We’re going to cover how to Calculate GPM of a new Chiller Evaporator or Condenser Coil or any New Coil or Heat Exchanger by a field measurement of the pressure drop compared to the rated pressure drop of the equipment.
If you prefer to watch a video on this topic, scroll to the bottom of this page for link.
In this equation GPM2 is the actual flow rate that we’re going to calculate, it could be for any coil whether in a chiller, fan coil or heat exchanger.
How to Calculate Flow (GPM) in a Coil
So the Rated GPM is GPM1, times the square root of PD2, which is the actual pressure drop that we are going to measure. This is different than the Rated Pressure drop, which is PD1, the pressure drop across the Chiller’s Evaporator, Condenser or Heat Exchanger that your trying to measure.
So what will happen to the GPM if the Pressure Drop (PD) increases or decreases?
For our example we’re going to take a new chiller that was Rated for 600 GPM at 14 feet of Pressure drop through the Evaporator.
Measuring Flow Across a Coil
When we go into the field and measure the actual pressure drop we get 13 Feet.
Field Measurement Confirming an Actual Pressure Drop of 13′ vs the Rated Value of 14′
Now what we are going to do is we’re going to take all the known Rated conditions of the new piece of equipment, in this example the evaporator of a chiller and the actual measured pressure drop from our field technician.
So here is our formula. All we’re going to do is put in the known Rated values for the evaporator. Which GPM1, the Rated GPM is 600, times the square root of the actual measured pressure drop, which came out to 13 feet divided by the rated pressure drop (PD) of 14 feet for our example evaporator.
All it is at this point is math. So, doing the math we come up to 578. So our 600 GPM at 14 feet pressure drop Rated condition is now only flowing 578 GPM due to the decrease in pressure drop.
How to Calculate Pressure Drop in a Chiller or Coil
So a decrease in pressure equals a decrease in GPM.
The refrigeration cycle is the heart of the HVACR industry. Just like the heart in the human body which circulates life giving blood, the refrigeration cycle circulates refrigerant using a compressor. Before we explain the refrigeration cycle we need to explain a few principles of physics to help you better understand and retain the information.
Moving Heat
The purpose of the Air Conditioner or Refrigerator is to MOVE heat from one space to another. In the case of an Air Conditioner, you want to move the heat out of the building. Those in the HVACR industry are professional movers of Heat, either by moving heat out of a building or bringing heat into a building. One of the ways to do this is with the refrigeration cycle.
Heat is either moved out of or into a Building
Heat Flow
Heat flows from the warmer medium to the cooler medium. So for the condenser to reject its heat to the outdoors using an air-cooled condenser the temperature of the condenser has to be greater than the surrounding outdoor air. This is why during really hot summer days when the temperature is above the temperature rating of the condenser, which is usually rated for 95 Degrees, the cooling capacity is reduced.
Heat Flows from Warmer to Cooler Mediums
The greater the outdoor summer temperature above 95 degrees the worse your air conditioner will operate as it makes it difficult for the condenser to transfer its heat to the outdoors when the temperature difference is reduced.
See the notice located in the Air Conditioners manufactures literature for the performance of the Air Conditioners rating as shown below, the outdoor temperature is figured to be 95 degrees. Which means anything above that will reduce the tonnage (reduce cooling) of the AC Unit.
Temperature Rating of Condenser
Heat seeks to equalize with the medium around it by transferring some of it heat to the cooler medium until they equalize. You can’t transfer heat to something that is warmer, only something cooler.
Temperature and Pressure Relationship
To understand the compressor, you’ll need to understand the relationship between Temperature & Pressure. It’s very simple as they act in the same direction in the refrigeration cycle. If you increase the pressure than the temperature also increases. If you decrease the pressure the temperature also decreases. Pressure and Temperature move in the same direction like a one-way highway.
Temperature – Pressure Relationship in Refrigeration Cycle
In the refrigerant cycle we have the LOW & HIGH pressure sides. The high side is shown in red, while the low side is shown in blue in the following pictures. There are two sides, like in martial arts or Tennis. You have the Low Side & the High Side which is created by the compressor.
Phases of Refrigerant
Now we need to know what phases the refrigerant is found in within the system. You’re familiar with water so we’ll use that as our initial example. Water basically has three phases that it can be found in as follows;
Solid: Ice
Liquid: Water
Gas: Vapor, Steam
Phases of Water – Solid, Liquid and Gas
In the refrigeration cycle the refrigerant will only exist in two phases, as a LIQUID or a GAS. This is what it’s all about. The refrigerant just keeps going in a circle from Gas to Liquid, then back to Gas again. This is the basic refrigerant cycle, the continuous changing of the state of the refrigerant from gas to liquid, liquid to gas, over and over again while in the process of moving heat around. The high pressure side is related to heat, while the low pressure side is related to cooling.
