boiler and the old system had 1 thermostat (1 zone). We would get fairly good boiler efficiency. We then decide to break the system

into multiple zones (thermostats). The properly sized boiler now becomes over sized as all the thermostats will not all call for heat the exact

same time and they will not all satisfy the exact same time. The cycling of the different zones will overlap causing the boiler to short cycle

reducing the boiler efficiency. The system efficiency will improve the the boiler efficiency will go down. There are a number of ways to control

the boiler cycle rate. Here is a short list of things that can be done to extend the cycle time and increase efficiency, buffer tanks, circulators,

modulating condensing boilers and outdoor reset to mention a few. I would suggest run times of at least 7 minutes to peak your efficiency.

The system efficiency is as, or more important then the appliance efficiency. I don't want to down play the boiler or furnace efficiency but the

distribution system and near boiler piping, I feel makes or breaks the total efficiency package. I have done jobs that had a properly sized 84%

efficient boiler, and the fuel bills were still outrageous. We kept the same boiler and re-piped the near boiler piping. The fuel savings per btu's

per degree day was 62% for the season. I had another job where the homeowner had an 88% efficient properly sized unit installed and saved a

decent amount on his annual fuel bill. We added 22,000 btu's of baseboard to the system, installed a boiler bypass pipe and reduced the fuel

bill by 9%. Purchase the proper sized appliance and get a good qualified contractor with a good understanding of systems to install and setup

the appliance. This in no way discounts replacing the appliance with a newer more efficient one. A new appliance properly sized and proper

near boiler piping should save between 20% - 40%

As in warm air furnaces, boilers come in a multitude of efficiencies. Boilers and furnaces that operate at efficiencies above 90% are called

condensing, modulating/condensing or mod/con units. Most units operating in this range modulate the gas input, as needed which can create

added fuel savings. Units operating below 90% are noncondensingcondensing units and do not modulate the fuel input. Mod/cons will try to

match the heat loss as the outside temperature changes. What this means is the gas input will actually change. As the day warms up the unit

will input less gas and if the temperature gets colder the unit will increase the gas input.

The efficiencies on today's equipment will range from about 80% to 95%+. The higher the efficiency the less fuel used.

With all that said you should first upgrade the home with new doors and windows, add insulation, re-caulk and replace weatherstripping if these

areas have not been addressed. This will reduce the boiler size required which will lower your installation cost plus use less fuel.

Boiler Horsepower * 0.069 = Gallons of Water Per Minute (GPM)

Sq Ft of EDR * 0.000637 = Gallons of Water Per Minute (GPM)

Boiler Horsepower * 33,479 = BTU

Boiler Horsepower * 108 = Equivalent Direct Radiation (EDR)

Lbs per Sq In * 2.31 = Feet of Water

Lbs per Sq In * 2.036 = Inches of Mercury

Feet of Water (Head) * 0.4335 = Pounds per Sq In

Inches of Mercury * 13.6 = Inches of Water Column

Gallons of Water * 8.34 = Pounds of Water

Cubic Feet of Water * 7.48 = Gallons of Water

Cubic Feet per Minute * 62.43 Pounds of Water per Minutes

Cubic Feet per mInute * 448.8 = Gallons per Hour

Pounds of Condensate x 4 = Sq Ft EDR

EDR/ 1000 * 0.5 = Evaporation Rate Gallons per minute (GPM)

Pounds of Steam/hr / 500 = Evaporation Rate Gallons per minute (GPM)

Evaporation Rate * 20 = Receiver Tank Size (Gallon Storage at 20 Minutes)

Example; 4,500,000 BTU Output / 33.479 = 134.5 BHP

134.5 BHP * .069 = 9.28 GPM

9.28 GPM * 1.85 = 17.1 Pump GPM Required

17.1 * 20 = 342 Gallons Storage

Pump GPM * 1 = Receiver Tank Size (Gallons Storage)