So this is how we will find the refrigerant inside a Refrigerator, Air Conditioner or Chiller. Its either on the low or high side and in either liquid or vapor form. There are only four possibilities. LOW, HIGH, LIQUID or VAPOR.
Components of Refrigeration Cycle
Before we explain the refrigeration cycle, we need to understand the four basic components which are:
Compressor
Condenser
Metering Device
Evaporator
Compressor (From Low Pressure Gas to High Pressure Gas)
The compressor is what does most of the hard work in a refrigerant cycle. It pumps the refrigerant around the system. It does this by creating a pressure difference between one side of the compressor and the other side. The compressor is what creates the low and high sides of the system.
There are different types of compressors which will cover in another video, but the purpose is to move the refrigerant around the system. The compressor will only work with GAS, no liquid, so its important that only vapor enters the compressor inlet or suction line.
Refrigerant Compressor
The compressor takes low pressure refrigerant and increases it to high pressure refrigerant. And as we learned when the pressure goes up so does the temperature of the refrigerant.
Condenser (From High Pressure Gas to High Pressure Liquid)
The condenser does just as the name implies, it condenses the refrigerant from a High Pressure Vapor back into a High Pressure liquid by removing the heat either to the air stream or through a water-cooled heat exchanger. Remember heat wants to balance its self out, so that everything is the same temperature, it does this by transfer heat to a cooler medium. From the hot refrigerant to the cooler air blowing over the condenser coil.
Condenser Section of Refrigeration Cycle
The air blowing over the condenser comes in contact with the hot surface of the copper coil (pipe) as represented in the image. This causes the refrigerant vapor in the condenser coil to condense back into a liquid.
Condenser Heat Rejection
Metering Device (Expansion Valve) (From High Pressure Liquid to Low Pressure Liquid)
The metering device only allows as much liquid refrigerant that is required to pass through from the high side to the low side. The metering device is like the Parking Cop at the Professional Ball Park, only allowing as many cars to pass as there are parking stalls available in that area. The metering device only allows refrigerant to get through according to the cooling requirements.
Refrigerant Expansion Valve – Metering Device
Evaporator (From Low Pressure Liquid to Low Pressure Gas)
The evaporator does as its name implies, and it evaporates the liquid into a gas.
Evaporator – Refrigerant Evaporates absorbing Heat in the Process
As warm air passes over the Evaporator coil the heat is absorbed by the liquid refrigerant causing the refrigerant to boil off into a gas. The air leaving the coil will be much cooler as it has given up some of its heat to the refrigerant. See diagram below.
Evaporator Process
As the liquid refrigerant passes through the evaporator the warm air passing over the evaporator gives up its heat to the liquid refrigerant passing through the coil tubing of the evaporator causing the refrigerant to evaporate (convert from liquid to gas). The air gives up some of its heat, thereby becoming cooler. Remember as long as there is a temperature difference between two items, the warmer one will transfer some of its heat to the cooler one.
What is IPLV (Integrated Part Load Values)? It is based on part load chiller efficiency values. The IPLV is calculated on the weighted percentage of assumed operational hours at each operating condition. So there are four operating points that are going to be measured. This is an average efficiency of a single chiller. A lot of chiller installations will have multiple chillers. So this is a rating for a single chiller.
If you prefer to watch a video on this topic, scroll to the bottom of this page for link.
The IPLV formula is one over the four operational points that we are calling A, B, C & D. These will be the efficiency of the chiller at the different partial loads, which we’ll get into next. Were going to take condition “A” at 1%, condition “B” at 42%, condition “C” at 45%, and condition “D” 12%. This means that the chiller runs at condition a 1% of the time and 42% of the time at condition “B” and so on.
So what does “A”, “B”, “C” and “D” stand for? We are going to take these four data points for the chiller under review. Data point “A” is 1% of the chiller conditions running at 100% full capacity. The four data points are as follows:
Four Data Points Used in IPLV Calculation (EER)
“A” 1% of the time Chiller is running at 100% Capacity
“B” 42% of the time Chiller is running at 75% Capacity
“C” 45% of the time Chiller is running at 50% Capacity
“D” 12% of the time Chiller is running at 25% Capacity
These values will be put into the calculation
“A” = COP or EER at 100% Capacity
“B” = COP or EER at 75% Capacity
“C” = COP or EER at 50% Capacity
“D” = COP or EER at 25% Capacity
So, when the chiller is running at 100%, 75%, 50% and 25% we’ll put in the EER that corresponds to that datapoint.
Step 1 is to determine Part-Load energy efficiency values for the chiller running at these four datapoints as discussed previously as 100%, 75%, 50% and 25% load. These operating points are the AHRI (Air Conditioning, Heating and Refrigeration Institute) standard-550 conditions. Standard-550 specifies the conditions under which these datapoints are measured.