Example ; 4,500,000 Output boiler / 240 = 18,750

18,750 EDR / 1000 x .5 = 9.37

9.37 GPM * 3 = 28 GPM Pump Capacity

Pump GPM * 1 = 28 Gallon Tank

will get a signal from a boiler pump controller to start and stop the pump. The boiler only gets water when it is needed. The boiler

does not overfill. If the near boiler piping is correct we get drier steam. The other difference is the make-up water for the boiler is fed

into the tank not the boiler. The feed water is controlled by a float. The benefit of this is as the boiler feed tank is hot due to

returning condensate it helps remove the damaging oxygen before getting to the boiler. It also give a place for dissolved solids to

drop out before getting to the boiler.

water in it to lift the float it makes a switch. This turns the pump on and pumps water back into the boiler. The problem is the boiler

may not need water at that time and it overfills the boiler. This will create wet steam. Wet steam moves slower and turns back into

condensate long before it should. You lose the heating capability of steam and must run the boiler to make more steam.

The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings.

But to really understand it, you first have to understand a couple of other

concepts:

Cavitation is what net positive suction head (NPSH) is all about, so you need to know a little about cavitation.

Vapor Pressure is another term we will be using. The product's vapor pressure varies with the fluid's temperature.

Specific gravity play an important part in all calculations involving liquid. You have to be familiar with the term.

You have to be able to read a pump curve to learn the N.P.S.H. required for your pump.

You need to understand how the liquid's velocity affects its pressure or head.

It is important to understand why we use the term Head instead of Pressure when we make our calculations.

Head loss is an awkward term, but you will need to understand it.

You will have to be able to calculate the head loss through piping, valves and fittings.

You must know the difference between gage pressure and absolute pressure.

Vacuum is often a part of the calculations, so you are going to have to be familiar with the terms we use to describe vacuum.

Lets look at each of these concepts in a little more detail :

Cavitation means cavities or holes in liquid. Another name for a hole in a liquid is a bubble, so cavitation is all about bubbles forming and collapsing.

Bubbles take up space so the capacity of our pump drops.

Collapsing bubbles can damage the impeller and volute. This makes cavitation a problem for both the pump and the mechanical seal.

Vapor pressure is about liquids boiling. If I asked you, "at what temperature does water boil ?" You could say 212° F. or 100° C., but that is only true at atmospheric pressure. Every product will boil (make bubbles) at some

combination of pressure and temperature. If you know the temperature of your product you need to know its vapor pressure to prevent boiling and the formation of bubbles. In the charts section of this web site you will find a vapor

pressure chart for several common liquids.

Specific gravity is about the weight of the fluid. Using 4°C (39° F) as our temperature standard we assign fresh water a value of one. If the fluid floats on this fresh water it has a specific gravity is less than one. If the fluid sinks in

this water the specific gravity of the fluid is greater than one.

Look at any pump curve and make sure you can locate the values for head, capacity, best efficiency point (B.E.P.), efficiency, net positive suction head (NPSH), and horse power required. If you cannot do this, have someone show

you where they are located.

Liquid velocity is another important concept. As a liquid's velocity increases, its pressure (90° to the flow) decreases. If the velocity decreases the pressure increases. The rule is : velocity times pressure must remain a constant.

"Head" is the term we use instead of pressure. The pump will pump any liquid to a given height or head depending upon the diameter and speed of the impeller. The amount of pressure you get depends upon the weight (specific

gravity) of the liquid. The pump manufacturer does not know what liquid the pump will be pumping so he gives you only the head that the pump will generate. You have to figure out the pressure using a formula described later on in

this paper.

Head (feet) is a convenient term because when combined with capacity (gallons or pounds per minute) you come up with the conversion for horsepower (foot pounds per minute).

"Head loss through the piping, valves and fittings" is another term we will be using. Pressure drop is a more comfortable term for most people, but the term "pressure" is not used in most pump calculations so you could substitute

the term "head drop" or "loss of head" in the system. To calculate this loss you will need to be able to read charts like those you will find in the "charts you can use" section in the home page of this web site. They are labeled Friction

loss for water and Resistance coefficients for valves and fittings.

Gage and absolute pressure. Add atmospheric pressure to the gage pressure and you get absolute pressure.

Vacuum is a pressure less than atmospheric. At sea level atmospheric pressure is 14.7 psi. (760 mm of Mercury). Vacuum gages are normally calibrated in inches or millimeters of mercury.

To calculate the net positive suction head (NPSH) of your pump and determine if you are going to have a cavitation problem, you will need access to several additional pieces of information:

The curve for your pump. This pump curve is supplied by the pump manufacturer. Someone in your plant should have a copy. The curve is going to show you the Net Positive Suction Head (NPSH) required for your pump at a given

capacity. Each pump is different so make sure you have the correct pump curve and use the numbers for the impeller diameter on your pump. Keep in mind that this NPSH required was for cold, fresh water.

A chart or some type of publication that will give you the vapor pressure of the fluid you are pumping. You can find a typical vapor pressure chart in the "charts you can use" section in the home page of this web site

If you would like to be a little more exact, you can use a chart to show the possible reduction in NPSH required if you are pumping hot water or light hydrocarbons. I will cover this subject in great detail in another paper.

You need to know the specific gravity of your fluid. Keep in mind that the number is temperature sensitive. You can get this number from a published chart, ask some knowledgeable person at your plant, or or take a reading on the

fluid using a hydrometer.

Charts showing the head loss through the size of piping you are using between the source and the suction eye of your pump. You will also need charts to calculate the loss in any fittings, valves, or other hardware that might have

been installed in the suction piping. You can find these charts in the "charts you can use" section in the home page of this web site

Is the tank you are pumping from at atmospheric pressure or is it pressurized in some manner? Maybe it is under a vacuum ?

You need to know the atmospheric pressure at the time you are making your calculation. We all know atmospheric pressure changes through out the day, but you have to start somewhere.

The formulas for converting pressure to head and head back to pressure in the imperial system are as follows:

sg. = specific gravity

pressure = pounds per square inch

head = feet

You also need to know the formulas that show you how to convert vacuum readings to feet of head. Here are a few of them:

To convert surface pressure to feet of liquid; use one of the following

formulas:

Inches of mercury x 1.133 / specific gravity = feet of liquid

Pounds per square inch x 2.31 / specific gravity = feet of liquid

Millimeters of mercury / (22.4 x specific gravity) = feet of liquid

There are different ways to think about net positive suction head (NPSH) but they all have two terms in common.

NPSHA (net positive suction head available)

NPSHR (net positive suction head required)

NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the

pump. This number is shown on your pump curve, but it is going to be too low if you are pumping hydrocarbon liquids or hot water.

Cavitation begins as small harmless bubbles before you get any indication of loss of head or capacity. This is called the point of incipient cavitation.

Testing has shown that it takes from two to twenty times the NPSHR (net positive suction head required) to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions.

To stop a product from vaporizing or boiling at the low pressure side of the pump the NPSHA (net positive suction head available) must be equal to or greater than the NPSHR (net positive suction head required).

As I mentioned at the beginning, NPSHA is defined as static head + surface pressure head - the vapor pressure of your product - loss in the piping, valves and fittings .

In the following paragraphs you will be using the above formulas to determine if you have a problem with NPSHA. Here is where you locate the numbers to put into the formula:

Static head. Measure it from the centerline of the pump suction to the top of the liquid level. If the level is below the centerline of the pump it will be a negative or minus number.

Surface pressure head. Convert the gage absolute pressure to feet of liquid using the formula:

Pressure = head x specific gravity / 2.31

Vapor pressure of your product . Look at the vapor pressure chart in the "charts you can use" section in the home page of this web site. You will have to convert the pressure to head. If you use the absolute pressure shown on the

left side of the chart, you can use the above formula

Specific gravity of your product. You can measure it with a hydrometer if no one in your facility has the correct chart or knows the number.

Loss of pressure in the piping, fittings and valves. Use the three charts in the "charts you can use" section in the home page of this web site

Find the chart for the proper pipe size, go down to the gpm and read across to the loss through one hundred feet of pipe directly from the last column in the chart. As an example: two inch pipe, 65 gpm = 7.69 feet of loss for

each 100 feet of pipe.

For valves and fittings look up the resistance coefficient numbers (K numbers) for all the valves and fittings, add them together and multiply the total by the V2/2g number shown in the fourth column of the friction loss piping

chart. Example: A 2 inch long radius screwed elbow has a K number of 0.4 and a 2 inch globe valve has a K number of 8. Adding them together (8 + 0.4) = 8.4 x 0.6 (for 65 gpm) = 5 feet of loss.