Step 2 is to insert the four data points into the IPLV formula. So now all you need is a sample of the EER for a particular Chiller that you want to analyze. We have a sample of the EER (Energy Efficiency Ratio) of a particular water-cooled chiller as shown here.
Example of Water-Cooled Chiller Efficiencies at differing Load Conditions
Based on the information obtained from the chiller manufacture as shown in the chart above, we can see that the chiller running at 100% as an EER (Energy Efficiency Ratio) of 0.63, and when running at 50% it has an EER of 0.29.
We take the information from above an insert it into our IPLV formula to derive at the IPLV number.
Chiller IPLV Formula
Remember that the chiller runs at full load only 1% of the time, so in the above example the 0.63 EER goes in the first column of the formula, followed by the EER of the chiller at 75% load and so on. You can see that the chiller has the worst EER at full load capacity and the best EER at 25% capacity. That’s why multi chiller plants may run several chillers at low load than to run one chiller at full load.
Below is a spreadsheet that we made to show various chiller manufacturers and how they compare in their total IPLV (Integrated Part Load Value).
EER Performance of 4 chiller manufacturers
You can see that the IPLV for chiller #1 comes out to be 0.335, while chiller #2 is 0.339 and the most efficient of the chillers is #4 at a IPLV of 0.311
AHRI Standard 550
AHRI (Air Conditioning, Heating and Refrigeration Institute) specifies the operating conditions under standard 550/590 that chillers are rated for as follows. According to AHRI the purpose and scope of the standard is as follows;
1.1 Purpose. The purpose of this standard is to establish for Water-chilling and Heat Pump Water-heating Packages using the vapor compression cycle: definitions; test requirements; rating requirements; minimum data requirements for Published Ratings; marking and nameplate data; conversions and calculations; nomenclature; and conformance conditions.
2.1 Scope. This standard applies to factory-made vapor compression refrigeration Water-chilling and Water-heating Packages including one or more compressors. These Water-chilling and Water-heating Packages include:
2.1.1 Water-cooled, Air-cooled, or Evaporatively-cooled Condensers
2.1.2 Water-cooled heat recovery condensers
2.1.3 Air-to-water heat pumps
2.1.4 Water-to-water heat pumps with a capacity greater or equal to 135,000 Btu/h. Water-to-water heat pumps with a capacity less than 135,000 Btu/h are covered by the latest edition of ASHRAE/ANSI/AHRI/ISO Standard 13256
AHRI Standard 550/590
You can see that the AHRI standard has the evaporator LWT (Leaving Water Temperature) at 44 F and various condenser entering water temperatures (EWT) for the various percentages of chiller loads corresponding to the four data points.
If a chiller is to run at other than AHRI Standard 550/590 conditions then its considered NPLV (Non-standard Part Load Values).
IPLV Calculation is Based on a single chiller. Data shows that 80% or more of the central plants have multiple chillers. This would then indicate that IPLV doesn’t accurately represent multi-chiller plant operating conditions. The use of energy analysis tools will provide a better solution. The use of the IPLV is used to compare unloading characteristics of similar chillers in a situation when there is only one chiller.
A chiller doesn’t run at full load most of the time. Chillers run less than full load most of the time, so this calculation is a better indicator of the chillers most likely operating efficiency.
Basically when you’re bidding a project that requires a bid bond it’s because the owner wants you to guarantee that you’re capable of fulfilling the contract requirements and that you’ll be able to secure a Performance & Payment Bond.
Usually a bid bond will roll into a performance bond.
Construction Bid Bond
So, what happens if you back out of a bid where you were the successful low bidder and for which a bid bond was issued on?
If you’re the low bidder and you decide not to execute the contract and you want to backout and you provided a bid bond to bid the project. The owner will file a claim against you and your surety company.
If you’re a small company most likely you put up some form of collateral or personal guarantee. You’re personally and financially responsible for the cost difference for the owner to secure the next lowest bidder up to your bond amount.
So, if you bid a $100,000 and the next lowest bidder was $110,000, and your bid bond was in the amount of 10%, your liability is the difference between your bid and the next lowest bidder, which makes you liable for the $10,000 difference if you decide not to execute the contract.
So, if you back out, you’re liable to your bond company for the claim amount plus attorney fees. So, they can come after you if you don’t pay the claim amount and the attorney fees. Read your contract to determine your liability exposure.
It’s almost better to perform the contract knowing you’re going to take a loss, at least you might be able to mitigate the loss with careful planning and great project management while hopefully there’ll be change-orders allowing you to recover some of the loss incurred on the base bid amount.
Bid bonds are usually in the range of 10%, but can vary depending on the contracting authority.
The bid bond percentage will be indicated in the bid documents.
Again, to reiterate. If you back out and you made a personal guarantee, then your personal assets could be at risk, so you want to read the fine print. You want the bond company to explain to you the terms and conditions of the bond.
Often there will be alternatives to a bid bond, such as submitting a cashiers check, certified check or a money order, or some other asset that can make good on your promise to execute the contract if you’re low bidder. Other assets that might be considered are liquid assets, that is something that can be converted into cash easily.
Bid Bond Alternatives
When are bid bonds required?
Most likely on all your Federal, State and Local Municipality projects.
Are bid bonds required on privately held or funded projects? A lot of project are now requiring bid bonds. We’ve seen as the larger the project gets, the greater the chances there will be a requirement to bond the project.
If you are going to be bidding in the Government sector or within the larger private sector projects than you need to build up your bonding capacity. You can’t just start a company and expect a surety company to issue millions of dollars’ worth of bonds on your behalf unless you back it up with collateral and have established record of completing bonded work, and have the area of experience required to complete the proposed project.
The bonding company will also tract how much bond exposure you have outstanding. This is for any work which is currently under bond coverage, and for which is yet to be completed, releasing the bond of any further risk.
So, how much do bid bonds cost?
For projects over $400,000, the bid bond could be free. If you have a good relationship with your bond company then it’s possible there won’t be any charge. For smaller projects there may be a nominal fee of say $100. It’s possible that they have an annual fee, which covers all the bid bonds you would need for the year. Check with your bonding company to discover how they structure their fees.
Bid Bond Cost
What happens if you submit a bid without a bid bond on a project that requires a bid bond. What will they do? Your bid will definitely be rejected. They will not accept your bid, even if you are low bidder.
What are Performance Bonds?
They’re used to ensure the project gets completed according to the construction documents. The bid bond usually also guarantees that your surety will issue a performance bond if you’re the successful responsible low bidder. Unlike the bid bond, the performance bond will definitely cost you some money. This is where the surety company gets their money, especially if they offered you the bid bond for free. They know that following a successful bid, you’ll need to pay for the performance bond.
Construction Performance Bond
Now with a Performance bond in place, what happens if you fail to perform? You’ve gotten this far and have been executed, that is signed the contract and are now required to perform. If you don’t perform then the bonding company will pay to have the project completed. So your bonding company will either payout to the owner or General Contractor whatever it takes to finish the project that you were contractually obligated to finish. The bonding company may pay another company to finish the contract, but they make risk having to pay more than what the original bond amount was issued for.
Maybe you file bankruptcy or something. They will make your surety company pay to complete the project. You can see why the surety companies want to make sure that you have the experience and financial resources required to complete the project that you are attempting.
The penalty is usually 100% of the contract amount.
What are Payment Bonds?
The payment bond is usually coupled with the performance bond. The payment bond ensures that you pay those who provide material or labor on your behalf for the proposed project. They want to be sure that you have paid all your subcontractors, equipment and material suppliers, including all the labor used on the project that the bond was issued for.
Construction Payment Bond
How much does a Performance & Payment Bond Cost?
Performance and payment bonds are often issued as one bond except in some locales where maybe it’s just a performance bond.
Bond Rates can vary from less than 1% up to 2% or more depending on the contract amount, your experience and credit rating.
For example, let’s say that you had a construction contract in the amount of $100,000 and your bond rate was $7.00 for every $1,000 of contract. Or stated as 0.7% of the total contract amount.
Bond Cost Calculation
So your cost calculation would look like this:
Calculation #1
Take the $100,000 divided by the $1,000 to get how many thousands there are. As each $1,000 of contract amount will cost you $7.00.
$100,000 / $1,000 = 100 x $7.00 = $700
Calculation #2
Take the $100,000 times 0.7$
$100,000 x 0.7% = $700
If you are just starting out then your bond rates will most likely be higher than this amount, but as you gain experience and grow your credit your rate can eventually be lower than this. Check with your bonding company for the rate structure.
How do you Qualify for a Bond?
Small companies may be require to have the owner pledge a personal guarantee. That is the small company owner personally guarantees to reimburse the bonding company for any claims against the bond. So you really want to think this through if you are a small company and are thinking of securing a bond for a project. You want to make sure that you have all your cost covered and you have an experienced team to execute the contract documents, including the funds to pay for the project as often accounts payable will exceed accounts receivables.
So if you’re a small company its possible you’ll have to provide a personal guarantee, and if your married than your spouse might also be required to sign the guarantee. Check with your bonding company or shop around for the best terms and conditions.
Large companies usually qualify based on their credit rating and experience. So until you get some experience and establish some credit behind you, you may have to give a personal guarantee.
In summary the bonds are insurance that you will do what you said you would, and that is, to execute the contract if you are low bidder and pay all those who helped you build the project, whether that was your subcontractors, or the vendors that sold you equipment or materials, and the workers that perform labor on your behalf.