In the following examples we will be looking only at the suction side of the pump. If we were calculating the pump's total head we would look at both the suction and discharge sides.

Let's go through the first example and see if our pump is going to cavitate:

Given:

Atmospheric pressure = 14.7 psi

Gage pressure =The tank is at sea level and open to atmospheric pressure.

Liquid level above pump centerline = 5 feet

Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.

Pumping =100 gpm. 68°F. fresh water with a specific gravity of one (1).

Vapor pressure of 68°F. Water = 0.27 psia from the vapor chart.

Specific gravity = 1

NPSHR (net positive suction head required, from the pump curve) = 9 feet

Now for the calculations:

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

Static head = 5 feet

Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute

Gage pressure = 0

Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg =

0.27 x 2.31/1 = 0.62 feet

Looking at the friction charts:

100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping

The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet

Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.

NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 =

36.04 feet

The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.

Example number 2 . This time we are going to be pumping from a tank under vacuum.

Given:

Gage pressure = - 20 inches of vacuum

Atmospheic pressure = 14.7 psi

Liquid level above pump centerline = 5 feet

Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.

Pumping = 100 gpm. 68°F fresh water with a specific gravity of one (1).

Vapor pressure of 68°F water = 0.27 psia from the vapor chart.

NPSHR (net positive suction head required) = 9 feet

Now for the calculations:

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

Atmospheric pressure = 14.7 psi x 2.31/sg. =34 feet

Static head = 5 feet

Gage pessure pressure = 20 inches of vacuum converted to head

inches of mercury x 1.133 / specific gravity = feet of liquid

-20 x 1.133 /1 = -22.7 feet of pressure head absolute

Vapor pressure of 68°F water = pressure x 2.31/sg. = 0.27 x 2.31/1 =

0.62 feet

Looking at the friction charts:

100 gpm flowing through 2.5 inch pipe shows a loss of 17.4 feet or each 100 feet of pipe or 17.4/10 = 1.74 feet loss in the piping

The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet

Adding these two numbers together: (1.74 + 0.6) = a total of 2.34 feet friction loss in the pipe and fitting.

NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 0.62 - 2.34 =

13.34 feet. This is enough to stop cavitation also.

For the third example we will keep everything the same except that we will be pumping 180° F. hot condensate from the vacuum tank.

The vapor pressure of 180°F condensate is 7 psi according to the chart. We get the specific gravity from another chart and find that it is 0.97 sg. for 180° F. Fresh water.

Putting this into the pressure conversion formula we get:

pressure x 2.31/sg. = 7 x 2.31 / 0.97 = 16.7 feet absolute

NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings

NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 16.7 - 2.34 =

-2.74 feet.

We need 9 feet, so the pump is going to cavitate for sure.

A few notes about this last example:

A negative NPSHA is physically impossible because it implies that the friction losses exceed the available head and that cannot happen. The rule when pumping a boiling fluid is: The NPSHA equals the Static Suction Head minus

the Suction friction head because the suction surface pressure and the vapor pressure equalize one another. The absolute pressure in the tank is 34

-22.7 = 11.3 ft. The vapor pressure of the condensate in the tank converts to 16.7 ft of head (see above) so the condensate is boiling /flashing and reaching a state of equilibrium.

When pumping a boiling liquid, the Static Head must exceed the Suction Friction Head (2.34 feet) by the amount of NPSH Required (9 feet) or: (9 ft.

+ 2.34 feet = 11.34 feet.) We can do this by raising the level in the

suction tank an additional 6.34 feet to get the 11.34 feet required (6.34 feet + 5 feet existing = 11.34 feet)

In some instances you could reduce the Suction Friction Head to get the same result, but in this example there is not enough friction head available to reduce.

This example also allows you to shortcut NPSHA calculations any time you are pumping from a tank where the liquid is at its vapor pressure. Oil refineries are full of these applications.

If you are given the absolute and vapor pressures in psia, and you forgot how to convet to feet of head; you can use the following formula, providing you know the specific weight of the liquid you are pumping :

Pp = Absolute pressure expressed in psia. In an open system, Pp equals atmospheric pressure, Pa, expressed in psia.

Pvpa = Vapor pressure expressed in psia.

W = Specific weight of liquid at the pumping temperature in pounds per cubic foot.